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Large light X-ray optics: basic ideas and concepts
Advances in Space Research 34 (2004) 2637–2645 www.elsevier.com/locate/asr
Large light X-ray optics: basic ideas and concepts O. Citterio a,*, M. Ghigo a, F. Mazzoleni a, G. Pareschi a, B. Aschenbach b, H. Braeuninger b, P. Friedrich b, G. Hasinger b, G. Parodi c b
a INAF-Osservatorio Astronomico di Brera – Via E. Bianchi 46 23807, Merate, Italy Max-Planck-Institut f€ur extraterrestrische Physik – Postfach 1312, Giessenbachstrasse 85741, Garching, Germany c BCV Progetti, Via Santa Orsola 1 – 20123, Milano, Italy
Received 4 February 2003; received in revised form 24 March 2003; accepted 31 March 2003
Abstract One of the main guidelines for future X-ray astronomy projects like, e.g., XEUS (ESA) and Generation-X (NASA) is to utilize grazing-incidence focusing optics with extremely large telescopes (several tens of m2 at 1 keV), with a dramatic increase in collecting area of about two order of magnitude compared to the current X-ray telescopes. In order to avoid the problem of the source’s confusion limit at low fluxes, the angular resolution required for these optics should be superb (a few arcsec at most). The enormous mirror dimensions together with the high imaging performances give rise to a number of manufacturing problems. It is basically impossible to realize so large mirrors from closed Wolter I shells which benefit from high mechanical stiffness. Instead the mirrors need to be formed as rectangular segments and a series of them will be assembled in a petal. Taking into account the realistic load capabilities of space launchers, to be able to put in orbit so large mirror modules the mass/geometric-area ratio of the optics should be very small. Finally, with a so large optics mass it would be very difficult to provide the electric power for an optics thermal active control, able to maintain the mirrors at the usual temperature of 20 C. Therefore, very likely, the optics will instead operate in extreme thermal conditions, with the mirror temperature oscillating between )30 and )40 C, that tends to exclude the epoxy replication approach (the mismatch between the CTE of the substrate and that of the resin would cause prohibitively large deformations of the mirror surface profiles). From these considerations light weight materials with high thermal–mechanical properties such as glass or ceramics become attractive to realize the mirrors of future Xray telescopes. In this paper, we will discuss a segments manufacturing method based on BorofloatTM glass. A series of finite element analysis concerning different aspects of the production, testing and integration of the optics are also presented as well. 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: X-ray optics; X-ray astronomy; X-ray telescopes
1. Introduction In Astronomy, like for many sciences, the advances in knowledge are strictly connected with the availability of new and powerful instrumentation. The development of a new generation of optical telescopes with diameters in the range of 8–10 m has delivered the cascade of astronomical discoveries of the last few years. The manufacturing of these large optics has been possible thank to new engineering ideas and radical approaches. For example the possibility to assemble a large mirror con*
Corresponding author. E-mail address: [email protected] (O. Citterio).
necting a set of smaller mirrors has permitted the creation of the twin Keck Telescopes that have a diameter of 10 m each. Also in the field of the X-ray Astronomy the need of larger optics (and hence collecting area) has pushed in the years the state of the art of the mirror manufacturing techniques. In Fig. 1, an overview of past and future X-ray missions from the point of view of their Effective Area (at 1 keV) and Angular Resolution (in terms of Half-Energy-Width) is visible. After the launch of XMM-Newton and Chandra (the main X-ray telescopes now operating and with diameters, respectively of 700 and 1200 mm), the next step will be the launch in 2010 by NASA of Constellation-X (Bookbinder, 2001). Four satellites, all of them co-orbiting in
0273-1177/$30 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2003.03.064
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Fig. 1. Overview of past and future X-ray missions.
the L2 libration point, will be simultaneously pointed toward the same celestial source, mimicking in some way the same philosophy followed by the VLT optical telescope that employs four 8 m telescopes linked together. Every one of the four satellites will host a soft X-ray telescope (SXT) with a diameter of 1.7 m and three hard X-ray telescopes (HXT), each with a diameter of 35 cm. Considering the different technology and manufacturing procedure necessary for the construction of a grazingincidence X-ray telescope with respect to an optical normal-incidence telescope, a diameter in the range of 1.7 m for the outer shells is probably outside of what is reasonable to build monolithically. For this reason the SXT optics will be built connecting many segments in a circular fashion to recreate the working shells. After Constellation-X it is clear that the next future missions using grazing-incidence focusing optics will have sizes and collecting areas in a range that will surely require the use of segments to recreate a full X-ray telescope. In this contest two future missions nowadays on the drawings boards are the X-ray Evolving Universe Spectroscopy mission (XEUS mission) and the Generation-X mission. They represent a quantum leap with respect to the previous missions and Constellation-X. The XEUS ESA mission (ESA, 2000a,b,c) is foreseen around the year 2015 and will be based on a single telescope in configuration Wolter I (Bavdaz et al., 1999, 2000) (ESA, 2001) having a diameter of 9.9 m, with a resolution goal of 200 HEW (Half Energy Width). The focal length will be of 50 m and this fact has pushed toward an innovative approach that foresees the use of two distinct spacecrafts, one for the optics and the other for the detectors, that will operate in formation flight, linked to each other by an active tracking system. Initially the core optics of XEUS (XEUS I) will be laun-
ched by an Ariane V rocket that will deliver in LEO orbit the two spacecrafts. In this first part of the mission the diameter of the optics will be of 4.5 m with an effective area of 6 m2 at 1 keV. For comparison, the sum of the four effective areas of the SXT modules of Constellation-X is of 1.5 m2 at that energy. After completion of the initial 4–6 years mission phase, XEUS will rendezvous with the International Space Station (ISS) for refurbishment and to allow the addition of extra mirror modules, bringing the total effective area to 30 m2 at 1 keV and the diameter to 9.9 m. After the separation from ISS it will begin the second phase of observations, named XEUS II, and that will last about 15 years. The NASA mission Generation-X (Zhang et al., 2001) is foreseen around 2020 or later and in the present configuration follows the same philosophy used for the Constellation-X mission: it should be launched to the L2 libration point and will consist of six identical satellites, each of them carrying a fraction of the required total collecting area of 150 m2 at 1 keV, minimizing both launch and component failure risks. Each telescope during the launch phase would be stowed in about 4.5 m in diameter but after deployment should be able to unfold and deliver 25 m2 of effective area. To achieve a broad band coverage in energy the focal length will be of about 100–150 m and the resolution goal will be of about 0.100 HEW. Part of the considerations that will follow are the subject of another article (Ghigo et al., 2004) and are here reported shortly for sake of completeness. We will use as guideline for the definition of a X-ray mirror technology mainly the XEUS mission optical requirements and constrains. Of course some of the proposed ideas and considerations could be useful as well for other future X-ray missions.
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2. Main technical difficulties and limits of present manufacturing techniques The X-Ray mirror technology required for the realization of Large Light X-ray Optics (LLXO) must solve the problem of deliver mirrors that satisfy the imaging resolution required and in the meantime stay within the weight budgets allocated for the optics. For example for the XEUS mission the goal in resolution is 200 HEW with a mass budget of just 25,000 kg. As an example in Fig. 2 it is shown the setup foreseen for the XEUS optics. Since it is practically impossible to realize X-ray optics so large using closed Wolter I shells, the mirrors will be formed by rectangular segments. A set of these segments will be mounted and aligned in a petal that will be next assembled circularly to form the telescope optics. The weight constrains, due to the limits of the launchers, imply that the mass/geometrical-area ratio of the optics should be very small and in the order of 0.08 kg/cm2 for XEUS. The total number of segments necessary for the full telescope amount to about 17,500 pieces. Another problem that must be solved for the manufacturing of LLXO is that the total mirror surface is so huge that the optics probably cannot be thermally controlled. This means that the optics will be manufactured at ambient temperature (20 C) but will operate in a much lower (about )35 C) and fluctuating range of temperatures, depending from the chosen orbit. This means also that, to avoid deformations, the segments
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and the supporting structure must be based on materials with CTE as equal as possible. Last but not least, the manufacturing of these optics requires an industrial process ‘‘fast and automated’’, able to manufacture and test the thousands of segments needed for the project in a few years. The development of some kind of replication or molding technique could take advantage from the fact that the LLXO are assembled from large numbers of equal segments. These techniques have obvious advantages in terms of speed and cost of production. The current manufacturing techniques for space Xray optics are listed hereafter: • Nickel electroforming.It has been used for making the mirrors of a number of X-ray missions like SAX (Citterio et al., 1988), ABRIXAS (Altmann et al., 1998), XMM (Citterio et al., 1992), JET-X (Citterio et al., 1996), SWIFT (Burrows et al., 2000). Therefore, it can be considered a mature technology. Due to the high density of Nickel and considering that the typical mass/geometric-area allocated for a XEUS segment is just 2.64 kg/m2 , the typical thickness of a XEUS segment should be in the order of 0.3 mm. This very small thickness will degrade the stiffness of the segment to an extent that very unlikely could reach the high angular resolutions required. • epoxy replication.(De Korte et al., 1981; Citterio et al., 1997; Citterio et al., 2000) As for Ni electroforming replication, this technique makes use of a
Fig. 2. XEUS optical setup.
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superpolished gold-coated master (mandrel). In this case the master is placed almost in contact with a carrier, leaving only a small gap (100 lm) between the two surfaces. This gap is then filled with epoxy resin. When the resin is cured the optic is separated from the master. Having the gold a better adhesion to the epoxy than to the master, it will follow the carrier, forming in this way the reflective surface of the optic. The master can of course be used to replicate a large number of equal mirrors. This procedure is therefore, attractive for the XEUS mirror production but suffers for a major drawback: a segment produced in this way is essentially a sandwich composed by epoxy and the carrier material (for example SiC or Glass). It is easy to understand that the difference in CTE of the two materials, once that the optics will be put in space at a temperature of )35 C or less, will create internal forces able to deform the shape of the segments. The working temperature of optics fabricated with this method has to be the same of the manufacturing process and this is not possible in absence of thermal control. This fact tends to exclude epoxy replication for the XEUS segments production. In another section of this paper more detailed results obtained with finite element analysis on simulated epoxy replicated segments are reported. • Direct polishing mirror technology.With this technique the optics of the CHANDRA telescope have been realized, that showed the best angular resolution (0.500 HEW) achieved so far (Weisskopf et al., 2000). This performance has been obtained at the price of using just a few (4) very thick (a couple of cm) heavy and stiff shells made in glass ceramics. The optics are directly figured to the required tolerances with classical optical grinding and polishing techniques. The low number of shells obviously means that the achievable collecting area is relatively small. This technique can reach the resolutions requested by LLXO but not within the mass budget allocated for the optics. • Thin foil mirror technology.It has been used for example in ASCA(Serlemitsos et al., 1995), and it is based on aluminum segmented mirrors having a very thin thickness. Their optical resolution is generally in the order of 10000 HEW. The obvious advantage is instead the very low weight of these optics. From all these considerations we can summarize the requirements that the technology necessary for the production of LLXO telescopes should satisfy: • It should be able to produce stiff optical segments with a very small mass/geometrical-area ratio and high performances in terms of angular resolution. • It should be possible to correctly assemble the segments to form a full optical telescope. • Segments and related supporting structure should have very similar CTEs to ensure the minimum amount of mechanical–optical distortion.
• Some kind of a replication or molding process is desirable to ease the production of the thousands of segments required (17,500 for XEUS II). However the classical epoxy replication technique cannot be used due to the CTE mismatch between the epoxy layer and the substrate material. • The production process must guarantee a high volume production, able to fabricate and test in an industrial realization chain all the segments requested.
3. Proposed manufacturing process of segments The choice among candidate materials to be used for making the LLXO mirrors should be based on the best tradeoff in terms of stiffness against the costs and complexity of production. A finite element analysis that has been performed on typical segments (1 0.5 m, weight ¼ 1.32 kg) made of different materials like Electroformed-Nickel, Al2 O3 , Glass, Silicon Carbide (SiC) and two sandwiches of Al2 O3 and SiC, has shown that purely from the point of view of the mechanical performances a segment made with a sandwich in SiC (two SiC skins and a SiC foam in-between – Novi et al., 2001) has the maximum Flexural Rigidity. This parameter describes the rigidity of a structure and the corresponding values for the different materials here considered are reported in Fig. 3, normalized to the Nickel one. It can be seen that a segment made in foamed SiC is 874 times stiffer than an equivalent Nickel segment. Even if this is an impressive performance we must consider also other facts like the cost of the material and the difficulties involved in the development of a sandwich technology. A manufacturing technique representing a good compromise between complexity and costs has been individuated. It makes use of thin (about 1 mm) sheets
Fig. 3. The Flexural Rigidity parameter for different materials in the case of a typical XEUS panel.
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of BorofloatTM glass to produce the necessary segments. The Flexural Rigidity of a glass segment is 23 times that of an equivalent Nickel segment having the same weight. The glass is also a well known material, easily workable and with low cost if compared to the SiC. Fig. 4 shows in general terms the proposed manufacturing sequence. A BorofloatTM glass sheet is produced floating the material on a liquid tin bath and then it is cutted to the required dimensions. The sheet is grinded and polished to obtain an uniform thickness. A concave master made in a ceramic material is then used for the slumping of the segment to the desired shape. The glass in fact is heated to a temperature that permits to soften it and, consequently, to change its shape. If the thickness of the sheet is perfectly constant, the optical shape of the master could be transferred also on the upper face (concave) of the sheet. In the reality this is not practically possible and the deviations from an uniform thickness will introduce profile errors on the concave reflecting side of the sheet. To correct for these errors a figuring phase is foresee that will use a vacuum support system to maintain in position the sheet. This system will also be used to transfer the finished segment to the support structure, maintaining its shape during the assembling in the telescope. Once the alignment is done and the segment fixed to the support structure by means, e.g., of epoxy resin, the vacuum support will be released. The viability of this procedure is under evaluation with a joint effort between the MPE (Max-Planck- Institut fur Extraterrestrische Physik-Germany) and the Osservatorio Astronomico di Brera (OAB).
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The slumping process is investigated with the support of the Schott Company (Germany). The results of a preliminary series of tests has been very encouraging. These tests have been performed on smaller scale starting from flat sheets of BorofloatTM glass having a size of 200 200 1 mm. The idea was to demonstrate the feasibility of the first steps of the procedure and to obtain curved segments with a curvature radius of 2000 mm and a maximum departure error of 10 l P–P from the final surface. This value is the maximum error acceptable by the Carl Zeiss company (Germany) that should develop the robotic polishing technique able to bring the surface within the requested optical tolerances. In order to test the process a mold having the same dimensions of the sheets and made in a ceramic material has been manufactured with a cylindrical surface having a maximum departure from the theoretical one of 5 l P– P. After some tests used to calibrate the thermal cycles necessary for the slumping process it has been obtained a P–P difference of 6 l on the segments produced, with respect to the shape of the mold. This result is already within the specifications of Carl Zeiss but since this result has been obtained with a relatively small number of tests we are confident that there is space for a further improvement of the slumping process. Of course the smaller is the error left before the figuring step, the lesser is the time necessary for the figuring correction. The figuring should also bring the microroughness of the BorofloatTM sheets to a value better than 0.5 nm rms, necessary for the X-ray reflection in the range of energy up to 10 keV. In Fig. 5 the results obtained with the
Fig. 4. Segments production and integration.
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Fig. 5. Slumping results obtained with different cooling rates.
slumping process using two different cooling rates (very critical factors in the process) are shown. The bi-dimensional profiles shown in the figure are the residual errors resulting after the subtraction of the segment shape from the mould. A perfect match should shown a flat plane. The slower cooling rate (right surface) offers the best result, with a P–P of 6 l, probably since it gives to the glass more time to relax and to move staying in contact with the mould surface. The next foreseen steps will be given by the figuring of a segment and its subsequent integration in a supporting frame able of maintaining its final shape. This phase will be the second part of the feasibility study.
4. Finite element analysis of various parameters affecting the segments optical performances The ability to manufacture single segments of an LLXO telescope obviously does not correspond to the capability of making a whole working telescope. In fact there are many parameters that influence the final optical performances of this system. To correctly evaluate the manufacturing requirements of all the aspects of construction (optical and mechanical) it is of paramount importance to perform a finite element analysis of the critical components. A similar study is important to select the best technological approaches and to understand the tolerances requested in each step of the realization. In our study, we used the output results of a finite element code as input parameters of an optical ray tracing software. In this way, every aspect investigated from the mechanical point of view (deformations, stresses and so on) is reflected on the optical performances of the simulated optics giving directly the imaging degradation in terms of HEW. All the ray tracings have been carried out assuming a source at infinite distance.
A first aspect that was initially evaluated was connected to the use of the epoxy replication method for the segments prediction. The interest for this manufacturing system is obvious and hence it is important to quantitatively understand the effect that the temperature variations would have on the optical performances. For these simulations we assumed the best available material, SiC, in two different possible configurations: SiC solid sheets and foamed SiC sandwiches. The CTE assumed for SiC was 3 106 C1 , and for the epoxy resin was 80 106 C1 . To try to compensate for the inevitable deformations arising from the temperature changes, two layers of epoxy were supposed to be applied to the SiC substrate, the inner one (the normal one supporting the reflecting gold) and the outer one, on the opposite side, having about the same thickness of the first (to simulate a thickness error in the manufacturing). The case without the epoxy layer on the outer side was also evaluated. In Table 1 are shown the most relevant results obtained. The HEW degradation is computed for a variation in temperature of 1 C. The best result is obtained for the case B, a foamed SiC sandwich. With 50 l of epoxy from the side of the gold and 40 l from the other side (we assume a 10 l tolerance for the epoxy thickness) the degradation per degree amounts to 0.2600 HEW assuming a linear trend of HEW vs. temperature. If we consider that the optics would be manufactured at about 20 C and it will work at )40 C, the 60 C difference in temperature this gives us a degradation of 15.600 HEW. This value is much larger than the 200 HEW requested for XEUS. It is hence unlikely that the epoxy replication (in the present version) can be used for the manufacturing of the segments. The situation would be even much worst using glass. The 60 C difference from the manufacturing and the XEUS operating temperature can potentially introduce
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also another source of error: the mismatch between the CTEs of the mirror segment and the supporting structure. An analysis has been made concerning the effects of a mirror and petal supporting structure CTE mismatch. As already mentioned, a number of individual segments will be assembled in self-contained angular sectors named ‘‘petal’’ that once circularly connected will form a corona of mirror shells. Since petals directly support the segments, any CTE difference between them could introduce non isotropic deformations affecting the segments shape. In Table 2 are shown the results of the analysis performed. In this case the simulation assumed a typical XEUS segment made in BorofloatTM with a CTE of 3.3 106 C1 . It has been inserted into an infinitely stiff structure having a CTE of 4.3 106 C1 . Hence the CTE mismatch assumed between the two materials were of 1 106 C1 . The distortions of the optical surface has been divided in radial displacement, axial slope and azimuthal slope. The more relevant data are the deformations along the slopes that heavily influences the optical performances. In fact we have a degradation of about 8000 either for the parabola and the hyperbola sections and of 42700 in azimuthal direction, assuming again a change in temperature of 60 C. These errors are the result of a CTE mismatch of only 1 106 C1 , and the conclusions that must be drawn are that the material
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of the segments and the material of the petals should be essentially the same. This requirement seems very important because otherwise the errors introduced would be much larger than the goal resolution of 200 HEW. It is interesting to analyze the influence of the gravity on a segment. This information is necessary, e.g., to define the best position and orientation of the petals during the X-ray measurements. We have simulated two BorofloatTM segments (MS1 located at a radius form the telescope axis of about 1320 mm and MS2 located at 4200 mm) composed by two separated sections: a parabola plus a hyperbole. The segments, with a thickness of 1 mm, have been placed and constrained in an infinitely stiff petal. The testing of the optics has been evaluated in their three possible orientations with respect to the gravity vector. The segments can be assembled and tested with the optics laterally oriented (gravity oriented along the shorter side of the segment, Fig. 6.1), normally oriented (gravity oriented perpendicularly to the surface of the segment, Fig. 6.2), axially oriented (gravity oriented along the longer side of the segment, Fig. 6.3). In Table 3 are shown the results obtained. The data obtained indicate that the best results are in the case of the axial gravity configuration. We see that in this case the inner segment shows a degradation of 0.3500 HEW and the outer segment has a degradation of 1.1500 HEW.
Table 1 Thermal effects on epoxy replicated shells CASE
A B C D
SiC carrier
Sandwich Sandwich Solid Solid
Inner (lm)
EPOXY thickness Outer (lm)
EPOXY CTE ( 106 K1 )
HEW (arcsec) 1 C
90EW (arcsec) 1C
Focal length change (mm)
40 50 40 50
0 40 0 40
80 80 80 80
0.60 0.26 2.25 0.81
1.99 0.58 10.87 3.09
)5.29 )1.53 )30.95 )8.89
Table 2 Effect of mirror and petal CTE mismatch for a AT of 60 C -6
-1
Ms data: Material = BOROFLOAT (CTE3.3 × 10 C ) . Radius at par-hyp interface = 1320mm . Focal length = 50,000mm . MS thickness = 1mm . Angular aperture 22.5˚C
Petal dimensions: 0.8m in radial direction; 0.5 + 0.5m invertical direction; 0.5m in tangential direction -6 -1 Petal CTE: 4.3 × 10 C
A temperature variation equal to – 60 ˚C has been assumed. The temperature value is uniform along the whole structure, no temperature gradients have been considered. Data relevevant to the optical surface distortions MS SURFACE
Parabola Hyperbola
REDIAL DISPLACEMENTS Peak to RMS Valley [µm] [µm] 494 150 493 149
AXIAL SLOPE Peak to RMS valley [arcsec] [arcsec] 340 80.6 341 81
AZIMUTHAL SLOPE RMS Peak to Valley [arcsec] [arcsec] 2268 427 2273 427
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Fig. 6. Flexure by gravity of a segment in an infinitely stiff petal.
Table 3 HEW degradation due to the gravity CASE
MS1 MS1 MS1 MS2 MS2 MS2
– – – – – –
Nominal focal plane
lateral gravity normal gravity axial gravity lateral gravity normal gravity axial gravity
HEW (arcsec)
90% HEW (arcsec)
3.7 0.74 0.35 25.0 1.9 1.15
9.0 1.61 0.85 47.7 4.1 2.84
of some critical aspects of their manufacturing, testing and assembling, a set of finite element analysis has been performed. The conclusions deriving from their results should be used to optimize the performances obtainable in the final assembled optics. Acknowledgements This research has been partially funded by the Italian Space Agency (ASI).
5. Conclusions We have presented a possible manufacturing process for the production of the segments for the optics of the future large X-ray space missions and in particular aimed to the XEUS mission. This approach is based on segments made in BorofloatTM slumped glass. This material offers the best trade-off between optical-mechanical performances and costs of production. To assess the influence on the segments optical performances
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Large light X-ray optics: basic ideas and concepts O. Citterio a,*, M. Ghigo a, F. Mazzoleni a, G. Pareschi a, B. Aschenbach b, H. Braeuninger b, P. Friedrich b, G. Hasinger b, G. Parodi c b
a INAF-Osservatorio Astronomico di Brera – Via E. Bianchi 46 23807, Merate, Italy Max-Planck-Institut f€ur extraterrestrische Physik – Postfach 1312, Giessenbachstrasse 85741, Garching, Germany c BCV Progetti, Via Santa Orsola 1 – 20123, Milano, Italy
Received 4 February 2003; received in revised form 24 March 2003; accepted 31 March 2003
Abstract One of the main guidelines for future X-ray astronomy projects like, e.g., XEUS (ESA) and Generation-X (NASA) is to utilize grazing-incidence focusing optics with extremely large telescopes (several tens of m2 at 1 keV), with a dramatic increase in collecting area of about two order of magnitude compared to the current X-ray telescopes. In order to avoid the problem of the source’s confusion limit at low fluxes, the angular resolution required for these optics should be superb (a few arcsec at most). The enormous mirror dimensions together with the high imaging performances give rise to a number of manufacturing problems. It is basically impossible to realize so large mirrors from closed Wolter I shells which benefit from high mechanical stiffness. Instead the mirrors need to be formed as rectangular segments and a series of them will be assembled in a petal. Taking into account the realistic load capabilities of space launchers, to be able to put in orbit so large mirror modules the mass/geometric-area ratio of the optics should be very small. Finally, with a so large optics mass it would be very difficult to provide the electric power for an optics thermal active control, able to maintain the mirrors at the usual temperature of 20 C. Therefore, very likely, the optics will instead operate in extreme thermal conditions, with the mirror temperature oscillating between )30 and )40 C, that tends to exclude the epoxy replication approach (the mismatch between the CTE of the substrate and that of the resin would cause prohibitively large deformations of the mirror surface profiles). From these considerations light weight materials with high thermal–mechanical properties such as glass or ceramics become attractive to realize the mirrors of future Xray telescopes. In this paper, we will discuss a segments manufacturing method based on BorofloatTM glass. A series of finite element analysis concerning different aspects of the production, testing and integration of the optics are also presented as well. 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: X-ray optics; X-ray astronomy; X-ray telescopes
1. Introduction In Astronomy, like for many sciences, the advances in knowledge are strictly connected with the availability of new and powerful instrumentation. The development of a new generation of optical telescopes with diameters in the range of 8–10 m has delivered the cascade of astronomical discoveries of the last few years. The manufacturing of these large optics has been possible thank to new engineering ideas and radical approaches. For example the possibility to assemble a large mirror con*
Corresponding author. E-mail address: [email protected] (O. Citterio).
necting a set of smaller mirrors has permitted the creation of the twin Keck Telescopes that have a diameter of 10 m each. Also in the field of the X-ray Astronomy the need of larger optics (and hence collecting area) has pushed in the years the state of the art of the mirror manufacturing techniques. In Fig. 1, an overview of past and future X-ray missions from the point of view of their Effective Area (at 1 keV) and Angular Resolution (in terms of Half-Energy-Width) is visible. After the launch of XMM-Newton and Chandra (the main X-ray telescopes now operating and with diameters, respectively of 700 and 1200 mm), the next step will be the launch in 2010 by NASA of Constellation-X (Bookbinder, 2001). Four satellites, all of them co-orbiting in
0273-1177/$30 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2003.03.064
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Fig. 1. Overview of past and future X-ray missions.
the L2 libration point, will be simultaneously pointed toward the same celestial source, mimicking in some way the same philosophy followed by the VLT optical telescope that employs four 8 m telescopes linked together. Every one of the four satellites will host a soft X-ray telescope (SXT) with a diameter of 1.7 m and three hard X-ray telescopes (HXT), each with a diameter of 35 cm. Considering the different technology and manufacturing procedure necessary for the construction of a grazingincidence X-ray telescope with respect to an optical normal-incidence telescope, a diameter in the range of 1.7 m for the outer shells is probably outside of what is reasonable to build monolithically. For this reason the SXT optics will be built connecting many segments in a circular fashion to recreate the working shells. After Constellation-X it is clear that the next future missions using grazing-incidence focusing optics will have sizes and collecting areas in a range that will surely require the use of segments to recreate a full X-ray telescope. In this contest two future missions nowadays on the drawings boards are the X-ray Evolving Universe Spectroscopy mission (XEUS mission) and the Generation-X mission. They represent a quantum leap with respect to the previous missions and Constellation-X. The XEUS ESA mission (ESA, 2000a,b,c) is foreseen around the year 2015 and will be based on a single telescope in configuration Wolter I (Bavdaz et al., 1999, 2000) (ESA, 2001) having a diameter of 9.9 m, with a resolution goal of 200 HEW (Half Energy Width). The focal length will be of 50 m and this fact has pushed toward an innovative approach that foresees the use of two distinct spacecrafts, one for the optics and the other for the detectors, that will operate in formation flight, linked to each other by an active tracking system. Initially the core optics of XEUS (XEUS I) will be laun-
ched by an Ariane V rocket that will deliver in LEO orbit the two spacecrafts. In this first part of the mission the diameter of the optics will be of 4.5 m with an effective area of 6 m2 at 1 keV. For comparison, the sum of the four effective areas of the SXT modules of Constellation-X is of 1.5 m2 at that energy. After completion of the initial 4–6 years mission phase, XEUS will rendezvous with the International Space Station (ISS) for refurbishment and to allow the addition of extra mirror modules, bringing the total effective area to 30 m2 at 1 keV and the diameter to 9.9 m. After the separation from ISS it will begin the second phase of observations, named XEUS II, and that will last about 15 years. The NASA mission Generation-X (Zhang et al., 2001) is foreseen around 2020 or later and in the present configuration follows the same philosophy used for the Constellation-X mission: it should be launched to the L2 libration point and will consist of six identical satellites, each of them carrying a fraction of the required total collecting area of 150 m2 at 1 keV, minimizing both launch and component failure risks. Each telescope during the launch phase would be stowed in about 4.5 m in diameter but after deployment should be able to unfold and deliver 25 m2 of effective area. To achieve a broad band coverage in energy the focal length will be of about 100–150 m and the resolution goal will be of about 0.100 HEW. Part of the considerations that will follow are the subject of another article (Ghigo et al., 2004) and are here reported shortly for sake of completeness. We will use as guideline for the definition of a X-ray mirror technology mainly the XEUS mission optical requirements and constrains. Of course some of the proposed ideas and considerations could be useful as well for other future X-ray missions.
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2. Main technical difficulties and limits of present manufacturing techniques The X-Ray mirror technology required for the realization of Large Light X-ray Optics (LLXO) must solve the problem of deliver mirrors that satisfy the imaging resolution required and in the meantime stay within the weight budgets allocated for the optics. For example for the XEUS mission the goal in resolution is 200 HEW with a mass budget of just 25,000 kg. As an example in Fig. 2 it is shown the setup foreseen for the XEUS optics. Since it is practically impossible to realize X-ray optics so large using closed Wolter I shells, the mirrors will be formed by rectangular segments. A set of these segments will be mounted and aligned in a petal that will be next assembled circularly to form the telescope optics. The weight constrains, due to the limits of the launchers, imply that the mass/geometrical-area ratio of the optics should be very small and in the order of 0.08 kg/cm2 for XEUS. The total number of segments necessary for the full telescope amount to about 17,500 pieces. Another problem that must be solved for the manufacturing of LLXO is that the total mirror surface is so huge that the optics probably cannot be thermally controlled. This means that the optics will be manufactured at ambient temperature (20 C) but will operate in a much lower (about )35 C) and fluctuating range of temperatures, depending from the chosen orbit. This means also that, to avoid deformations, the segments
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and the supporting structure must be based on materials with CTE as equal as possible. Last but not least, the manufacturing of these optics requires an industrial process ‘‘fast and automated’’, able to manufacture and test the thousands of segments needed for the project in a few years. The development of some kind of replication or molding technique could take advantage from the fact that the LLXO are assembled from large numbers of equal segments. These techniques have obvious advantages in terms of speed and cost of production. The current manufacturing techniques for space Xray optics are listed hereafter: • Nickel electroforming.It has been used for making the mirrors of a number of X-ray missions like SAX (Citterio et al., 1988), ABRIXAS (Altmann et al., 1998), XMM (Citterio et al., 1992), JET-X (Citterio et al., 1996), SWIFT (Burrows et al., 2000). Therefore, it can be considered a mature technology. Due to the high density of Nickel and considering that the typical mass/geometric-area allocated for a XEUS segment is just 2.64 kg/m2 , the typical thickness of a XEUS segment should be in the order of 0.3 mm. This very small thickness will degrade the stiffness of the segment to an extent that very unlikely could reach the high angular resolutions required. • epoxy replication.(De Korte et al., 1981; Citterio et al., 1997; Citterio et al., 2000) As for Ni electroforming replication, this technique makes use of a
Fig. 2. XEUS optical setup.
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superpolished gold-coated master (mandrel). In this case the master is placed almost in contact with a carrier, leaving only a small gap (100 lm) between the two surfaces. This gap is then filled with epoxy resin. When the resin is cured the optic is separated from the master. Having the gold a better adhesion to the epoxy than to the master, it will follow the carrier, forming in this way the reflective surface of the optic. The master can of course be used to replicate a large number of equal mirrors. This procedure is therefore, attractive for the XEUS mirror production but suffers for a major drawback: a segment produced in this way is essentially a sandwich composed by epoxy and the carrier material (for example SiC or Glass). It is easy to understand that the difference in CTE of the two materials, once that the optics will be put in space at a temperature of )35 C or less, will create internal forces able to deform the shape of the segments. The working temperature of optics fabricated with this method has to be the same of the manufacturing process and this is not possible in absence of thermal control. This fact tends to exclude epoxy replication for the XEUS segments production. In another section of this paper more detailed results obtained with finite element analysis on simulated epoxy replicated segments are reported. • Direct polishing mirror technology.With this technique the optics of the CHANDRA telescope have been realized, that showed the best angular resolution (0.500 HEW) achieved so far (Weisskopf et al., 2000). This performance has been obtained at the price of using just a few (4) very thick (a couple of cm) heavy and stiff shells made in glass ceramics. The optics are directly figured to the required tolerances with classical optical grinding and polishing techniques. The low number of shells obviously means that the achievable collecting area is relatively small. This technique can reach the resolutions requested by LLXO but not within the mass budget allocated for the optics. • Thin foil mirror technology.It has been used for example in ASCA(Serlemitsos et al., 1995), and it is based on aluminum segmented mirrors having a very thin thickness. Their optical resolution is generally in the order of 10000 HEW. The obvious advantage is instead the very low weight of these optics. From all these considerations we can summarize the requirements that the technology necessary for the production of LLXO telescopes should satisfy: • It should be able to produce stiff optical segments with a very small mass/geometrical-area ratio and high performances in terms of angular resolution. • It should be possible to correctly assemble the segments to form a full optical telescope. • Segments and related supporting structure should have very similar CTEs to ensure the minimum amount of mechanical–optical distortion.
• Some kind of a replication or molding process is desirable to ease the production of the thousands of segments required (17,500 for XEUS II). However the classical epoxy replication technique cannot be used due to the CTE mismatch between the epoxy layer and the substrate material. • The production process must guarantee a high volume production, able to fabricate and test in an industrial realization chain all the segments requested.
3. Proposed manufacturing process of segments The choice among candidate materials to be used for making the LLXO mirrors should be based on the best tradeoff in terms of stiffness against the costs and complexity of production. A finite element analysis that has been performed on typical segments (1 0.5 m, weight ¼ 1.32 kg) made of different materials like Electroformed-Nickel, Al2 O3 , Glass, Silicon Carbide (SiC) and two sandwiches of Al2 O3 and SiC, has shown that purely from the point of view of the mechanical performances a segment made with a sandwich in SiC (two SiC skins and a SiC foam in-between – Novi et al., 2001) has the maximum Flexural Rigidity. This parameter describes the rigidity of a structure and the corresponding values for the different materials here considered are reported in Fig. 3, normalized to the Nickel one. It can be seen that a segment made in foamed SiC is 874 times stiffer than an equivalent Nickel segment. Even if this is an impressive performance we must consider also other facts like the cost of the material and the difficulties involved in the development of a sandwich technology. A manufacturing technique representing a good compromise between complexity and costs has been individuated. It makes use of thin (about 1 mm) sheets
Fig. 3. The Flexural Rigidity parameter for different materials in the case of a typical XEUS panel.
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of BorofloatTM glass to produce the necessary segments. The Flexural Rigidity of a glass segment is 23 times that of an equivalent Nickel segment having the same weight. The glass is also a well known material, easily workable and with low cost if compared to the SiC. Fig. 4 shows in general terms the proposed manufacturing sequence. A BorofloatTM glass sheet is produced floating the material on a liquid tin bath and then it is cutted to the required dimensions. The sheet is grinded and polished to obtain an uniform thickness. A concave master made in a ceramic material is then used for the slumping of the segment to the desired shape. The glass in fact is heated to a temperature that permits to soften it and, consequently, to change its shape. If the thickness of the sheet is perfectly constant, the optical shape of the master could be transferred also on the upper face (concave) of the sheet. In the reality this is not practically possible and the deviations from an uniform thickness will introduce profile errors on the concave reflecting side of the sheet. To correct for these errors a figuring phase is foresee that will use a vacuum support system to maintain in position the sheet. This system will also be used to transfer the finished segment to the support structure, maintaining its shape during the assembling in the telescope. Once the alignment is done and the segment fixed to the support structure by means, e.g., of epoxy resin, the vacuum support will be released. The viability of this procedure is under evaluation with a joint effort between the MPE (Max-Planck- Institut fur Extraterrestrische Physik-Germany) and the Osservatorio Astronomico di Brera (OAB).
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The slumping process is investigated with the support of the Schott Company (Germany). The results of a preliminary series of tests has been very encouraging. These tests have been performed on smaller scale starting from flat sheets of BorofloatTM glass having a size of 200 200 1 mm. The idea was to demonstrate the feasibility of the first steps of the procedure and to obtain curved segments with a curvature radius of 2000 mm and a maximum departure error of 10 l P–P from the final surface. This value is the maximum error acceptable by the Carl Zeiss company (Germany) that should develop the robotic polishing technique able to bring the surface within the requested optical tolerances. In order to test the process a mold having the same dimensions of the sheets and made in a ceramic material has been manufactured with a cylindrical surface having a maximum departure from the theoretical one of 5 l P– P. After some tests used to calibrate the thermal cycles necessary for the slumping process it has been obtained a P–P difference of 6 l on the segments produced, with respect to the shape of the mold. This result is already within the specifications of Carl Zeiss but since this result has been obtained with a relatively small number of tests we are confident that there is space for a further improvement of the slumping process. Of course the smaller is the error left before the figuring step, the lesser is the time necessary for the figuring correction. The figuring should also bring the microroughness of the BorofloatTM sheets to a value better than 0.5 nm rms, necessary for the X-ray reflection in the range of energy up to 10 keV. In Fig. 5 the results obtained with the
Fig. 4. Segments production and integration.
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Fig. 5. Slumping results obtained with different cooling rates.
slumping process using two different cooling rates (very critical factors in the process) are shown. The bi-dimensional profiles shown in the figure are the residual errors resulting after the subtraction of the segment shape from the mould. A perfect match should shown a flat plane. The slower cooling rate (right surface) offers the best result, with a P–P of 6 l, probably since it gives to the glass more time to relax and to move staying in contact with the mould surface. The next foreseen steps will be given by the figuring of a segment and its subsequent integration in a supporting frame able of maintaining its final shape. This phase will be the second part of the feasibility study.
4. Finite element analysis of various parameters affecting the segments optical performances The ability to manufacture single segments of an LLXO telescope obviously does not correspond to the capability of making a whole working telescope. In fact there are many parameters that influence the final optical performances of this system. To correctly evaluate the manufacturing requirements of all the aspects of construction (optical and mechanical) it is of paramount importance to perform a finite element analysis of the critical components. A similar study is important to select the best technological approaches and to understand the tolerances requested in each step of the realization. In our study, we used the output results of a finite element code as input parameters of an optical ray tracing software. In this way, every aspect investigated from the mechanical point of view (deformations, stresses and so on) is reflected on the optical performances of the simulated optics giving directly the imaging degradation in terms of HEW. All the ray tracings have been carried out assuming a source at infinite distance.
A first aspect that was initially evaluated was connected to the use of the epoxy replication method for the segments prediction. The interest for this manufacturing system is obvious and hence it is important to quantitatively understand the effect that the temperature variations would have on the optical performances. For these simulations we assumed the best available material, SiC, in two different possible configurations: SiC solid sheets and foamed SiC sandwiches. The CTE assumed for SiC was 3 106 C1 , and for the epoxy resin was 80 106 C1 . To try to compensate for the inevitable deformations arising from the temperature changes, two layers of epoxy were supposed to be applied to the SiC substrate, the inner one (the normal one supporting the reflecting gold) and the outer one, on the opposite side, having about the same thickness of the first (to simulate a thickness error in the manufacturing). The case without the epoxy layer on the outer side was also evaluated. In Table 1 are shown the most relevant results obtained. The HEW degradation is computed for a variation in temperature of 1 C. The best result is obtained for the case B, a foamed SiC sandwich. With 50 l of epoxy from the side of the gold and 40 l from the other side (we assume a 10 l tolerance for the epoxy thickness) the degradation per degree amounts to 0.2600 HEW assuming a linear trend of HEW vs. temperature. If we consider that the optics would be manufactured at about 20 C and it will work at )40 C, the 60 C difference in temperature this gives us a degradation of 15.600 HEW. This value is much larger than the 200 HEW requested for XEUS. It is hence unlikely that the epoxy replication (in the present version) can be used for the manufacturing of the segments. The situation would be even much worst using glass. The 60 C difference from the manufacturing and the XEUS operating temperature can potentially introduce
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also another source of error: the mismatch between the CTEs of the mirror segment and the supporting structure. An analysis has been made concerning the effects of a mirror and petal supporting structure CTE mismatch. As already mentioned, a number of individual segments will be assembled in self-contained angular sectors named ‘‘petal’’ that once circularly connected will form a corona of mirror shells. Since petals directly support the segments, any CTE difference between them could introduce non isotropic deformations affecting the segments shape. In Table 2 are shown the results of the analysis performed. In this case the simulation assumed a typical XEUS segment made in BorofloatTM with a CTE of 3.3 106 C1 . It has been inserted into an infinitely stiff structure having a CTE of 4.3 106 C1 . Hence the CTE mismatch assumed between the two materials were of 1 106 C1 . The distortions of the optical surface has been divided in radial displacement, axial slope and azimuthal slope. The more relevant data are the deformations along the slopes that heavily influences the optical performances. In fact we have a degradation of about 8000 either for the parabola and the hyperbola sections and of 42700 in azimuthal direction, assuming again a change in temperature of 60 C. These errors are the result of a CTE mismatch of only 1 106 C1 , and the conclusions that must be drawn are that the material
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of the segments and the material of the petals should be essentially the same. This requirement seems very important because otherwise the errors introduced would be much larger than the goal resolution of 200 HEW. It is interesting to analyze the influence of the gravity on a segment. This information is necessary, e.g., to define the best position and orientation of the petals during the X-ray measurements. We have simulated two BorofloatTM segments (MS1 located at a radius form the telescope axis of about 1320 mm and MS2 located at 4200 mm) composed by two separated sections: a parabola plus a hyperbole. The segments, with a thickness of 1 mm, have been placed and constrained in an infinitely stiff petal. The testing of the optics has been evaluated in their three possible orientations with respect to the gravity vector. The segments can be assembled and tested with the optics laterally oriented (gravity oriented along the shorter side of the segment, Fig. 6.1), normally oriented (gravity oriented perpendicularly to the surface of the segment, Fig. 6.2), axially oriented (gravity oriented along the longer side of the segment, Fig. 6.3). In Table 3 are shown the results obtained. The data obtained indicate that the best results are in the case of the axial gravity configuration. We see that in this case the inner segment shows a degradation of 0.3500 HEW and the outer segment has a degradation of 1.1500 HEW.
Table 1 Thermal effects on epoxy replicated shells CASE
A B C D
SiC carrier
Sandwich Sandwich Solid Solid
Inner (lm)
EPOXY thickness Outer (lm)
EPOXY CTE ( 106 K1 )
HEW (arcsec) 1 C
90EW (arcsec) 1C
Focal length change (mm)
40 50 40 50
0 40 0 40
80 80 80 80
0.60 0.26 2.25 0.81
1.99 0.58 10.87 3.09
)5.29 )1.53 )30.95 )8.89
Table 2 Effect of mirror and petal CTE mismatch for a AT of 60 C -6
-1
Ms data: Material = BOROFLOAT (CTE3.3 × 10 C ) . Radius at par-hyp interface = 1320mm . Focal length = 50,000mm . MS thickness = 1mm . Angular aperture 22.5˚C
Petal dimensions: 0.8m in radial direction; 0.5 + 0.5m invertical direction; 0.5m in tangential direction -6 -1 Petal CTE: 4.3 × 10 C
A temperature variation equal to – 60 ˚C has been assumed. The temperature value is uniform along the whole structure, no temperature gradients have been considered. Data relevevant to the optical surface distortions MS SURFACE
Parabola Hyperbola
REDIAL DISPLACEMENTS Peak to RMS Valley [µm] [µm] 494 150 493 149
AXIAL SLOPE Peak to RMS valley [arcsec] [arcsec] 340 80.6 341 81
AZIMUTHAL SLOPE RMS Peak to Valley [arcsec] [arcsec] 2268 427 2273 427
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Fig. 6. Flexure by gravity of a segment in an infinitely stiff petal.
Table 3 HEW degradation due to the gravity CASE
MS1 MS1 MS1 MS2 MS2 MS2
– – – – – –
Nominal focal plane
lateral gravity normal gravity axial gravity lateral gravity normal gravity axial gravity
HEW (arcsec)
90% HEW (arcsec)
3.7 0.74 0.35 25.0 1.9 1.15
9.0 1.61 0.85 47.7 4.1 2.84
of some critical aspects of their manufacturing, testing and assembling, a set of finite element analysis has been performed. The conclusions deriving from their results should be used to optimize the performances obtainable in the final assembled optics. Acknowledgements This research has been partially funded by the Italian Space Agency (ASI).
5. Conclusions We have presented a possible manufacturing process for the production of the segments for the optics of the future large X-ray space missions and in particular aimed to the XEUS mission. This approach is based on segments made in BorofloatTM slumped glass. This material offers the best trade-off between optical-mechanical performances and costs of production. To assess the influence on the segments optical performances
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