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EUVITA — An extreme UV imaging telescope array with spectral capability
Adv. Space Res. Vol. 13, No. 12, pp. (12)299—(12)302, 1993 Printed in Great Britain. All rightsresa~ve&
0273—1177i~3$6.00 + 0.00 Copyright @ 1993 COSPAR
EUVITA AN EXTREME UV IMAGING TELESCOPE ARRAY WiTH SPECTRAL CAPABILITY -
T. J.-L. Courvoisier,1 A. Orr,1 P. Btthler,1’2 A. Zehnder,2R. Henneck,2 F. Stauffacher,2 J. Biakhowski,2 N. Schlumpf,2 W. Schoeps,2 A. Mchedlishvili,2 R. Sunyaev,3 V. Arefev,3 A. Yascovich,3 G. Babalyan,3 M. Pavlinsky,3 I. P. Delaboudinière,4 T. Carone,5 0. Siegmund,5 J. Warren,5 D. Leahy,6 N. Salaschenko7 and J. Platonov7 1Geneva Observatory, 1290 Sauverny, Switzerland 2 Scherrer Institute, 5232 Villigen-PSI, Switzerland 3Russian Academy ofSciences, Space Research Institute (1K!), Moscow, 4lnstitut d’Astrophysique Spatiale, France 5 Science Laboratory (SSL), Berkeley, USA 6university of Calgary, Canada 7 ofApplied Physics, Nizhny Novgorod, CIS
CIS
Abstract EUVITA is a set of 8 extreme UV normal incidence imaging telescopes, each of them sensitive in a narrow band (A/L~\ = 15 to 80), centered at wavelengths between 50 and 175 A. Each telescope has an effective area of a few cm2, a field of view of 1.2°and a spatial resolution of 10 arcsec. EUVITA will be flown on the Russian mission SPECTRUM X-G. This satellite will be launched in a highly eccentric orbit with a period of 4 days, allowing long, uninterrupted observations (e.g. iO~seconds). EUVITA’s narrow spectral bands allow the measurement of source parameters such as temperature or power law index as well as interstellar absorption, and will resolve groups ofstrong lines emitted by optically thin hot plasmas.
Introduction The EUVITA (Extreme UV Imaging Telescope Array) experiment will be flown on the Soviet Spectrum-X-G satellite along with several other experiments such as SODART (covering the range 0.2 to 20 keV, i.e. 0.6-60 A), JET-X (0.3-10 keV, or 1.2-40 A), MART-LIME (4-100 keV, 0.12-3 A) and TAUVEX (0.004-0.008 keV, 1500-3000 A). EUVITA is sensitive from 0.06 to 0.26 keV (i.e. 45-200 A), with a spatial resolution of 10 arcsec FWHM, and a spectral resolution of A/z.~.\= 15 to 80, and is thus complementary to the other experiments aboard. The 8 EUVITA imaging telescopes will be placed along the principal axis of Spectrum-X-G, co-aligned with the JET-X and SODART experiments. This combination allows a very wide spectral coverage of the objects to be observed. This coverage is essential for understanding the emission mechanisms of the various sources and for deconvolving the emitted spectrum from interstellar effects. The EUVITA bandpasses axe at energies corresponding to the maximum emission of blackbodies with temperatures T = 3 x i0~to 106 K. All EUVITA telescopes will be operated simultaneously and so the variability of the spectral properties of sources will be monitored continuously. This is more difficult with instruments in which wavelengths are successively selected by filter wheels (as with EXOSAT or ROSAT). JASR 13:12-U
(12)299
(12)300
T.J.-L Courvoisier ef aL
In comparison to other EUV experiments the EUVITA telescope array is the only imaging experiment with spectroscopic capability. Its good spatial resolution competes with the best imaging X-ray telescopes. All instruments together give a total effective area of about 25 cm2. Furthermore, the 4 day orbit and the excellent pointing capabilities of the Spectrum-X-G satellite will allow observation times of up to iO~seconds, considerably longer than the standard observations possible on ROSAT or EUVE. There is a wide range of astrophysical objects for which EUVITA observations can bring data essential for the understanding of their nature. These include objects in the solar system, single stars, single or interacting compact stars, interstellar medium, galaxies, active galactic nuclei and clusters of galaxies.
Instrument characteristics Each telescope has a normal incidence multilayer coated parabolic meter of the mirrors is 20 cm with a focal length of 140 cm. The of the individual layers of mirror coating defines the reflectivity as wavelength. It is possible to produce multilayers for the EUV with of 10 to 45 % and a bandwith smaller than 10 % FWHM.
mirror. The diarelative thickness a function of the peak reflectivities
The EUVITA-detector will be mounted on axis at the prime focus of the mirror in the detector electronic box supported by a spider arrangement. See Courvoisier et al. (in preparation) for a more detailed description of the instrument. A broad band metallic filter is mounted close to the front of the MCP stack so as to block off photons with energies lower than the bandpass peaks of the mirrors, in particular in the far UV region where the mirror reflectivity increases. The geometrical mirror area is 221 cm2 on axis, taking into account the shadow of the detector and support. The field of view is 1.2 degrees in diameter. The spatial resolution of the telescope is given by the aberration of the mirror, the resolution of the detector and the jitter of the satellite and is roughly 10 arcsec. The design of the individual telescopes is identical. The telescopes differ only in the multilayer coatings of the mirrors, in the filters and in the photocathode materials. These three elements define the bandpass of each telescope and are chosen in such a way as to optimize the overall sensitivities.
Effective areas The current design EUVITA bandpasses are described in table 1. Globally, the mirror reflectivities follow Lorentzian profiles with red leaks determined by the reflectivity of the mirror substrate (Silicium). The reflectivities used in this paper are based on both measurements and calculations performed in the EUV by the Nizhny Novgorod group. The effective area for each telescope is plotted as a function of wavelength in figure 1. Beyond 1000 A, the effective areas of the detectors have dropped well over 6 orders of magnitude below peak areas. Presently MgF2, CsI, KBr and NaBr photocathodes are being considered.
EUVITA - An ExtremeUV Imaging Telescope
telescope number field of view (FOV) [0] spatial resolution [arcsec] mirror 2] geom. area [cm central wavelength Ao [A] resolution Ao/&~ max. refi. [%] ifiter
1
2
3
(12)301
4
6 1.2 ~ 10 normal incidence multilayer mirror _____ _______ _______ 221 _______ 175 149 132 102 83 70 15 20 30 50 60 60 30 45 40 10 10 10 Al/C Lex/Be Lex/Be Lex/Ti/B Lex/Sn Lex/Sn _____________________ global filter mesh and ion grid transmisivity_= 0.64 maximum effective areas [cm2] with different photocathodes: MgF2 4.3 4 5 0.8 1 1.6 CsI 4.6 5.3 8.4 1.3 0.8 1 KBr 3.9 4.1 5.8 1.5 1.9 3 NaBr 7.5 7.7 10.4 1.8 2.1 3.2
7
_______
59 60 10 Lex/Sn ________
1.2 0.9 2.4 2.8
8
_______
49 80 10 Lex/Sn ________
0.7 1 1.9 2.5
Table 1: Parameters describing the EU VITA bandpasses: the central wavelength of the peak, bandwith of the peaks with L~A= FWHM, the peak reflectivities, and the filters used for each telescope.
Minimum detectable fluxes The signal to noise ratio ~ is given by: I N
=
where S is the number of
S
N
counts per detection area (see below), N~d the background counts per detection area, and k is the number of detection areas from which the average sky and detector noise background are measured. In the present case k is large enough so that (1 + ~) 1. ‘~-‘
We consider a circular detection area of radius 40 pm on the MCP surface. The total background is expected to be of the order of 10 counts/sec over the 7 cm2 surface of the MCPs. From the expression for ~ one obtains the minimal detectable number of counts per detection area. A minimum of 14 counts per detection area is necessary to detect a source with a signal to noise ratio = 3 in t = iO~sec. The minimum detectable flux for a pointlike source can be calculated with: Fmjir, = SurfacexPxtxl where P is the percentage of all the detected counts falling in one detection area (P = 83%), t is the observation time, and Surface is the geometrical area. I is the integral over all wavelengths of the product of the mirror reflectivity, the filter transmissivity and the photocathode quantum efficiency. The corresponding minimal fluxes Fmin [10-6 photons/cm2 sec A] for a NaBr photocathode and an observation time of iO~sec. are: telescope number NaBr photocathode
1 1.5
2 1.9
3 2.3
4 20
5 39
6 29
7
37
8 65
T. J..L. Courvoi~icret aL
(12)302 —
.
I
•
I
I
•
3— -
0
-
2T 40
—
—
I
I
1
-
60
80
100
lambda (A) 10—S
:‘~
•
9—
100
—
150
200
lambda (A) Figure 1: Effective areas for the EUVITA telescopes with photocathodes ofMgF2 (full line), CsI (dots), KBr (dashes) and NaBr (dash-dots).
Conclusion EUVITA has a good spatial resolution (10 arcsec) over a field of view of 1°.2,with an
effective area comparable to that of presently flown instruments. The novel feature of EUVITA is that it combines these ‘standard’ properties with a spectral resolution in the order of a few A. This should allow to resolve prominent groups of emission lines. We have demonstrated that EUVITA will be able to detect several classes of galactic and in favourable cases extra-galactic sources, at signal to noise levels which are sufficient to allow a measurement of the source parameters in the EUV (Courvoisier et al. in preparation). For known sources, the bandpass combination allows to measure independently the source characteristics and the intervening column density. -
The EUV domain is still largely unexplored. In addition to the determination of astrophysical quantities in a new spectral region for known sources, we expect that instruments of the class of EUVITA will also provide a number of unexpected results. It is indeed true that each time that a new spectral window has been opened to astronomical investigations, new and exciting physical phenomena have been unveiled.
0273—1177i~3$6.00 + 0.00 Copyright @ 1993 COSPAR
EUVITA AN EXTREME UV IMAGING TELESCOPE ARRAY WiTH SPECTRAL CAPABILITY -
T. J.-L. Courvoisier,1 A. Orr,1 P. Btthler,1’2 A. Zehnder,2R. Henneck,2 F. Stauffacher,2 J. Biakhowski,2 N. Schlumpf,2 W. Schoeps,2 A. Mchedlishvili,2 R. Sunyaev,3 V. Arefev,3 A. Yascovich,3 G. Babalyan,3 M. Pavlinsky,3 I. P. Delaboudinière,4 T. Carone,5 0. Siegmund,5 J. Warren,5 D. Leahy,6 N. Salaschenko7 and J. Platonov7 1Geneva Observatory, 1290 Sauverny, Switzerland 2 Scherrer Institute, 5232 Villigen-PSI, Switzerland 3Russian Academy ofSciences, Space Research Institute (1K!), Moscow, 4lnstitut d’Astrophysique Spatiale, France 5 Science Laboratory (SSL), Berkeley, USA 6university of Calgary, Canada 7 ofApplied Physics, Nizhny Novgorod, CIS
CIS
Abstract EUVITA is a set of 8 extreme UV normal incidence imaging telescopes, each of them sensitive in a narrow band (A/L~\ = 15 to 80), centered at wavelengths between 50 and 175 A. Each telescope has an effective area of a few cm2, a field of view of 1.2°and a spatial resolution of 10 arcsec. EUVITA will be flown on the Russian mission SPECTRUM X-G. This satellite will be launched in a highly eccentric orbit with a period of 4 days, allowing long, uninterrupted observations (e.g. iO~seconds). EUVITA’s narrow spectral bands allow the measurement of source parameters such as temperature or power law index as well as interstellar absorption, and will resolve groups ofstrong lines emitted by optically thin hot plasmas.
Introduction The EUVITA (Extreme UV Imaging Telescope Array) experiment will be flown on the Soviet Spectrum-X-G satellite along with several other experiments such as SODART (covering the range 0.2 to 20 keV, i.e. 0.6-60 A), JET-X (0.3-10 keV, or 1.2-40 A), MART-LIME (4-100 keV, 0.12-3 A) and TAUVEX (0.004-0.008 keV, 1500-3000 A). EUVITA is sensitive from 0.06 to 0.26 keV (i.e. 45-200 A), with a spatial resolution of 10 arcsec FWHM, and a spectral resolution of A/z.~.\= 15 to 80, and is thus complementary to the other experiments aboard. The 8 EUVITA imaging telescopes will be placed along the principal axis of Spectrum-X-G, co-aligned with the JET-X and SODART experiments. This combination allows a very wide spectral coverage of the objects to be observed. This coverage is essential for understanding the emission mechanisms of the various sources and for deconvolving the emitted spectrum from interstellar effects. The EUVITA bandpasses axe at energies corresponding to the maximum emission of blackbodies with temperatures T = 3 x i0~to 106 K. All EUVITA telescopes will be operated simultaneously and so the variability of the spectral properties of sources will be monitored continuously. This is more difficult with instruments in which wavelengths are successively selected by filter wheels (as with EXOSAT or ROSAT). JASR 13:12-U
(12)299
(12)300
T.J.-L Courvoisier ef aL
In comparison to other EUV experiments the EUVITA telescope array is the only imaging experiment with spectroscopic capability. Its good spatial resolution competes with the best imaging X-ray telescopes. All instruments together give a total effective area of about 25 cm2. Furthermore, the 4 day orbit and the excellent pointing capabilities of the Spectrum-X-G satellite will allow observation times of up to iO~seconds, considerably longer than the standard observations possible on ROSAT or EUVE. There is a wide range of astrophysical objects for which EUVITA observations can bring data essential for the understanding of their nature. These include objects in the solar system, single stars, single or interacting compact stars, interstellar medium, galaxies, active galactic nuclei and clusters of galaxies.
Instrument characteristics Each telescope has a normal incidence multilayer coated parabolic meter of the mirrors is 20 cm with a focal length of 140 cm. The of the individual layers of mirror coating defines the reflectivity as wavelength. It is possible to produce multilayers for the EUV with of 10 to 45 % and a bandwith smaller than 10 % FWHM.
mirror. The diarelative thickness a function of the peak reflectivities
The EUVITA-detector will be mounted on axis at the prime focus of the mirror in the detector electronic box supported by a spider arrangement. See Courvoisier et al. (in preparation) for a more detailed description of the instrument. A broad band metallic filter is mounted close to the front of the MCP stack so as to block off photons with energies lower than the bandpass peaks of the mirrors, in particular in the far UV region where the mirror reflectivity increases. The geometrical mirror area is 221 cm2 on axis, taking into account the shadow of the detector and support. The field of view is 1.2 degrees in diameter. The spatial resolution of the telescope is given by the aberration of the mirror, the resolution of the detector and the jitter of the satellite and is roughly 10 arcsec. The design of the individual telescopes is identical. The telescopes differ only in the multilayer coatings of the mirrors, in the filters and in the photocathode materials. These three elements define the bandpass of each telescope and are chosen in such a way as to optimize the overall sensitivities.
Effective areas The current design EUVITA bandpasses are described in table 1. Globally, the mirror reflectivities follow Lorentzian profiles with red leaks determined by the reflectivity of the mirror substrate (Silicium). The reflectivities used in this paper are based on both measurements and calculations performed in the EUV by the Nizhny Novgorod group. The effective area for each telescope is plotted as a function of wavelength in figure 1. Beyond 1000 A, the effective areas of the detectors have dropped well over 6 orders of magnitude below peak areas. Presently MgF2, CsI, KBr and NaBr photocathodes are being considered.
EUVITA - An ExtremeUV Imaging Telescope
telescope number field of view (FOV) [0] spatial resolution [arcsec] mirror 2] geom. area [cm central wavelength Ao [A] resolution Ao/&~ max. refi. [%] ifiter
1
2
3
(12)301
4
6 1.2 ~ 10 normal incidence multilayer mirror _____ _______ _______ 221 _______ 175 149 132 102 83 70 15 20 30 50 60 60 30 45 40 10 10 10 Al/C Lex/Be Lex/Be Lex/Ti/B Lex/Sn Lex/Sn _____________________ global filter mesh and ion grid transmisivity_= 0.64 maximum effective areas [cm2] with different photocathodes: MgF2 4.3 4 5 0.8 1 1.6 CsI 4.6 5.3 8.4 1.3 0.8 1 KBr 3.9 4.1 5.8 1.5 1.9 3 NaBr 7.5 7.7 10.4 1.8 2.1 3.2
7
_______
59 60 10 Lex/Sn ________
1.2 0.9 2.4 2.8
8
_______
49 80 10 Lex/Sn ________
0.7 1 1.9 2.5
Table 1: Parameters describing the EU VITA bandpasses: the central wavelength of the peak, bandwith of the peaks with L~A= FWHM, the peak reflectivities, and the filters used for each telescope.
Minimum detectable fluxes The signal to noise ratio ~ is given by: I N
=
where S is the number of
S
N
counts per detection area (see below), N~d the background counts per detection area, and k is the number of detection areas from which the average sky and detector noise background are measured. In the present case k is large enough so that (1 + ~) 1. ‘~-‘
We consider a circular detection area of radius 40 pm on the MCP surface. The total background is expected to be of the order of 10 counts/sec over the 7 cm2 surface of the MCPs. From the expression for ~ one obtains the minimal detectable number of counts per detection area. A minimum of 14 counts per detection area is necessary to detect a source with a signal to noise ratio = 3 in t = iO~sec. The minimum detectable flux for a pointlike source can be calculated with: Fmjir, = SurfacexPxtxl where P is the percentage of all the detected counts falling in one detection area (P = 83%), t is the observation time, and Surface is the geometrical area. I is the integral over all wavelengths of the product of the mirror reflectivity, the filter transmissivity and the photocathode quantum efficiency. The corresponding minimal fluxes Fmin [10-6 photons/cm2 sec A] for a NaBr photocathode and an observation time of iO~sec. are: telescope number NaBr photocathode
1 1.5
2 1.9
3 2.3
4 20
5 39
6 29
7
37
8 65
T. J..L. Courvoi~icret aL
(12)302 —
.
I
•
I
I
•
3— -
0
-
2T 40
—
—
I
I
1
-
60
80
100
lambda (A) 10—S
:‘~
•
9—
100
—
150
200
lambda (A) Figure 1: Effective areas for the EUVITA telescopes with photocathodes ofMgF2 (full line), CsI (dots), KBr (dashes) and NaBr (dash-dots).
Conclusion EUVITA has a good spatial resolution (10 arcsec) over a field of view of 1°.2,with an
effective area comparable to that of presently flown instruments. The novel feature of EUVITA is that it combines these ‘standard’ properties with a spectral resolution in the order of a few A. This should allow to resolve prominent groups of emission lines. We have demonstrated that EUVITA will be able to detect several classes of galactic and in favourable cases extra-galactic sources, at signal to noise levels which are sufficient to allow a measurement of the source parameters in the EUV (Courvoisier et al. in preparation). For known sources, the bandpass combination allows to measure independently the source characteristics and the intervening column density. -
The EUV domain is still largely unexplored. In addition to the determination of astrophysical quantities in a new spectral region for known sources, we expect that instruments of the class of EUVITA will also provide a number of unexpected results. It is indeed true that each time that a new spectral window has been opened to astronomical investigations, new and exciting physical phenomena have been unveiled.