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New applications of X-ray optical techniques
Nuclear Instruments and Methods in Physics Research 221 (1984) 251-264 North-Holland, Amsterdam
251
Section VII. Appfications of X- and y-ray imaging systems in astronomy N E W A P P L I C A T I O N S O F X-RAY O P T I C A L T E C H N I Q U E S J.L. C U L H A N E 1, W. C A S H 2 a n d R.C. C A T U R A 3 i Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey RH5 6NT, England 2 Laboratoryfor Atmospheric and Space Physics, University of Colorado, USA s Lockheed Palo Alto Research Laboratory, Palo Alto, CA, USA
The large Cosmic X-ray Telescope (LCXT), a diamond turned Wolter 1 system, was successfully flown on an Aries rocket in October 1980. A brief account of its calibration and flight will be presented. A larger, nested, telescope system is currently being built for another Aries rocket flight. This telescope will be equipped with an array of objective reflection diffraction gratings used in the extreme "off-plane" or "conical" diffraction mode. The operation of this system will be described. The rocket spectrometer will act as a prototype for the low energy instrument on the proposed UK High Throughput Spectroscopy mission, so the design and expected performance of the proposed spectrometer will be described. Recent work on the use of multilayer coatings to enhance the reflectivity of X-ray optical systems will be presented and, finally, a brief account will be given of the proposed X-ray Multi-Mirror mission (XMM), which is currently being studied by the European Space Agency.
1. Introduction High angular resolution X-ray telescopes (which have arc second performance), such as the one flown on the NASA Einstein Observatory, are difficult and expensive to figure and finish. Recognising that for some applications, particularly those involving large collecting area, it might be possible to manufacture telescopes of 10" to 30" resolution more cheaply, the Lockheed and MSSL groups embarked on a programme to develop large aperture nested X-ray telescopes which would be produced by diamond turning on large precision air-bearing lathes, rather than by the traditional methods. At the beginning, this work was aimed mainly at developing X-ray optical elements for the NASA Large Area Modular Array of Reflectors (LAMAR) program. Since then, a number of other programs have emerged as possible users of diamond turned X-ray optical systems. In this review we will discuss the design and performance of the first X-ray telescope constructed by these techniques. This was a single Wolter I (paraboloid-hyperboloid) system which was successfully flown on a NASA Aries sounding rocket in October 1980. The design of a second Aries telescope, consisting of three nested Wolter I systems, will also be described. This telescope will include an objective reflection grating X-ray spectrometer whose design will also be discussed. The telescope and grating system will serve as a prototype for a possible new UK X-ray spectroscopy satellite (the High Throughput Spectrometer) whose design and performance will be outlined. The possibility of enhancing the reflectivity of diamond turned X-ray telescopes at high energies by means of multilayer coatings will be 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
discussed. The review will conclude with a brief description of the X-ray Multi-Mirror (XMM) mission which is at present being studied by the European Space Agency.
2. The Large Cosmic X-ray Telescope (LCXT) A Wolter type I X-ray telescope which consisted of a single paraboloid-hyperboloid mirror pair has been design, fabricated and flown on a NASA Aries rocket in October 1980. These mirrors shown schematically in fig. 1, were made from rolled ring forgings of 5083 aluminium alloy. The mirror blanks were figured by the process of diamond turning, which utilized a precision air-bearing lathe at the US Oak Ridge National LaboraPARABOLOID
HYPERSOLOID
Fig. 1. Cross section of the Wolter Type I telescope flown on a NASA Aries rocket in October 1980. VII. APPLICATIONS IN ASTRONOMY
J.L. Culhane et aL / X-ray optical techniques
252
tory and a diamond cutting tool to machine the required curves to within 1 micron of the desired figure. These figured surfaces were then plated with a thin coating of electroless nickel and polished on a lapping machine, at the National Physical Laboratory, to obtain the final X-ray reflecting surfaces. The telescope had a focal length of 2.3 m, a grazing angle of 1.9 ° , an entrance aperture 66 cm in diameter and a geometrical collecting area of 380 cm ~. The measurement of the telescope figure is described by Stedman elsewhere in these proceedings. Prior to flight, the telescope performance was measured in the 300 m X-ray calibration facility at the Marshall Space Flight Center. measurements were made at a number of X-ray energies. Preliminary results of the measured effective area which the mirrors present to a distant point source are shown in fig. 2. The solid disks indicate the effective area calculated from the X-ray optical constants (Ershov et al. [1]) of the nickel surface, while the crosses show the measured values. The statistical uncertainty in the measurements is negligible compared to the size of the data points. The measured effective area at 0.933 keV falls well above the calculated value because this energy is in the vicinity of the L-shell X-ray absorption edges in the nickel reflecting surfaces where the optical constants are very poorly
500
I
I
I •
l
CalculaLed
x
Measured
X x
I00 X
<
50
p
&3
0
l0
i
1
i
i
ARIES X - R A Y TELESCOPE MSFC C a l i b r a t i o n
103 I~
Analysis
Point Spread Function: T e s t l 3
T=41]
E=.277
\
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g
4 7
Z
o Z
\
101
2
\ h
Q
\
5 n10 o
o 10 -1
10 -2 20
40
80
613
RADIUS FROM CENTROID (PIXELS)
Fig. 3. Point Spread Function of the Aries telescope at 1.5 keV. One pixel is equivalent to 14.5 arc sac.
known. At 0.572, 0.705, 1.5 and 2.05 keV the measured values average 63% of those calculated. Since there are two reflections in the telescope, these data indicate that the polishing process has achieved - 80% of theoretical reflection efficiency. The Point Spread Function (PSF) of the telescope (mirrors plus IPC) measured at 1.5 keV is shown in fig. 3. T-he entire plot covers the central 0.4 ° radius of the field. The IPC spatial resolution is limited by noise in the resistive sheet read-out so it degrades roughly as E 1. Also, the IPC has a 3 mm axial depth for X-ray absorption which produces an energy dependent image blur. The measured imaging properties of the telescope are summarized in table 1. The IPC resolution has been calculated from a semi-empirical expression which was normalized to the value measured for a 100 micron diameter pencil beam at 1.5 keV.
Table 1 X-ray telescope and mirror resolution X I
0.5
I
1.0
I
1.5
Energy (keV)
Measured talescope fwhm (arc sac)
IPC resolution fwhm (arc sac)
Deduced X-ray mirror diam. fwhm (arc sac)
0.57 1.5 2.1
57 43 49
46 30 30
34 31 39
I
2.0
E N E R G Y (keV)
Fig. 2. Effective area of the Aries telescope plotted against photon energy for paraxial rays.
J.L. Culhane et al. / X-ray optical techniques Although, as indicated above, it is difficult to determine mirror resolution, particularly at low energies, because of the IPC performance, the results above 1 keV where scattering effects would be noticeable indicate that the target figure of a blur circle radius of 20 arc sec has been bettered. The telescope performance was verified in flight by an observation of the point source Cygnus X-I, indicating that the flight environment did not affect the mirror performance.
3. The Soft
X-ray Telescope (SXT)
It is proposed to add two additional mirror pairs to the LCXT. These mirrors are designed to be nested inside the LCXT. Parameters of the complete mirror array for the Aries telescope are given in table 2. While the primary aim of the flight is to demonstrate the ability to nest a number of diamond turned telescopes, a novel type of reflection grating spectrometer, which is described below, will also be flown. The diamond turning will be undertaken at the Cranfield Unit for Precision Engineering using the large diamond turning lathe established at Cranfield by the U K Science and Engineering Research Council (SERC). The flight configuration is indicated in fig. 4 where the mirror-pair presently available and the two proposed for diamond turning at Cranfield are shown cross-hatched. The remaining mirrors are for future expansion of the telescope. In reference to the central support plate, the upper part of fig. 4 is a section through one of six radial support ribs, while the lower
253
Table 2 SXT mirror parameters Mirror
Joint radius (cm)
Nominal grazing angle (degrees)
Geometrical area (cm2)
1a 2b 3c 4d 5d 6~
43.3 37.0 31.3 26.1 21.3 16.9
2.64 2.27 1.92 1.61 1.32 1.04
730 550 290 * 260 180 110
a) Mirror-pairs for possible future construction. h) Available for possible later inclusion of a mirror-pair being fabricated by Dr. Gordon Garmire of Penn. State University. ~) Existing mirror-pair flown on the first Aries. d) Mirror-pairs proposed for diamond turning. * Includes effects of vignetting from paraboloid-hyperboloid separation.
half indicates a section of the X-ray transmitting portion of the plate. The central support plate also serves as an alignment fixture for the nested array. This alignment is obtained by utilizing the precision machining capability of the diamond turning facility to machine the mating surfaces of the paraboloids, hyperboloids and the support plate. The mating flange surfaces of each mirror are machined at the same time as its interior surface is figured and thus the flange and its outside diameter provide a true reference for the mirror axis. Surfaces of the support plate are diamond turned to be
Fig. 4. Cross section of the proposed Aries telescope system. VII. APPLICATIONS IN ASTRONOMY
254
J.L. Culhane et aL / X-ray optical techniques
flat and parallel, for axial positioning of the mirrors and angular alignment of their axes. The inner surfaces of raised lugs, shown in the top half of fig. 4, present on the six radial webs are also diamond turned to provide mirror alignment in a direction normal to the telescope axis. This alignment technique utilizes the precision of the diamond turning machine to reduce greatly what otherwise would be a time consuming effort in the telescope production. A new and very promising technique will be used to obtain the final surface finish on the mirrors. Developed by Dr. Peter Serlemitsos of Goddard Space Flight Center, it involves coating the substrate with a thin ( ~ 10 micron) layer of acrylic lacquer. The lacquer provides a very smooth finish by the effects of surface tension in much the same way that occurs in the production of float glass. When a metal layer is applied by vacuum deposition over the lacquer coating, a highly efficient and low scatter X-ray reflecting surface results. An imaging X-ray telescope using this technique and has been built at GSFC and its properties measured at the MSFC X-ray calibration facility (Serlemitsos [2]).The measurements show excellent X-ray reflectivity with no appreciable scattering observed up to E = 10 keV with an angular resolution of 2 arc min. This technique, at present being established at Lockheed, is to be used in finishing mirrors 4 and 5 of table 2. In addition to demonstrating the use of nested diamond turned X-ray optical systems, the flight will also carry a new type of Objective Reflection Grating Spectrometer (ORGS). This spectrometer differs from existing grazing incidence spectrometers by utilizing reflection gratings in the conical diffraction mode. The grating equation appropriate for the conventional "in-plane" mounting is nX
= sin a + sin ft.
(1)
In this equation, X is the X-ray wavelength, d is the ruling spacing of the grating and n is the order of the diffracted light. The definitions of a and fl are shown in fig. 5. Both are measured from the normal to the grating, where a is the angle of the incoming rays, and for a given n, X, d and a, eq. (1) determines fl, the angle of the diffracted light. The direction of zeroth order is that of rays specularly reflected from the macroscopic grating surface. Determination of the fraction of light which goes into each order is a complicated function of the physical properties of the grating as well as of the angles at which it is used, and will be discussed later. It is assumed that the grooves in grating surfaces are triangular in profile. The groove profile has no effect on the geometry of where the light goes; however, it plays a fundamental role in determining the efficiency of each order. Triangular groove gratings are among the most efficient possible and are readily available through mac-
DISPERSED LIGHT
LIGHT~RUER ~m
Fig. 5. Notation for gratings in the conventional in-plane mount. The ruling direction is normal to the plane of the figure. Relationship of the angles to the grating facets is shown inset.
hine ruling or holographic fabrication techniques. A triangular groove grating provides maximum efficiency at wavelengths which satisfy eq. (1) for 13 =/3,. such that the angle, tim, represents specular reflection from facets of the grating grooves as shown in the inset of fig. 5. Reflection gratings have a number of advantages over transmission gratings. Firstly, they have higher efficiency; where a typical gratings. Firstly, they have higher efficiency; where a typical transmission grating has 10 to 20% efficiency split evenly between the plus and minus first orders (e.g. Predehl et al. [3]), a reflection grating (used in conical diffraction) typically delivers 40% to the plus first order only (Cash and Kohnert [4]; Werner [5]). Secondly, reflection gratings provide higher resolution because they have higher dispersions. The high dispersion is achievable because reflection gratings are ruled on a thick blank while transmission gratings must be free standing cobwebs. Currently transmission gratings with 1000 lines/mm have been used, but it is unclear whether high efficiency gratings with substantially higher ruling density can be fabricated. On the other hand, reflection gratings with 6000 lines/mm are routinely manufactured and there is no doubt that gratings with up to 20000 lines/mm can be made with existing ruling engines or by holographic techniques. There is yet another reason that reflection gratings can achieve higher dispersion. Their blaze function throws all the diffracted light to one side of the zero order while transmission gratings have symmetric response. Since imaging optics, which are a necessary part of these systems, have a restricted field of view, the transmission grating must place zero order in the center
J.L. Culhane et a L / X-ray optical techniques DISPERSED LIGHT i t c ~ i1~1 r n M ~
Fig. 6. Geometry of conical diffraction. Relationship of the angles to the grating facets is shown in the inset at right.
of the field and keep dispersion low enough that both orders can be viewed. The reflection grating loses nothing by moving the zero order entirely out of the field and centering on the blazed order. This leads to an effective increase in dispersion of about a factor of four to six. In the extreme off-plane or conical diffraction mode proposed for the Aries spectrometer, X-rays are incident on the grating in a plane parallel to the ruling direction, indicated in fig. 6, rather than in a plane normal to the rulings a in fig. 5. High efficiencies for grazing incidence conical diffraction have been measured at X-ray wavelengths by Werner [5] and more recently by Cash and Kohnert [4]. Also, their properties have been investigated theoretically by Neviere et al. [6], Neviere et al. [7] and Vincent et al. [8]. Eq. (1) is a special case of the more general grating equation which describes the off-plane mounting: n?, -~- = sin y(sin a + sin fl),
255
and hence the term conical diffraction. The angles a and fl, as in fig. 5, are measured in a plane normal to the ruling direction for the in-plane mounting. In conical diffraction, however, the incident and diffracted rays no longer lie in this plane and and measure the projection of these rays onto the plane as shown in fig. 6. They are no azimuthal angles within the cones and thus, like Right Ascension in the sky, they are not measured along great circles. To calculate the final direction of a diffracted ray, the incoming vector must be reduced into its components parallel and perpendicular to the direction of the rulings. The direction cosine parallel to rulings is cos y, and the azimuthal angle of the component perpendicular to the ruling defines a. All the light will re-emerge with a direction cosine equal to cos y parallel to the rulings. The zero-order light will have an azimuthal angle a in the emerging cone as if it were specularly reflected off the macroscopic plane of the grating, just as in fig. 5. The diffracted light will have an azimuthal angle fl which may be calculated as a function of y from eq. (2). As in fig. 5 the grating has maximum efficiency for wavelength y such that specular reflection occurs from the facets of the grooves as shown in the inset of fig. 6. That is, when X-rays are incident so that a = tim, where tim is the blaze angle of the grating. The peak in the diffracted beam occurs when fl = tim" The intensity distribution of the diffracted light as a
CONICAL DIFFRACTION RESPONSE FOR THREE ORDERS
..........
0.B
3rd
Order
..........
2nd
-
1st
-
Order Order
0.6
//j :J
0.4
/i
1 !
,
(2) 0.2
where y is the angle in space between the direction of the incoming ray and the direction of the rulings on the grating. It is measured for the incoming ray, and has the same value for all outgoing rays; the angle between the direction of the rulings and the direction of all outgoing rays, diffracted or not, has the same value, y. Thus, light is dispersed into a cone whose center is the direction of the rulings and whose half-angle is y, as shown in fig. 6,
if
~
:i 0,0 I~ -10
0
I0
20
30
"%
40
50
60
70
80
BETA (degrees)
Fig. 7. Intensity distribution for rays diffracted from a grating used in conical mode. VII. APPLICATIONS IN ASTRONOMY
J.L. Culhane et al. / X-ray optical techniques
256
function of ti is that of Fraunhofer diffraction from a single facet of the grating (Bardas et al. [9]). The peak occurs in the blaze direction and the distribution is given by:
l(fl)
(sin ,5 )2 A
'
(3)
~d A = ~--cos ft,, sin[(fl -- tim)Sin 3'].
(4)
where
Using the expression for X / d from eq. (2) and the small angle approximation for 3' one obtains: cos ti..(ti
- tim)
D = -n (sin tim + sin ti) "
(5)
The function 1(,8) is plotted in fig. 7 for the first three diffraction orders for a grating with tim = 16°, the angle where peak efficiency was measured for the gratings proposed for use in the spectrometer. The curves in fig. 7 are normalized to unity, however, thus the peak efficiency of a grating is limited by the reflectivity of its surface. This depends on surface quality, on the metal coating and on the grazing angle, 3'. The gratings proposed for use are replicas of an existing 6000 groove/ram master which has been ruled by Hyperfine Inc. Gratings with 10000 and 20000 grooves/mm are being developed and will be available for future applications. The 6000 groove master has been fabricated for the High Resolution Spectrograph on the Space Telescope and is 12.6 × 15.4 cm with a nominal blaze angle of 21°. The performance of a gold coated test sample replicated from a submaster of this
j
grating has been investigated by Cash and Kohnert [4]. The peak of the intensity distribution was measured at an effective blaze angle of 16 °, They have observed very high first-order diffraction efficiencies of 0.34, 0.38 and 0.40 at wavelengths of 44, 24 and 13.3 ,~ respectively. Because of this excellent performance we propose to use such gratings formed by the replication process in the spectrometer. The Objective Reflection Grating Spectrometer (ORGS) consists of the gratings, the X-ray mirrors and an imaging proportional counter as shown in fig. 8. X-rays from an astronomical source are parallel as they enter the instrument and the first optical component they encounter is the array of flat, co-aligned reflection gratings. The gratings are aligned so the diffracted ray corresponding to the angle tim in fig. 6 is parallel to the axis of the telescope. Thus, the band of X-ray wavelengths diffracted with maximum efficiency lies at the center of the telescope field where angular resolution is best. During an observation, the ORGS must be pointed such that the source lies on the incident ray in fig. 6. The gratings diffract the X-rays so that the direction in space of the emerging light is a function of its wavelength. The telescope has the property of converting this angular dispersion into position in the focal plane. The telescope images the conically diffracted X-rays of fig. 6 so they appear as an arc in the image plane. The radius of this arc, r, is given by f sin 3', where f is the telescope focal length, and wavelength is dispersed along its length, s, of the arc. In this case, spectral resolution, R, of the ORGS is limited by image blur, 8s, in the focal plane, it can be shown from eq. (2) that the spectral resolution of
C o - a l i g n e Flat d Reflection Gratings
IPC
/
~
Collimated Light from Point Source
TOP VIEW Gratings in front of
Parallel
Entrance Annulus
Fig. 8. A schematic diagram of the Objective Reflection Grating Spectrometer (ORGS).
257
J . L Culhane et al. / X-ray optical techniques
the ORGS is given by R=
sin a + sin fl ,~X
cos
(6)
B~B
Also. the following relationships are valid: 8s = rgfl = f sin ygfl;
8s =f~,~ = f sin y~fl, 8fl = 8@/sin y,
(7)
where $~ is the telescope angular resolution, and thus R
=
sin 7(sin a + sin fl) cos fiB@
=
nX d cos fig@
nX dS@
(8)
for small ft. In this approximation the ORGS resolution depends only on diffraction order, wavelength of interest, groove spacing on the grating and telescope resolution. We assume on diffraction order, wavelength of interest, groove spacing on the grating and telescope resolution. We assume that nearly all of the reflected X-rays from the proposed telescope will lie within a resolution element of 40 arc sec. Thus for 8~ = 2 × 10 -4 rad, and using replicas of the existing 6000 l / m m master, d = 1667 A will provide an O R G S resolution of R = 3 n h . By employing first-order diffraction for wavelengths at the oxygen edge near 25 ,~ (y = 1.33 °) a spectral resolution of 75 can be obtained. Operating at this wavelength
in second order ( y = 2.7°), where the diffraction efr ficiency is somewhat lower, the resolution would be - 1 5 0 . If, in an extreme and unlikely case, centroid finding leads to no further improvement in the 350 micron (fwhm) spatial resolution of the detector that has already been demonstrated, the resolution values at 25 ,~ would be 60 and 120 in first and second orders respectively. However, a spatial resolution of 200 micron (fwhm), which is more than a factor two worse than the value which would be achieved if an improvement factor of V~ were realised, would degrade the mirror limited resolution values by only 10%. The calculated effective area of the ORGS is shown in fig. 9. We have assumed six grating panels 15.4 × 50 cm which are arranged tangentially to the mirror apertures, two per mirror pair, as shown in fig. 8. At y = 1.33 °, this provides slightly over 100 cm 2 of geometrical collecting area. The calculations in these figures include reflectivity of gratings and mirrors, IPC efficiency and the blaze function of fig. 7. In fig. 9, which puts 21 ,~ at the peak of the blaze in first order with 7 = 1.33°, the maximum effective area of 12 cm 2 occurs in first order. Higher orders are reduced because of the reduced telescope efficiency below the nickel L absorption edges near 14 ,~.
1670 Lines/rnm gratings
__3___ ORGS
RESPONSE FOR TWO DIFFRACTION
Wolter type nested mirrors ~ =
ORDERS
slight angular
!
12
........
2nd Order
- -
1st Order
Gamma
= 1.3)deg.
offset
Detectorl
I0
<
~
10,000 Linea/mm gratings
B
<
wevelength spectrum Long
g
wavelength spectrum]l
Shaft
6
la
8,.v O
4
Gratings of 10,000 Lines/ram
Slight angular offset 0
5
10
15
20
25
30
35
40
t*5
WAVELENGTH (X)
Fig. 9. ORGS effective area plotted against wavelength for ?~= 21 ,~., N = 1 and 7 = 1.33°.
FOCAL PLANE FIELD OF VIEW
OCatings of 1670 lines/ram
TOP VIEW Parallel gratings in front Entrance Annulus
of
Fig. 10. Schematic diagram of the ORGS proposed for the UK High Throughput Spectroscopy (HTS) satellite. VI|. APPLICATIONS IN ASTRONOMY
258
J . L Culhane et al. / X-ray optical techniques
103 tl II
I I II I I III
I
this much larger system is illustrated in fig. 10. In addition to gratings of two different ruling space, there are seven nested diamond turned X-ray telescopes and either an imaging proportional counter or a micro-channel plate to register the short and long wavelength X-rays respectively. The maximum mirror element diameter is 1 m while the focal length of the array is 3 m. The system will have a geometric area of 1700 cm 2, a field of view of 1.5 ° and an angular resolution of 20 arc sec. The effective aperture of the complete short wavelength spectrometer (gratings, mirrors and detectors) is shown plotted against wavelength in fig, 11. Data for the Einstein focal plane crystal and objective grating spectrometers are shown for comparison. It is clear that the proposed objective reflection grating system has an effective area greater by one to two orders of magnitude over a wide range of X-ray wavelengths•
I1[
//
(crn 2)
102 ~ "
. . . . ~
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4. The use of muitilayers for X-ray reflectivity enhancement at short wavelengths
30
WAVELENGTH (A)
Fig. 11. The effective aperture of the HTS plotted against wavelength.
A spectrometer of similar design to the SXT system described above is being studied as part of the proposed U K High Throughput Spectroscopy mission which, if proceeded with, would be launched on a free-flying satellite at the end of the decade. The overall design of
The technology for depositing extremely thin alternating layers of high and low density materials, which act as Bragg diffractors, is now becoming well established (Underwood et al. [10]; Spiller et al. [11]). These multilayered coatings are applied by vacuum deposition with highly uniform layer thicknesses down to 10 ~,, essentially replicating the surface finish of the underlying substrate. Deposition of these multilayer diffraction coatings on the reflecting surfaces of a Wolter Type I X-ray telescope offers the potential of providing additional effective areas in a selected high energy bandpass. If the underlying mirror finish is preserved by the multilayers, the low energy specular reflection of the telescope will not be diminished. However, it is neces-
Table 3 Angular properties of the Space Shuttle Telescope for paraxial rays Mirror
1 2 3 4 5 6 7 8 9 10
Hyperboloid
Surface material
Paraboloid Grazing angle at center of element (degrees)
Spread in grazing angle front to back (degrees)
Grazing angle at center element (degrees)
Spread in grazing angle front to back (degrees)
Ni Ni Ni Ni Au Au Au Au Au Au
1.693 1.546 1.404 1.267 1.134 1.006 0.882 0.763 0.648 0.538
0.041 0.038 0.034 0.031 0.028 0.024 0.022 0.018 0.016 0.013
1.785 1.630 1.481 1.336 1.196 1.061 0.930 0.805 0.684 0.567
0.124 0.114 0.103 0.093 0.083 0.075 0.064 0.057 0.047 0.039
259
J.L. Culhane et al. / X-ray optical techniques
sary to arrange the high energy band pass from Bragg reflection well above the energy at which the normal mirror reflectivity cuts off, since interference effects will otherwise degrade the specular reflection. For the outer mirrors of a large Wolter Type I telescope, whose response normally cuts off in the 2-3 keV energy range, it should be possible to enhance their response for cosmic iron-line emission near 6.7 keV without disturbing their low energy performance. Since studies of the spatial distribution of iron-line X-ray emission from galaxy clusters and supernova remnants can provide information on the origin and evolution of these objects, it is of interest to obtain X-ray images of this emission. A large diamond turned nested telescope was studied for possible flight on the Space Shuttle (Catura et al. [12]) and a theoretical study of the possibility of adding multilayers to the outer members of the nest was undertaken by Catura et al. [13]. Details of this telescope system are given in table 3. The telescope focal length is 3.6 m and the mirror diameters range from 30 cm to 90 cm with grazing angles in the range 1.7 ° to 0.5 °. The Bragg diffraction properties of gold and carbon multilayer coatings have been investigated with the aid of a computer program developed for thin film applications at visible wavelengths. This program performs a matrix calculation of the electric and magnetic field vectors at each boundary in the multilayer, assuming an incident plane wave and layers which are semi-infinite in extent normal to their thickness. The optical constants, 7 and fl (parameters in the complex refractive index of the multilayer materials) and parameters of the multilayer stack are required as input data. These optical constants are obtained from the following relationships:
spread in grazing angles along the mirror shown in table 3. Because of the tandem reflections, the bandpass of the hyperboloid coating must be well matched to the spectral distribution off the paraboloid or this second multilayer will act as a filter to reduce the transmission of the telescope. In the present calculations, the multilayer thickness on each of the mirror elements has been obtained by requiring the bandpass to peak for the grazing angle at the center of each element given in table 3. These angles, the wavelength of interest, and the Bragg equation fix the layer spacing on each element. The materials in the multilayer and the number of layer-pairs are the remaining free parameters which determine its rocking curve. Since two reflections are involved, the telescope efficiency depends on the product of the multilayer reflectivities of the paraboloid and hyperboloid. Also, the rocking curve width should be as broad as possible, both to encompass the spread in grazing angles on the mirror elements and to reflect the widest possible band of incident X-rays. However, the peak reflectivity and rocking curve width are competing parameters in the multilayer design and so they must be optimized. For the calculations presented here, we sought to provide a rocking curve fwhm greater than or equal to the spread in grazing angles on the hyperboloids given in the last column of table 3, while maintaining a minimum peak reflectivity of 0.4. We
0.8
n=l -y-ifl, )~2Ne2 3' -- 21rmc2 ,
(9) >-
0.6
O~
0.4
fl= 4~, where n is the complex refractive index of the material, is the X-ray wavelength, e 2 / m c 2 is the classical electron radius, N is the electron density and # the linear X-ray absorption coefficient of the material. The above equations are valid when ~, is far away from any critical absorption edges of the material. The program provides as output of the X-ray intensities which are reflected, transmitted and absorbed by the multilayer for both polarization states as a function of incident angle. A Wolter Type I X-ray telescope involves tandem reflections from the paraboloidal and hyperboloidal mirror elements. For incident X-rays with a continuous wavelength distribution, a multilayer deposited on the paraboloid will diffract X-rays in a narrow wavelength band determined by its rocking curve width and the
0.2
0.0
0.5
1.0
1.5
2.0
2.5
ANGLE (degrees)
Fig. 12. Reflectivity of 5 .~ X-rays from a thick nickel coating (dashed curve). The solid curve indicates the reflectivity of a gold-carbon multilayer on mirror No. 4, overcoated with 150 A of nickel. VII. APPLICATIONS IN ASTRONOMY
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J.L. Culhane et al. / X-ray optical techniques
have selected gold and carbon as the multilayer materials because they provide a high contrast in electron density. Also, the same muhilayer structure has been taken for each paraboloid-hyperboloid mirror pair, because the design does not change rapidly with grazing angle. Since many cosmic X-ray sources have strong iron-line emission near 1.85 ,~, we have chosen to enhance the telescope response in a bandpass at this wavelength. The outer 4 mirrors have nickel reflecting surfaces whose specular reflectivity drops very sharply in the 4 - 6 ,~ range because of the relatively large grazing angles involved. Since this cut-off in response occurs appreciably below the bandpass at 1.85 ,~, where the muhilayer bandpass is desired, it is possible to prevent interference effects of the multilayer from disrupting the specular reflection at long wavelengths by a thin nickel overcoating. The thickness of this nickel overcoating has been optimized to provide minimum attenuation for 1.85 ~, X-rays while maintaining high efficiency for specular reflection at longer wavelengths. The response of mirror 4 due to specular reflection cuts off at about 4 A for a thick nickel surface. Thus, we require that the performance of this mirror at 5 ,/~ be undisturbed by the nickel coated muhilayer. The computer program described previously is flexible enough to calculate performance of a multilayer overcoated by nickel and has been used to select a thickness of 150 J~ for the nickel.
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Fig. 14. Response of the multilayer of fig. 13 plotted on an expanded angular scale.
Results of these calculations are shown in fig. 12. An appreciable difference exists for the two configurations above an angle of 1.5 °, where the 150 ,~ nickel coating on the multilayer begins to transmit 5 ,~ X-rays. At the grazing angle of 1.3 °, at which mirror 4 operates, there is no appreciable difference in the two response curves. In fact, the 150 A coating has slightly higher reflectivity because of constructive interference from X-rays reflected off the lower boundary of the nickel coating. The transmission of a 150 ~, nickel layer at 1.85 ,~ is 0.94, making this thickness a suitable choice for overcoating the multilayers. The rocking curve calculated for the elements of mirror 1, using the above assumptions, is shown in fig. 13. This curve is the response of a gold-carbon multilayer composed of 16 layer-pairs with 16 ,~ layer thickness, whose reflectivity is optimized for 1.85 A X-rays. The fwhm of this rocking curve is 0.12 °, well matched to the spread in grazing angle on the hyperboloid of mirror 1. Fig. 14 shows the response of this multilayer over an expanded wavelength range, clearly indicating the specular reflection from the nickel coating at low grazing angles and the multilayer response near 1.7 °. Similar calculations have been made for mirrors 2 - 4 of the telescope and in these cases a peak reflectivity greater than that for mirror 1 is possible, while still maintaining a fwhm larger than the angular spread for the mirrors shown in the last column of table 3. This is due to the increased reflectivity associated with the
J.L. Culhane et aL / X-ray optical techniques
261
Table 4 Multilayer design for 1.85 A bandpass Mirror
Number of layer-pairs
Layer thickness (A)
Peak reflectivity
Angular width fwhm (degrees)
1 2 3 4
16 14 15 14
16.2 17.9 19.7 22.5
0.376 0.429 0.463 0.517
0.121 0.113 0.115 0.109
smaller grazing angles of these mirrors. Parameters of multilayers which have been optimized for the outer four mirrors of the telescope are shown in table 4. The effective area as a function of X-ray energy has been calculated for the X-ray telescope outlined in table 3. In this calculation, we have assumed that the paraboloid-hyperboloid elements of the outer four mirrors have been coated with the multilayers shown in table 4, that the low energy specular reflection of these mirrors has not been disturbed and that the inner 6 mirrors are gold coated. The effective area is calculated with a computer program for tracing rays through the telescope. For specular reflection this program calculates the reflectivity appropriate to the X-ray energy and the grazing optical constants are taken from Lukurskii et al.
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[14] in the 23-113 ,~ range and from Ershov et al. [1] in the 7 - 4 4 A band. At other wavelengths these parameters are calculated from eq. (9). In the region near the peak of the multilayer response a much finer grid of wavelengths (0.03 ,~ intervals) is used for the computations and the program includes Bragg reflectivity from the multilayers. The effective area of the H R L telescope, calculated in this manner, is shown in fig. 15. The telescope response near 6.7 keV (1.85 ,~) is clearly enhanced, with a peak effective area of 160 cm 2 and a bandpass having a fwhm of 0.4 keV. The effective area at 6.7 keV from specular reflection is 15 cm 2, so that the addition of the multilayers enhances the response at this energy by a factor of 10. The off-axis response is also shown in fig. 14 at angles of 0.1 ° and 0.3 ° , and indicates that the peak effective area from the multilayers degrades a factor of two more rapidly with angle than that for specular reflection at this energy. Thus if the reflecting surfaces of large grazing incidence telescopes could be coated with multilayers in the manner described above, the diamond turned Wolter I telescope could have its reflectivity at 1.85 ,~ usefully enhanced.
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In the mid-seventies, Paul Gorenstein and his coworkers proposed a large array of Kirkpatrick-Baez telescopes for high throughput X-ray spectroscopy and variability studies (the L A M A R mission mentioned earlier). X-ray telescopes of ultimate angular resolution (0.5-2 arc sec) such as those flown in N A S A ' s Einstein observatory and proposed for the Advanced X-ray Astronomy Facility are very expensive to produce and can support only one focal plane instrument at a time. The advantage of the array of Kirkpatrick-Baez telescopes lay in its ability to provide very large collecting areas (ten times greater than A X A F ) and multiple focal planes for simultaneous use. However, the design of the telescope modules probably limits the achievable angular resolution to about 2 arc min and there are several astrophysically important problems (e.g. ability to discriminate between point sources and distant clusters of VII. APPLICATIONS IN ASTRONOMY
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J.L Culhane et al. / X-ray optical techniques
Table 5 XMM model payload
105L
X-ray reflector array property
10 arc sec low energy array
30 arc sec high energy array
Mirror type No. of members (Pairs/member) Focal length Field of view Energy range Angular resolution
Wolter I 20 (12)
Wolter I (conical) 7 (70)
4m 1° × 1 ° 0.1-5.0 keV 10 arc sec
0.50)<0.5 ° 0.1-10.0 keV 30 arc sec
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1.2 m2 (1 keV) 0.9 m2 (2 keY) 0.1 m2 (10 keV)
........ ----......
OPTICS
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diameter 1 - 2 arc min) which require that the angular resolution even of large arrays be better than 20 arc sec. Thus the proposed X-ray Multi-Mirror Mission ( X M M ) n o w being studied will use arrays of telescopes of the Wolter I configuration made either by the d i a m o n d turning or replication techniques. The latter technique has been successfully employed in the production of the E X O S A T telescopes (De Korte et al. [15]) where an angular resolution of better than 15 arc sec has been achieved with telescopes of 25-30 cm diameter. However, diamond turning may be the only practical low cost method of achieving this angular resolution for the larger X-ray mirrors.
(keV)
Fig. 16. Effective area plotted against photon energy for both the low and high energy XMM telescope arrays.
The X M M payload presently being studied by the European Space Agency t consists of two different arrays of X-ray reflectors which are outlined in table 5. The low energy arrays could employ a mixture of repli-
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105
Fig. 17. Limiting sensitivity for point source detection by the 20 element XMM low energy (10 arc see) array. + Study team members: J.A.M, Bleeker, J.L. Culhane, L. Koch, H.W. Schnopper, B.G. Taylor.
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cated and diamond turned mirrors as discussed above. The high energy arrays are of a quite different design. They are constructed from a large number of very thin aluminium foil shells. The shells are cylindrically curved in one dimension but tilted in the path of the incoming beam to provide conical approximations to the paraboloid and hyperboloid figure of a Wolter I telescope. Since individual shells are very thin, much more of the available aperture can be filled than would be possible for the more conventional Wolter I telescopes described above (Serlemitsos [2]). A number of possible focal plane instruments are indicated in this table. A plot of effective area against photon energy is shown in fig. 16 for both systems. It is clear that the XMM arrays have more than ten times the effective aperture of the proposed AXAF telescope. This is reflected in an increased sensitivity for source detection, as shown in fig. 17. An outline of the proposed mission is given in table 6, while a method of mounting the two arrays in the Space Shuttle for injection into orbit is indicated in fig. 18. If undertaken by ESA, the XMM should provide European astronomers with a world class X-ray facility in the next decade.
6. Conclusions
In this very broad review we have emphasised the role of low cost X-ray optical systems in a variety of applications. The design, construction, calibration and performance in flight of a Wolter I imaging X-ray telescope is described. A second rocket payload which will contain a nest of three Wolter I telescopes is also discussed. This instrument will include an objective reflection grating spectrometer of novel design, and the operation and performance of this system is described in some detail, including its possible use in the proposed new UK High Throughput Spectroscopy satellite. A theoretical investigation into the use of multilayers (layered synthetic microstructures) to enhance the high energy (i.e. 6.7 keV) reflectivity of grazing incidence telescopes with a specular reflection cut-off in the 2-3 keV range is presented. Finally, a brief description is given of the X-ray Multi-Mirror Mission (XMM) which is at present being studied by the European Space Agency. In all of this work, the systems that are described make use of X-ray optical systems made by techniques which emphasise low cost at the expense of angular VII. APPLICATIONS IN ASTRONOMY
264
J.L. Culhane et al. / X-ray optical techniques
resolution. Nevertheless, if resolutions in the range 1 0 - 2 0 arc sec c o m b i n e d with effective apertures of 1 2.10 4 cm 2 can eventually be achieved, it is clear that the large high throughput X-ray observatories required in the next decade will be constructed using X-ray optical systems of the kind described. Work at the Mullard Space Science Laboratory is supported by a grant from the U K Science and Engineering Research Council. Work on gratings at the laboratory for A t m o s p h e r i c and Space Physics is supported by N A S A grant No. NAG-5-96. We are grateful for the efforts of B. Bach of Hyperfine Inc. in producing the reflection gratings. The research at Lockheed is supported by N A S A contracts NAS-5-23563 and NAS-5-26090 and by the Lockheed I n d e p e n d e n t Research Program.
References [1] O.A. Ershov, I.A. Brytov and A.P. Lukurskii, Opt. Spectr. 22 (1964) 66. [2] P.J. Serlemitsos, X-ray astronomy in the 1980's, ed. S.S. Holt, NASA Tech. Memorandum 83848 (1982) pp. 441-460.
[3] P. Predehl, K. Beuermann and H. Brauninger, Appl. Opt. 19 (1980) 190. [4] W. Cash and R. Kohnert, Appl. Opt. 21 (1982) 17. [5] W. Werner, Appl. Opt. 16 (1977) 2078. [6] M. Neviere, P. Vincent and D. Mazstre, Appl. Opt. 17 (1978) 843. [7] M. Neviere, D. Mazstre and W.R. Hunter, J. Opt. Soc. Amer. 68 (1978) 1106. [8] P. Vincent, M. Neviere and D. Mazstre, Appl. Opt. 18 (1979) 1790. [9] D. Bardas, L.N. Mertz and W.J. Rozenberg, unpublished results (1981). [10] J.H. Underwood, T.W. Barbee and D.C. Keith, SPIE Proc. 184 (1979) 123. [11] E. Spiller and A. Segmuller, Appl. Phys. Lett. 37 (1980) 1048. [12] R.C. Catura, L.W. Acton, W.A. Brown, C.W. Gilbreth, L.A. Springer, J.R. Vieira, J.L. Culhane, I.M. Mason, O. Siegmund, T.J. Patrick, P.H Sheather, K.A. Pounds, B.A. Cooke, K. Evans, J. Pye, G. Smith, A. Wells, J.E. Spragg, C.H. Whitford, A. Franks, B. Gale, K. Lindsey, M. Steadman, G. Garmire, B. Margon and A. Fabian, SPIE Proc. 284 (1981) 169. [13] R.C. Catura, W.A. Brown, G. Joki and R. Nobles, Opt. Eng. 22 (1981) 140. [14] A.P. Lukirskii, E.P. Savinov, O.A. Ershov and Y.F. Shepelev, Opt. Spectr. 16 (1964) 168. [15] P.A.J. de Korte J.A.M. Bleeker, A.J.F. Den Boggende, G. Branduardi-Raymont, A.C. Brinkman, J.L. Culhane, E.H.B..M. Gronenschild, I. Mason and S.P. McKechnie, Space Sci. Rev. 30 (1981) 495.
251
Section VII. Appfications of X- and y-ray imaging systems in astronomy N E W A P P L I C A T I O N S O F X-RAY O P T I C A L T E C H N I Q U E S J.L. C U L H A N E 1, W. C A S H 2 a n d R.C. C A T U R A 3 i Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey RH5 6NT, England 2 Laboratoryfor Atmospheric and Space Physics, University of Colorado, USA s Lockheed Palo Alto Research Laboratory, Palo Alto, CA, USA
The large Cosmic X-ray Telescope (LCXT), a diamond turned Wolter 1 system, was successfully flown on an Aries rocket in October 1980. A brief account of its calibration and flight will be presented. A larger, nested, telescope system is currently being built for another Aries rocket flight. This telescope will be equipped with an array of objective reflection diffraction gratings used in the extreme "off-plane" or "conical" diffraction mode. The operation of this system will be described. The rocket spectrometer will act as a prototype for the low energy instrument on the proposed UK High Throughput Spectroscopy mission, so the design and expected performance of the proposed spectrometer will be described. Recent work on the use of multilayer coatings to enhance the reflectivity of X-ray optical systems will be presented and, finally, a brief account will be given of the proposed X-ray Multi-Mirror mission (XMM), which is currently being studied by the European Space Agency.
1. Introduction High angular resolution X-ray telescopes (which have arc second performance), such as the one flown on the NASA Einstein Observatory, are difficult and expensive to figure and finish. Recognising that for some applications, particularly those involving large collecting area, it might be possible to manufacture telescopes of 10" to 30" resolution more cheaply, the Lockheed and MSSL groups embarked on a programme to develop large aperture nested X-ray telescopes which would be produced by diamond turning on large precision air-bearing lathes, rather than by the traditional methods. At the beginning, this work was aimed mainly at developing X-ray optical elements for the NASA Large Area Modular Array of Reflectors (LAMAR) program. Since then, a number of other programs have emerged as possible users of diamond turned X-ray optical systems. In this review we will discuss the design and performance of the first X-ray telescope constructed by these techniques. This was a single Wolter I (paraboloid-hyperboloid) system which was successfully flown on a NASA Aries sounding rocket in October 1980. The design of a second Aries telescope, consisting of three nested Wolter I systems, will also be described. This telescope will include an objective reflection grating X-ray spectrometer whose design will also be discussed. The telescope and grating system will serve as a prototype for a possible new UK X-ray spectroscopy satellite (the High Throughput Spectrometer) whose design and performance will be outlined. The possibility of enhancing the reflectivity of diamond turned X-ray telescopes at high energies by means of multilayer coatings will be 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
discussed. The review will conclude with a brief description of the X-ray Multi-Mirror (XMM) mission which is at present being studied by the European Space Agency.
2. The Large Cosmic X-ray Telescope (LCXT) A Wolter type I X-ray telescope which consisted of a single paraboloid-hyperboloid mirror pair has been design, fabricated and flown on a NASA Aries rocket in October 1980. These mirrors shown schematically in fig. 1, were made from rolled ring forgings of 5083 aluminium alloy. The mirror blanks were figured by the process of diamond turning, which utilized a precision air-bearing lathe at the US Oak Ridge National LaboraPARABOLOID
HYPERSOLOID
Fig. 1. Cross section of the Wolter Type I telescope flown on a NASA Aries rocket in October 1980. VII. APPLICATIONS IN ASTRONOMY
J.L. Culhane et aL / X-ray optical techniques
252
tory and a diamond cutting tool to machine the required curves to within 1 micron of the desired figure. These figured surfaces were then plated with a thin coating of electroless nickel and polished on a lapping machine, at the National Physical Laboratory, to obtain the final X-ray reflecting surfaces. The telescope had a focal length of 2.3 m, a grazing angle of 1.9 ° , an entrance aperture 66 cm in diameter and a geometrical collecting area of 380 cm ~. The measurement of the telescope figure is described by Stedman elsewhere in these proceedings. Prior to flight, the telescope performance was measured in the 300 m X-ray calibration facility at the Marshall Space Flight Center. measurements were made at a number of X-ray energies. Preliminary results of the measured effective area which the mirrors present to a distant point source are shown in fig. 2. The solid disks indicate the effective area calculated from the X-ray optical constants (Ershov et al. [1]) of the nickel surface, while the crosses show the measured values. The statistical uncertainty in the measurements is negligible compared to the size of the data points. The measured effective area at 0.933 keV falls well above the calculated value because this energy is in the vicinity of the L-shell X-ray absorption edges in the nickel reflecting surfaces where the optical constants are very poorly
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known. At 0.572, 0.705, 1.5 and 2.05 keV the measured values average 63% of those calculated. Since there are two reflections in the telescope, these data indicate that the polishing process has achieved - 80% of theoretical reflection efficiency. The Point Spread Function (PSF) of the telescope (mirrors plus IPC) measured at 1.5 keV is shown in fig. 3. T-he entire plot covers the central 0.4 ° radius of the field. The IPC spatial resolution is limited by noise in the resistive sheet read-out so it degrades roughly as E 1. Also, the IPC has a 3 mm axial depth for X-ray absorption which produces an energy dependent image blur. The measured imaging properties of the telescope are summarized in table 1. The IPC resolution has been calculated from a semi-empirical expression which was normalized to the value measured for a 100 micron diameter pencil beam at 1.5 keV.
Table 1 X-ray telescope and mirror resolution X I
0.5
I
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I
1.5
Energy (keV)
Measured talescope fwhm (arc sac)
IPC resolution fwhm (arc sac)
Deduced X-ray mirror diam. fwhm (arc sac)
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57 43 49
46 30 30
34 31 39
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E N E R G Y (keV)
Fig. 2. Effective area of the Aries telescope plotted against photon energy for paraxial rays.
J.L. Culhane et al. / X-ray optical techniques Although, as indicated above, it is difficult to determine mirror resolution, particularly at low energies, because of the IPC performance, the results above 1 keV where scattering effects would be noticeable indicate that the target figure of a blur circle radius of 20 arc sec has been bettered. The telescope performance was verified in flight by an observation of the point source Cygnus X-I, indicating that the flight environment did not affect the mirror performance.
3. The Soft
X-ray Telescope (SXT)
It is proposed to add two additional mirror pairs to the LCXT. These mirrors are designed to be nested inside the LCXT. Parameters of the complete mirror array for the Aries telescope are given in table 2. While the primary aim of the flight is to demonstrate the ability to nest a number of diamond turned telescopes, a novel type of reflection grating spectrometer, which is described below, will also be flown. The diamond turning will be undertaken at the Cranfield Unit for Precision Engineering using the large diamond turning lathe established at Cranfield by the U K Science and Engineering Research Council (SERC). The flight configuration is indicated in fig. 4 where the mirror-pair presently available and the two proposed for diamond turning at Cranfield are shown cross-hatched. The remaining mirrors are for future expansion of the telescope. In reference to the central support plate, the upper part of fig. 4 is a section through one of six radial support ribs, while the lower
253
Table 2 SXT mirror parameters Mirror
Joint radius (cm)
Nominal grazing angle (degrees)
Geometrical area (cm2)
1a 2b 3c 4d 5d 6~
43.3 37.0 31.3 26.1 21.3 16.9
2.64 2.27 1.92 1.61 1.32 1.04
730 550 290 * 260 180 110
a) Mirror-pairs for possible future construction. h) Available for possible later inclusion of a mirror-pair being fabricated by Dr. Gordon Garmire of Penn. State University. ~) Existing mirror-pair flown on the first Aries. d) Mirror-pairs proposed for diamond turning. * Includes effects of vignetting from paraboloid-hyperboloid separation.
half indicates a section of the X-ray transmitting portion of the plate. The central support plate also serves as an alignment fixture for the nested array. This alignment is obtained by utilizing the precision machining capability of the diamond turning facility to machine the mating surfaces of the paraboloids, hyperboloids and the support plate. The mating flange surfaces of each mirror are machined at the same time as its interior surface is figured and thus the flange and its outside diameter provide a true reference for the mirror axis. Surfaces of the support plate are diamond turned to be
Fig. 4. Cross section of the proposed Aries telescope system. VII. APPLICATIONS IN ASTRONOMY
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J.L. Culhane et aL / X-ray optical techniques
flat and parallel, for axial positioning of the mirrors and angular alignment of their axes. The inner surfaces of raised lugs, shown in the top half of fig. 4, present on the six radial webs are also diamond turned to provide mirror alignment in a direction normal to the telescope axis. This alignment technique utilizes the precision of the diamond turning machine to reduce greatly what otherwise would be a time consuming effort in the telescope production. A new and very promising technique will be used to obtain the final surface finish on the mirrors. Developed by Dr. Peter Serlemitsos of Goddard Space Flight Center, it involves coating the substrate with a thin ( ~ 10 micron) layer of acrylic lacquer. The lacquer provides a very smooth finish by the effects of surface tension in much the same way that occurs in the production of float glass. When a metal layer is applied by vacuum deposition over the lacquer coating, a highly efficient and low scatter X-ray reflecting surface results. An imaging X-ray telescope using this technique and has been built at GSFC and its properties measured at the MSFC X-ray calibration facility (Serlemitsos [2]).The measurements show excellent X-ray reflectivity with no appreciable scattering observed up to E = 10 keV with an angular resolution of 2 arc min. This technique, at present being established at Lockheed, is to be used in finishing mirrors 4 and 5 of table 2. In addition to demonstrating the use of nested diamond turned X-ray optical systems, the flight will also carry a new type of Objective Reflection Grating Spectrometer (ORGS). This spectrometer differs from existing grazing incidence spectrometers by utilizing reflection gratings in the conical diffraction mode. The grating equation appropriate for the conventional "in-plane" mounting is nX
= sin a + sin ft.
(1)
In this equation, X is the X-ray wavelength, d is the ruling spacing of the grating and n is the order of the diffracted light. The definitions of a and fl are shown in fig. 5. Both are measured from the normal to the grating, where a is the angle of the incoming rays, and for a given n, X, d and a, eq. (1) determines fl, the angle of the diffracted light. The direction of zeroth order is that of rays specularly reflected from the macroscopic grating surface. Determination of the fraction of light which goes into each order is a complicated function of the physical properties of the grating as well as of the angles at which it is used, and will be discussed later. It is assumed that the grooves in grating surfaces are triangular in profile. The groove profile has no effect on the geometry of where the light goes; however, it plays a fundamental role in determining the efficiency of each order. Triangular groove gratings are among the most efficient possible and are readily available through mac-
DISPERSED LIGHT
LIGHT~RUER ~m
Fig. 5. Notation for gratings in the conventional in-plane mount. The ruling direction is normal to the plane of the figure. Relationship of the angles to the grating facets is shown inset.
hine ruling or holographic fabrication techniques. A triangular groove grating provides maximum efficiency at wavelengths which satisfy eq. (1) for 13 =/3,. such that the angle, tim, represents specular reflection from facets of the grating grooves as shown in the inset of fig. 5. Reflection gratings have a number of advantages over transmission gratings. Firstly, they have higher efficiency; where a typical gratings. Firstly, they have higher efficiency; where a typical transmission grating has 10 to 20% efficiency split evenly between the plus and minus first orders (e.g. Predehl et al. [3]), a reflection grating (used in conical diffraction) typically delivers 40% to the plus first order only (Cash and Kohnert [4]; Werner [5]). Secondly, reflection gratings provide higher resolution because they have higher dispersions. The high dispersion is achievable because reflection gratings are ruled on a thick blank while transmission gratings must be free standing cobwebs. Currently transmission gratings with 1000 lines/mm have been used, but it is unclear whether high efficiency gratings with substantially higher ruling density can be fabricated. On the other hand, reflection gratings with 6000 lines/mm are routinely manufactured and there is no doubt that gratings with up to 20000 lines/mm can be made with existing ruling engines or by holographic techniques. There is yet another reason that reflection gratings can achieve higher dispersion. Their blaze function throws all the diffracted light to one side of the zero order while transmission gratings have symmetric response. Since imaging optics, which are a necessary part of these systems, have a restricted field of view, the transmission grating must place zero order in the center
J.L. Culhane et a L / X-ray optical techniques DISPERSED LIGHT i t c ~ i1~1 r n M ~
Fig. 6. Geometry of conical diffraction. Relationship of the angles to the grating facets is shown in the inset at right.
of the field and keep dispersion low enough that both orders can be viewed. The reflection grating loses nothing by moving the zero order entirely out of the field and centering on the blazed order. This leads to an effective increase in dispersion of about a factor of four to six. In the extreme off-plane or conical diffraction mode proposed for the Aries spectrometer, X-rays are incident on the grating in a plane parallel to the ruling direction, indicated in fig. 6, rather than in a plane normal to the rulings a in fig. 5. High efficiencies for grazing incidence conical diffraction have been measured at X-ray wavelengths by Werner [5] and more recently by Cash and Kohnert [4]. Also, their properties have been investigated theoretically by Neviere et al. [6], Neviere et al. [7] and Vincent et al. [8]. Eq. (1) is a special case of the more general grating equation which describes the off-plane mounting: n?, -~- = sin y(sin a + sin fl),
255
and hence the term conical diffraction. The angles a and fl, as in fig. 5, are measured in a plane normal to the ruling direction for the in-plane mounting. In conical diffraction, however, the incident and diffracted rays no longer lie in this plane and and measure the projection of these rays onto the plane as shown in fig. 6. They are no azimuthal angles within the cones and thus, like Right Ascension in the sky, they are not measured along great circles. To calculate the final direction of a diffracted ray, the incoming vector must be reduced into its components parallel and perpendicular to the direction of the rulings. The direction cosine parallel to rulings is cos y, and the azimuthal angle of the component perpendicular to the ruling defines a. All the light will re-emerge with a direction cosine equal to cos y parallel to the rulings. The zero-order light will have an azimuthal angle a in the emerging cone as if it were specularly reflected off the macroscopic plane of the grating, just as in fig. 5. The diffracted light will have an azimuthal angle fl which may be calculated as a function of y from eq. (2). As in fig. 5 the grating has maximum efficiency for wavelength y such that specular reflection occurs from the facets of the grooves as shown in the inset of fig. 6. That is, when X-rays are incident so that a = tim, where tim is the blaze angle of the grating. The peak in the diffracted beam occurs when fl = tim" The intensity distribution of the diffracted light as a
CONICAL DIFFRACTION RESPONSE FOR THREE ORDERS
..........
0.B
3rd
Order
..........
2nd
-
1st
-
Order Order
0.6
//j :J
0.4
/i
1 !
,
(2) 0.2
where y is the angle in space between the direction of the incoming ray and the direction of the rulings on the grating. It is measured for the incoming ray, and has the same value for all outgoing rays; the angle between the direction of the rulings and the direction of all outgoing rays, diffracted or not, has the same value, y. Thus, light is dispersed into a cone whose center is the direction of the rulings and whose half-angle is y, as shown in fig. 6,
if
~
:i 0,0 I~ -10
0
I0
20
30
"%
40
50
60
70
80
BETA (degrees)
Fig. 7. Intensity distribution for rays diffracted from a grating used in conical mode. VII. APPLICATIONS IN ASTRONOMY
J.L. Culhane et al. / X-ray optical techniques
256
function of ti is that of Fraunhofer diffraction from a single facet of the grating (Bardas et al. [9]). The peak occurs in the blaze direction and the distribution is given by:
l(fl)
(sin ,5 )2 A
'
(3)
~d A = ~--cos ft,, sin[(fl -- tim)Sin 3'].
(4)
where
Using the expression for X / d from eq. (2) and the small angle approximation for 3' one obtains: cos ti..(ti
- tim)
D = -n (sin tim + sin ti) "
(5)
The function 1(,8) is plotted in fig. 7 for the first three diffraction orders for a grating with tim = 16°, the angle where peak efficiency was measured for the gratings proposed for use in the spectrometer. The curves in fig. 7 are normalized to unity, however, thus the peak efficiency of a grating is limited by the reflectivity of its surface. This depends on surface quality, on the metal coating and on the grazing angle, 3'. The gratings proposed for use are replicas of an existing 6000 groove/ram master which has been ruled by Hyperfine Inc. Gratings with 10000 and 20000 grooves/mm are being developed and will be available for future applications. The 6000 groove master has been fabricated for the High Resolution Spectrograph on the Space Telescope and is 12.6 × 15.4 cm with a nominal blaze angle of 21°. The performance of a gold coated test sample replicated from a submaster of this
j
grating has been investigated by Cash and Kohnert [4]. The peak of the intensity distribution was measured at an effective blaze angle of 16 °, They have observed very high first-order diffraction efficiencies of 0.34, 0.38 and 0.40 at wavelengths of 44, 24 and 13.3 ,~ respectively. Because of this excellent performance we propose to use such gratings formed by the replication process in the spectrometer. The Objective Reflection Grating Spectrometer (ORGS) consists of the gratings, the X-ray mirrors and an imaging proportional counter as shown in fig. 8. X-rays from an astronomical source are parallel as they enter the instrument and the first optical component they encounter is the array of flat, co-aligned reflection gratings. The gratings are aligned so the diffracted ray corresponding to the angle tim in fig. 6 is parallel to the axis of the telescope. Thus, the band of X-ray wavelengths diffracted with maximum efficiency lies at the center of the telescope field where angular resolution is best. During an observation, the ORGS must be pointed such that the source lies on the incident ray in fig. 6. The gratings diffract the X-rays so that the direction in space of the emerging light is a function of its wavelength. The telescope has the property of converting this angular dispersion into position in the focal plane. The telescope images the conically diffracted X-rays of fig. 6 so they appear as an arc in the image plane. The radius of this arc, r, is given by f sin 3', where f is the telescope focal length, and wavelength is dispersed along its length, s, of the arc. In this case, spectral resolution, R, of the ORGS is limited by image blur, 8s, in the focal plane, it can be shown from eq. (2) that the spectral resolution of
C o - a l i g n e Flat d Reflection Gratings
IPC
/
~
Collimated Light from Point Source
TOP VIEW Gratings in front of
Parallel
Entrance Annulus
Fig. 8. A schematic diagram of the Objective Reflection Grating Spectrometer (ORGS).
257
J . L Culhane et al. / X-ray optical techniques
the ORGS is given by R=
sin a + sin fl ,~X
cos
(6)
B~B
Also. the following relationships are valid: 8s = rgfl = f sin ygfl;
8s =f~,~ = f sin y~fl, 8fl = 8@/sin y,
(7)
where $~ is the telescope angular resolution, and thus R
=
sin 7(sin a + sin fl) cos fiB@
=
nX d cos fig@
nX dS@
(8)
for small ft. In this approximation the ORGS resolution depends only on diffraction order, wavelength of interest, groove spacing on the grating and telescope resolution. We assume on diffraction order, wavelength of interest, groove spacing on the grating and telescope resolution. We assume that nearly all of the reflected X-rays from the proposed telescope will lie within a resolution element of 40 arc sec. Thus for 8~ = 2 × 10 -4 rad, and using replicas of the existing 6000 l / m m master, d = 1667 A will provide an O R G S resolution of R = 3 n h . By employing first-order diffraction for wavelengths at the oxygen edge near 25 ,~ (y = 1.33 °) a spectral resolution of 75 can be obtained. Operating at this wavelength
in second order ( y = 2.7°), where the diffraction efr ficiency is somewhat lower, the resolution would be - 1 5 0 . If, in an extreme and unlikely case, centroid finding leads to no further improvement in the 350 micron (fwhm) spatial resolution of the detector that has already been demonstrated, the resolution values at 25 ,~ would be 60 and 120 in first and second orders respectively. However, a spatial resolution of 200 micron (fwhm), which is more than a factor two worse than the value which would be achieved if an improvement factor of V~ were realised, would degrade the mirror limited resolution values by only 10%. The calculated effective area of the ORGS is shown in fig. 9. We have assumed six grating panels 15.4 × 50 cm which are arranged tangentially to the mirror apertures, two per mirror pair, as shown in fig. 8. At y = 1.33 °, this provides slightly over 100 cm 2 of geometrical collecting area. The calculations in these figures include reflectivity of gratings and mirrors, IPC efficiency and the blaze function of fig. 7. In fig. 9, which puts 21 ,~ at the peak of the blaze in first order with 7 = 1.33°, the maximum effective area of 12 cm 2 occurs in first order. Higher orders are reduced because of the reduced telescope efficiency below the nickel L absorption edges near 14 ,~.
1670 Lines/rnm gratings
__3___ ORGS
RESPONSE FOR TWO DIFFRACTION
Wolter type nested mirrors ~ =
ORDERS
slight angular
!
12
........
2nd Order
- -
1st Order
Gamma
= 1.3)deg.
offset
Detectorl
I0
<
~
10,000 Linea/mm gratings
B
<
wevelength spectrum Long
g
wavelength spectrum]l
Shaft
6
la
8,.v O
4
Gratings of 10,000 Lines/ram
Slight angular offset 0
5
10
15
20
25
30
35
40
t*5
WAVELENGTH (X)
Fig. 9. ORGS effective area plotted against wavelength for ?~= 21 ,~., N = 1 and 7 = 1.33°.
FOCAL PLANE FIELD OF VIEW
OCatings of 1670 lines/ram
TOP VIEW Parallel gratings in front Entrance Annulus
of
Fig. 10. Schematic diagram of the ORGS proposed for the UK High Throughput Spectroscopy (HTS) satellite. VI|. APPLICATIONS IN ASTRONOMY
258
J . L Culhane et al. / X-ray optical techniques
103 tl II
I I II I I III
I
this much larger system is illustrated in fig. 10. In addition to gratings of two different ruling space, there are seven nested diamond turned X-ray telescopes and either an imaging proportional counter or a micro-channel plate to register the short and long wavelength X-rays respectively. The maximum mirror element diameter is 1 m while the focal length of the array is 3 m. The system will have a geometric area of 1700 cm 2, a field of view of 1.5 ° and an angular resolution of 20 arc sec. The effective aperture of the complete short wavelength spectrometer (gratings, mirrors and detectors) is shown plotted against wavelength in fig, 11. Data for the Einstein focal plane crystal and objective grating spectrometers are shown for comparison. It is clear that the proposed objective reflection grating system has an effective area greater by one to two orders of magnitude over a wide range of X-ray wavelengths•
I1[
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i Oi~i 20
4. The use of muitilayers for X-ray reflectivity enhancement at short wavelengths
30
WAVELENGTH (A)
Fig. 11. The effective aperture of the HTS plotted against wavelength.
A spectrometer of similar design to the SXT system described above is being studied as part of the proposed U K High Throughput Spectroscopy mission which, if proceeded with, would be launched on a free-flying satellite at the end of the decade. The overall design of
The technology for depositing extremely thin alternating layers of high and low density materials, which act as Bragg diffractors, is now becoming well established (Underwood et al. [10]; Spiller et al. [11]). These multilayered coatings are applied by vacuum deposition with highly uniform layer thicknesses down to 10 ~,, essentially replicating the surface finish of the underlying substrate. Deposition of these multilayer diffraction coatings on the reflecting surfaces of a Wolter Type I X-ray telescope offers the potential of providing additional effective areas in a selected high energy bandpass. If the underlying mirror finish is preserved by the multilayers, the low energy specular reflection of the telescope will not be diminished. However, it is neces-
Table 3 Angular properties of the Space Shuttle Telescope for paraxial rays Mirror
1 2 3 4 5 6 7 8 9 10
Hyperboloid
Surface material
Paraboloid Grazing angle at center of element (degrees)
Spread in grazing angle front to back (degrees)
Grazing angle at center element (degrees)
Spread in grazing angle front to back (degrees)
Ni Ni Ni Ni Au Au Au Au Au Au
1.693 1.546 1.404 1.267 1.134 1.006 0.882 0.763 0.648 0.538
0.041 0.038 0.034 0.031 0.028 0.024 0.022 0.018 0.016 0.013
1.785 1.630 1.481 1.336 1.196 1.061 0.930 0.805 0.684 0.567
0.124 0.114 0.103 0.093 0.083 0.075 0.064 0.057 0.047 0.039
259
J.L. Culhane et al. / X-ray optical techniques
sary to arrange the high energy band pass from Bragg reflection well above the energy at which the normal mirror reflectivity cuts off, since interference effects will otherwise degrade the specular reflection. For the outer mirrors of a large Wolter Type I telescope, whose response normally cuts off in the 2-3 keV energy range, it should be possible to enhance their response for cosmic iron-line emission near 6.7 keV without disturbing their low energy performance. Since studies of the spatial distribution of iron-line X-ray emission from galaxy clusters and supernova remnants can provide information on the origin and evolution of these objects, it is of interest to obtain X-ray images of this emission. A large diamond turned nested telescope was studied for possible flight on the Space Shuttle (Catura et al. [12]) and a theoretical study of the possibility of adding multilayers to the outer members of the nest was undertaken by Catura et al. [13]. Details of this telescope system are given in table 3. The telescope focal length is 3.6 m and the mirror diameters range from 30 cm to 90 cm with grazing angles in the range 1.7 ° to 0.5 °. The Bragg diffraction properties of gold and carbon multilayer coatings have been investigated with the aid of a computer program developed for thin film applications at visible wavelengths. This program performs a matrix calculation of the electric and magnetic field vectors at each boundary in the multilayer, assuming an incident plane wave and layers which are semi-infinite in extent normal to their thickness. The optical constants, 7 and fl (parameters in the complex refractive index of the multilayer materials) and parameters of the multilayer stack are required as input data. These optical constants are obtained from the following relationships:
spread in grazing angles along the mirror shown in table 3. Because of the tandem reflections, the bandpass of the hyperboloid coating must be well matched to the spectral distribution off the paraboloid or this second multilayer will act as a filter to reduce the transmission of the telescope. In the present calculations, the multilayer thickness on each of the mirror elements has been obtained by requiring the bandpass to peak for the grazing angle at the center of each element given in table 3. These angles, the wavelength of interest, and the Bragg equation fix the layer spacing on each element. The materials in the multilayer and the number of layer-pairs are the remaining free parameters which determine its rocking curve. Since two reflections are involved, the telescope efficiency depends on the product of the multilayer reflectivities of the paraboloid and hyperboloid. Also, the rocking curve width should be as broad as possible, both to encompass the spread in grazing angles on the mirror elements and to reflect the widest possible band of incident X-rays. However, the peak reflectivity and rocking curve width are competing parameters in the multilayer design and so they must be optimized. For the calculations presented here, we sought to provide a rocking curve fwhm greater than or equal to the spread in grazing angles on the hyperboloids given in the last column of table 3, while maintaining a minimum peak reflectivity of 0.4. We
0.8
n=l -y-ifl, )~2Ne2 3' -- 21rmc2 ,
(9) >-
0.6
O~
0.4
fl= 4~, where n is the complex refractive index of the material, is the X-ray wavelength, e 2 / m c 2 is the classical electron radius, N is the electron density and # the linear X-ray absorption coefficient of the material. The above equations are valid when ~, is far away from any critical absorption edges of the material. The program provides as output of the X-ray intensities which are reflected, transmitted and absorbed by the multilayer for both polarization states as a function of incident angle. A Wolter Type I X-ray telescope involves tandem reflections from the paraboloidal and hyperboloidal mirror elements. For incident X-rays with a continuous wavelength distribution, a multilayer deposited on the paraboloid will diffract X-rays in a narrow wavelength band determined by its rocking curve width and the
0.2
0.0
0.5
1.0
1.5
2.0
2.5
ANGLE (degrees)
Fig. 12. Reflectivity of 5 .~ X-rays from a thick nickel coating (dashed curve). The solid curve indicates the reflectivity of a gold-carbon multilayer on mirror No. 4, overcoated with 150 A of nickel. VII. APPLICATIONS IN ASTRONOMY
260
J.L. Culhane et al. / X-ray optical techniques
have selected gold and carbon as the multilayer materials because they provide a high contrast in electron density. Also, the same muhilayer structure has been taken for each paraboloid-hyperboloid mirror pair, because the design does not change rapidly with grazing angle. Since many cosmic X-ray sources have strong iron-line emission near 1.85 ,~, we have chosen to enhance the telescope response in a bandpass at this wavelength. The outer 4 mirrors have nickel reflecting surfaces whose specular reflectivity drops very sharply in the 4 - 6 ,~ range because of the relatively large grazing angles involved. Since this cut-off in response occurs appreciably below the bandpass at 1.85 ,~, where the muhilayer bandpass is desired, it is possible to prevent interference effects of the multilayer from disrupting the specular reflection at long wavelengths by a thin nickel overcoating. The thickness of this nickel overcoating has been optimized to provide minimum attenuation for 1.85 ~, X-rays while maintaining high efficiency for specular reflection at longer wavelengths. The response of mirror 4 due to specular reflection cuts off at about 4 A for a thick nickel surface. Thus, we require that the performance of this mirror at 5 ,/~ be undisturbed by the nickel coated muhilayer. The computer program described previously is flexible enough to calculate performance of a multilayer overcoated by nickel and has been used to select a thickness of 150 J~ for the nickel.
I
i
I
~
l
I
I
1.4
1.5
1.6
1.7
1.8
1.9
2.0
0.35
0.30
0.25
)_~
_>
t~
J
0.20
0.15
0.i0
0.05
0.00
ANGLE
2.1
(degrees)
Fig. 13. Calculated rocking curve due to Bragg diffraction of the gold-carbon muhilayer for 1.85 ,~ X-rays. A 150 ~, nickel overcoat is included.
1.0
0.8
0.6
C-
w od
0.4
0.2
0.0 0.0
0.5
1.0
1.5 ANGLE
210
215
I 3.0
(degrees)
Fig. 14. Response of the multilayer of fig. 13 plotted on an expanded angular scale.
Results of these calculations are shown in fig. 12. An appreciable difference exists for the two configurations above an angle of 1.5 °, where the 150 ,~ nickel coating on the multilayer begins to transmit 5 ,~ X-rays. At the grazing angle of 1.3 °, at which mirror 4 operates, there is no appreciable difference in the two response curves. In fact, the 150 A coating has slightly higher reflectivity because of constructive interference from X-rays reflected off the lower boundary of the nickel coating. The transmission of a 150 ~, nickel layer at 1.85 ,~ is 0.94, making this thickness a suitable choice for overcoating the multilayers. The rocking curve calculated for the elements of mirror 1, using the above assumptions, is shown in fig. 13. This curve is the response of a gold-carbon multilayer composed of 16 layer-pairs with 16 ,~ layer thickness, whose reflectivity is optimized for 1.85 A X-rays. The fwhm of this rocking curve is 0.12 °, well matched to the spread in grazing angle on the hyperboloid of mirror 1. Fig. 14 shows the response of this multilayer over an expanded wavelength range, clearly indicating the specular reflection from the nickel coating at low grazing angles and the multilayer response near 1.7 °. Similar calculations have been made for mirrors 2 - 4 of the telescope and in these cases a peak reflectivity greater than that for mirror 1 is possible, while still maintaining a fwhm larger than the angular spread for the mirrors shown in the last column of table 3. This is due to the increased reflectivity associated with the
J.L. Culhane et aL / X-ray optical techniques
261
Table 4 Multilayer design for 1.85 A bandpass Mirror
Number of layer-pairs
Layer thickness (A)
Peak reflectivity
Angular width fwhm (degrees)
1 2 3 4
16 14 15 14
16.2 17.9 19.7 22.5
0.376 0.429 0.463 0.517
0.121 0.113 0.115 0.109
smaller grazing angles of these mirrors. Parameters of multilayers which have been optimized for the outer four mirrors of the telescope are shown in table 4. The effective area as a function of X-ray energy has been calculated for the X-ray telescope outlined in table 3. In this calculation, we have assumed that the paraboloid-hyperboloid elements of the outer four mirrors have been coated with the multilayers shown in table 4, that the low energy specular reflection of these mirrors has not been disturbed and that the inner 6 mirrors are gold coated. The effective area is calculated with a computer program for tracing rays through the telescope. For specular reflection this program calculates the reflectivity appropriate to the X-ray energy and the grazing optical constants are taken from Lukurskii et al.
10 4
i
i
i
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i
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I
i
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5. The European Space Agency's X-ray Multi-Mirror Mission
1O 2
I01
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i0 "I
10"
[14] in the 23-113 ,~ range and from Ershov et al. [1] in the 7 - 4 4 A band. At other wavelengths these parameters are calculated from eq. (9). In the region near the peak of the multilayer response a much finer grid of wavelengths (0.03 ,~ intervals) is used for the computations and the program includes Bragg reflectivity from the multilayers. The effective area of the H R L telescope, calculated in this manner, is shown in fig. 15. The telescope response near 6.7 keV (1.85 ,~) is clearly enhanced, with a peak effective area of 160 cm 2 and a bandpass having a fwhm of 0.4 keV. The effective area at 6.7 keV from specular reflection is 15 cm 2, so that the addition of the multilayers enhances the response at this energy by a factor of 10. The off-axis response is also shown in fig. 14 at angles of 0.1 ° and 0.3 ° , and indicates that the peak effective area from the multilayers degrades a factor of two more rapidly with angle than that for specular reflection at this energy. Thus if the reflecting surfaces of large grazing incidence telescopes could be coated with multilayers in the manner described above, the diamond turned Wolter I telescope could have its reflectivity at 1.85 ,~ usefully enhanced.
] 1
I 2
I 3
I 4
I 5
I 6
I 7
I B
I 9
ENERGY (keV)
Fig. 15. Calculated effective area of the spacelab telescope (table 3) plotted against energy. The outer four mirrors are coated with the multilayers described in table 4. The six inner mirrors are gold coated.
In the mid-seventies, Paul Gorenstein and his coworkers proposed a large array of Kirkpatrick-Baez telescopes for high throughput X-ray spectroscopy and variability studies (the L A M A R mission mentioned earlier). X-ray telescopes of ultimate angular resolution (0.5-2 arc sec) such as those flown in N A S A ' s Einstein observatory and proposed for the Advanced X-ray Astronomy Facility are very expensive to produce and can support only one focal plane instrument at a time. The advantage of the array of Kirkpatrick-Baez telescopes lay in its ability to provide very large collecting areas (ten times greater than A X A F ) and multiple focal planes for simultaneous use. However, the design of the telescope modules probably limits the achievable angular resolution to about 2 arc min and there are several astrophysically important problems (e.g. ability to discriminate between point sources and distant clusters of VII. APPLICATIONS IN ASTRONOMY
262
J.L Culhane et al. / X-ray optical techniques
Table 5 XMM model payload
105L
X-ray reflector array property
10 arc sec low energy array
30 arc sec high energy array
Mirror type No. of members (Pairs/member) Focal length Field of view Energy range Angular resolution
Wolter I 20 (12)
Wolter I (conical) 7 (70)
4m 1° × 1 ° 0.1-5.0 keV 10 arc sec
0.50)<0.5 ° 0.1-10.0 keV 30 arc sec
1.2 m2 (1 keV) 0.9 mz (2 keV) 0.2 m2 (5 keV)
1.2 m2 (1 keV) 0.9 m2 (2 keY) 0.1 m2 (10 keV)
........ ----......
OPTICS
8m
V 10 .1
1
Focal plane instruments Position sensitive proportional counter Micro-channel plate CCD Gratings
I
iO t*
(A,t,)
Effective collecting area (A EVE)
i ........ LOW ENERGY HIGH E N E R G Y
10
ENERGY
Gas scintillation proportional counter Solid state det/CCD Bragg spectrometer Bragg polarimeter
diameter 1 - 2 arc min) which require that the angular resolution even of large arrays be better than 20 arc sec. Thus the proposed X-ray Multi-Mirror Mission ( X M M ) n o w being studied will use arrays of telescopes of the Wolter I configuration made either by the d i a m o n d turning or replication techniques. The latter technique has been successfully employed in the production of the E X O S A T telescopes (De Korte et al. [15]) where an angular resolution of better than 15 arc sec has been achieved with telescopes of 25-30 cm diameter. However, diamond turning may be the only practical low cost method of achieving this angular resolution for the larger X-ray mirrors.
(keV)
Fig. 16. Effective area plotted against photon energy for both the low and high energy XMM telescope arrays.
The X M M payload presently being studied by the European Space Agency t consists of two different arrays of X-ray reflectors which are outlined in table 5. The low energy arrays could employ a mixture of repli-
10 -13 I
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.
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Table 6 Mission outline Lifetime in orbit Orbit/launch vehicle
Overall size/weight
Power Data
Attitude control/ measurement
x
:10 years or greater :500-600 km circular 0-30 ° inclination Shuttle/Ariane 4 :3.6-4.4 m diam 9 m length 2800 kg payload, 5400 kg total :700 W :4-24 kbps depending on nature of on-board processing. Max on-board storage 150 mbit per orbit. :Target acquisition < 2 arc min Stability < 1 arc min/104s measurement < 5 arc sec
I0 -16 n~
10-17 102
I
i
103 184 OBSERVATIONTIME (seconds)
105
Fig. 17. Limiting sensitivity for point source detection by the 20 element XMM low energy (10 arc see) array. + Study team members: J.A.M, Bleeker, J.L. Culhane, L. Koch, H.W. Schnopper, B.G. Taylor.
263
J.L. Culhane et al. / X-ray optical techniques C
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cated and diamond turned mirrors as discussed above. The high energy arrays are of a quite different design. They are constructed from a large number of very thin aluminium foil shells. The shells are cylindrically curved in one dimension but tilted in the path of the incoming beam to provide conical approximations to the paraboloid and hyperboloid figure of a Wolter I telescope. Since individual shells are very thin, much more of the available aperture can be filled than would be possible for the more conventional Wolter I telescopes described above (Serlemitsos [2]). A number of possible focal plane instruments are indicated in this table. A plot of effective area against photon energy is shown in fig. 16 for both systems. It is clear that the XMM arrays have more than ten times the effective aperture of the proposed AXAF telescope. This is reflected in an increased sensitivity for source detection, as shown in fig. 17. An outline of the proposed mission is given in table 6, while a method of mounting the two arrays in the Space Shuttle for injection into orbit is indicated in fig. 18. If undertaken by ESA, the XMM should provide European astronomers with a world class X-ray facility in the next decade.
6. Conclusions
In this very broad review we have emphasised the role of low cost X-ray optical systems in a variety of applications. The design, construction, calibration and performance in flight of a Wolter I imaging X-ray telescope is described. A second rocket payload which will contain a nest of three Wolter I telescopes is also discussed. This instrument will include an objective reflection grating spectrometer of novel design, and the operation and performance of this system is described in some detail, including its possible use in the proposed new UK High Throughput Spectroscopy satellite. A theoretical investigation into the use of multilayers (layered synthetic microstructures) to enhance the high energy (i.e. 6.7 keV) reflectivity of grazing incidence telescopes with a specular reflection cut-off in the 2-3 keV range is presented. Finally, a brief description is given of the X-ray Multi-Mirror Mission (XMM) which is at present being studied by the European Space Agency. In all of this work, the systems that are described make use of X-ray optical systems made by techniques which emphasise low cost at the expense of angular VII. APPLICATIONS IN ASTRONOMY
264
J.L. Culhane et al. / X-ray optical techniques
resolution. Nevertheless, if resolutions in the range 1 0 - 2 0 arc sec c o m b i n e d with effective apertures of 1 2.10 4 cm 2 can eventually be achieved, it is clear that the large high throughput X-ray observatories required in the next decade will be constructed using X-ray optical systems of the kind described. Work at the Mullard Space Science Laboratory is supported by a grant from the U K Science and Engineering Research Council. Work on gratings at the laboratory for A t m o s p h e r i c and Space Physics is supported by N A S A grant No. NAG-5-96. We are grateful for the efforts of B. Bach of Hyperfine Inc. in producing the reflection gratings. The research at Lockheed is supported by N A S A contracts NAS-5-23563 and NAS-5-26090 and by the Lockheed I n d e p e n d e n t Research Program.
References [1] O.A. Ershov, I.A. Brytov and A.P. Lukurskii, Opt. Spectr. 22 (1964) 66. [2] P.J. Serlemitsos, X-ray astronomy in the 1980's, ed. S.S. Holt, NASA Tech. Memorandum 83848 (1982) pp. 441-460.
[3] P. Predehl, K. Beuermann and H. Brauninger, Appl. Opt. 19 (1980) 190. [4] W. Cash and R. Kohnert, Appl. Opt. 21 (1982) 17. [5] W. Werner, Appl. Opt. 16 (1977) 2078. [6] M. Neviere, P. Vincent and D. Mazstre, Appl. Opt. 17 (1978) 843. [7] M. Neviere, D. Mazstre and W.R. Hunter, J. Opt. Soc. Amer. 68 (1978) 1106. [8] P. Vincent, M. Neviere and D. Mazstre, Appl. Opt. 18 (1979) 1790. [9] D. Bardas, L.N. Mertz and W.J. Rozenberg, unpublished results (1981). [10] J.H. Underwood, T.W. Barbee and D.C. Keith, SPIE Proc. 184 (1979) 123. [11] E. Spiller and A. Segmuller, Appl. Phys. Lett. 37 (1980) 1048. [12] R.C. Catura, L.W. Acton, W.A. Brown, C.W. Gilbreth, L.A. Springer, J.R. Vieira, J.L. Culhane, I.M. Mason, O. Siegmund, T.J. Patrick, P.H Sheather, K.A. Pounds, B.A. Cooke, K. Evans, J. Pye, G. Smith, A. Wells, J.E. Spragg, C.H. Whitford, A. Franks, B. Gale, K. Lindsey, M. Steadman, G. Garmire, B. Margon and A. Fabian, SPIE Proc. 284 (1981) 169. [13] R.C. Catura, W.A. Brown, G. Joki and R. Nobles, Opt. Eng. 22 (1981) 140. [14] A.P. Lukirskii, E.P. Savinov, O.A. Ershov and Y.F. Shepelev, Opt. Spectr. 16 (1964) 168. [15] P.A.J. de Korte J.A.M. Bleeker, A.J.F. Den Boggende, G. Branduardi-Raymont, A.C. Brinkman, J.L. Culhane, E.H.B..M. Gronenschild, I. Mason and S.P. McKechnie, Space Sci. Rev. 30 (1981) 495.