There recently has been significantly increasing interestin x-ray mirror telescopes with grazing-incidence optics.Such telescopes installed on spacecraft make it possible tostudy astrophysical objects in the x-ray region.1,2 Recent ad-vances in the technology of fabricating metallic x-ray mir-rors have allowed a number of projects involving variousx-ray telescopes with grazing-incidence mirror optics to becarried out.1–4 Especially significant results have been ob-tained in the XMM and AXAF projects. One of the mostpromising x-ray observatory projects is the Spectrum-Röntgen-Gamma �SRG� automatic spacecraft developed inRussia, with a complex of telescopes created in a wide-basedinternational collaboration for studies in high-energy astro-physics. This observatory includes several telescopes withgrazing-incidence mirror optics.5 The most significant ofthem is the SODART x-ray telescope.
The goal of this paper is to describe the layout of thistelescope.
When x rays are incident on the surface of metallic mir-rors at small angles that do not exceed the total-internal-reflection angles at the vacuum–mirror boundary, these raysbehave like ordinary light rays; this circumstance is used forfocusing them in grazing-incidence telescopes.1 In this case,two-mirror paraboloid + hyperboloid systems are used toconstruct an image. To achieve high efficiency of the mirrorsurface area, a system of coaxially placed mirrors is used,consisting of a front unit �a paraboloid� and a back unit �ahyperboloid�. The construction of such a mirror system2
makes it possible to achieve a focal length of 8–10 m, andthe well-known multimirror AXAF and XMM telescopes1,2
have such a length. It is extremely difficult to fabricate para-bolic and hyperbolic thin-film mirrors and then to adjustthem in optical systems having several tens of mirrors.Therefore, instead of paraboloid + hyperboloid mirrors, it ispromising to use thin-film conical mirrors made from alumi-num foil with a gold coating, as proposed in Ref. 5. A usableangular resolution can be achieved in this case, while thehigh packing density makes it possible to obtain a large ef-fective area of the mirror system. Such a mirror system,
42 J. Opt. Technol. 73 �1�, January 2006 1070-9762/2006/0
called XSPECT and created by the Danish Space ResearchInstitute, is used in the SODART telescope.6
A wide international collaboration participated in creat-ing this telescope: Russia �Space Research Institute of theRussian Academy of Sciences, FGUP S. A. Lavochkina Sci-entific Manufacturing Organization�, the USA, Israel, Den-mark, Finland, and the FRG.
The design layout of the telescope is determined by threemain factors: the optical structure, the operating conditions,and the limitations imposed by the possibilities of the satel-lite.
OPTICAL STRUCTURE OF THE SODART TELESCOPE
The SODART x-ray telescope has multimirror grazing-incidence optics with an area of the mirror system of 65 m2,a spectral range of 0.1–20 keV, and an angular resolution of2�.
The optical structure of the telescope �Fig. 1� includesthe following: two XSPECT multimirror systems �1�; theBXSPECT objective Bragg spectrometer �2�, two indepen-dent systems of focal devices, consisting of eight devices,including four LEPC/HEPC position-sensitive gas counters�3�, the SXRP X-ray polarimeter �4�, two KFRD position-sensitive gas proportional counters �5�, and the SIXAsilicon–lithium spectrometer �6�. The telescope includes theTAUVEX optical monitor �7�, which makes it possible toidentify the field of observation of the SODART telescopeand to combine the measurement data of the optical andx-ray ranges.
Such an optical structure for the telescope makes it pos-sible to solve a number of problems, including the following:construction of the image of weak x-ray sources in a widespectral region �LEPC/HEPC, KFRD�, the spectroscopy ofweak x-ray sources �SIXA�, high-resolution spectroscopy ofbright x-ray sources �BXSPECT�, polarization measurementsof x-ray radiation �SXPR�, and comparative measurementsof radiation sources in both the x-ray and UV regions �TAU-VEX�.
The presence of two independent mirror systems withdifferent focal devices makes it possible to carry out mutu-ally complementary measurements of radiation sources with
various types of devices and to increase the statistical reli-ability of the given measurements by simultaneously observ-ing a source using two mirror systems of the telescope.
FUNCTIONAL COMPOSITION OF THE TELESCOPE
The SODART grazing-angle x-ray telescope is a com-plex controllable optomechanical design that includes thefollowing: two mirror systems; the panel of the Bragg spec-trometer, with a high-accuracy rotation system; a complex offocal apparatus; an optical monitor; a transformable high-accuracy carbon-fiber-reinforced plastic telescope structure�because of limitations imposed by the size of the mainshroud�, which extends the mirror systems to a given focallength when the telescope is brought from the transport po-sition into the working position; a mechanism for exchang-ing the focal devices and moves the focal devices to the focalpoint; a system of ensuring the thermal regime �SETR� of themirror systems and the Bragg spectrometer and carriages,intended to maintain the calculated temperature and allow-
FIG. 1. Optical structure of the SODART telescope �see text forexplanation�.
FIG. 2. Schematic diagram of the SODART telescope. 1—AAS, 2—mirror spanel, 5—suspension subassemblies of the BS panel, 6—TAUVEX optical mB, 13—platform with scientific apparatus, 14—carbon-fiber-reinforced plast
43 J. Opt. Technol. 73 �1�, January 2006
able temperature gradients in the mirror units and the crystalsof the Bragg spectrometer and the focal devices; an SETR ofthe solid-state �Si�Li�� cooled spectrometer, which cools itduring operation to a temperature of 100 K; an active adjust-ment system �AAS� for monitoring the optogeometrical char-acteristics of the telescope �a laser complex, mounted on themirror system�, used both in the assembly of the telescopeand for determining the position of the optical axis and thegeometrical characteristics of the telescope during operation;a service systems control unit �SSCU� for the telescope, con-sisting of subsystems that put the telescope into operationand operate the telescope �exchanging the focal devices,maintaining the necessary thermal regime, controlling thetelescope covers, etc.�; and a cable network.
THE LAYOUT OF THE SODART TELESCOPE DESIGN
The SODART x-ray telescope is mounted on the plat-form with the scientific apparatus of the SRG spacecraft.Designwise, the telescope is divided into two parts: the upperpart, mounted on the platform like the other telescopes of theSRG craft, and a system for exchanging the focal devices ofthe SODART telescope �Fig. 2�, mounted under the platform.Because of the size limitations imposed by the main shroudof the SRG spacecraft, the telescope is designed to be trans-formable, having two positions—the transport position andthe working position.
The upper part of the SODART telescope consists of theoptical unit and the telescope support, articulated with eachother, as well as deployment drives with telescope-securingmechanisms. In the transport position, the optical unit of thetelescope is located inside the support. In the working posi-tion, the optical unit is turned 180° relative to its transportposition.
The optical unit of the telescope consists of a centralplate with two mirror systems �XSPECT� mounted on it, twoupper carbon-fiber-reinforced plastic tubes—the housing ofthe Bragg spectrometer �BXSPECT� and the housing of themirror system—connected by the upper panel, and two iden-tical conical carbon-fiber-reinforced plastic tubes. On oneside is the aperture of the BXSPECT, covered with a reus-able cover. The BXSPECT panel is mounted on the side
, 3—unit for controlling the drives of the Bragg spectrometer �BS�, 4—BSle, 7—SETR, 8—SSCU, 9—body, 10—screen, 11 and 12—carriages A andescope support.
ystemodu
ic tel
43V. K. Sysoev
walls of the housing on two special suspension subassem-blies, containing a rotation system and rotational-angle sen-sors. Two covers are mounted on the upper panel: a reusablecover above BXSPECT and an expendable cover aboveXSPECT.
Inside each housing above the XSPECT systems aremounted three rows of cross-shaped thermal collimators, thelowest of which is active and incorporates thin-film electricheaters. The panel with the TAUVEX optical monitor and theelectronic units of the various systems is mounted outside theoptical unit.
The lower conical carbon-fiber-reinforced plastic shellsof the optical unit are the structural elements that form theoptical bench of the telescope. Inside each shell under themirror systems are also mounted three rows of cross-shapedthermal collimators, the upper of which has thin-film electricheaters. In their lower part, the shells are connected by apanel with two expendable protective covers and telescope-securing mechanisms.
At all stages of operation with the optical unit, from theinstant of its final assembly and testing in a clean room untilthe beginning of the scientific observations, it is in an ampu-lized state; i.e., all the covers are vacuum-tight, and the in-ternal cavity communicates with the ambient mediumthrough a valve-filter and a pressure-relief valve. The inter-nal cavity of the optical unit is opened only once undernonclean-room conditions, using an air curtain around theadjustment ports in the lower protective covers when carry-ing out the final assembly of the telescope, using the AASmounted on the mirror systems.
The telescope support forms a large part of the opticalbench of the telescope and is made from carbon-fiber-reinforced plastic tubes 120 mm in diameter, with low ther-mal coefficients of linear expansion. In the upper part of thesupport is placed a horseshoe-shaped frame, kept closed forgreater rigidity of the tube, whose axis coincides with theaxis of rotation of the telescope when it is deployed. A se-curing mechanism is placed on the frame that actuates afterthe telescope is fully opened. The telescope support is cov-ered on the outside with a multilayer �metal–polymer� pro-tective screen to attenuate the x-ray background and preventthe focal devices from being illuminated by reflected solarradiation. To position the protective screen on the side to-ward which the telescope is deployed, a light frame structureis used that opens only in the period in which the telescope isbrought into working position. Drives that duplicate eachother, located on the optical unit and the telescope support,are used to deploy the telescope.
THE FOCAL PLANE OF THE SODART X-RAY TELESCOPE
When planning the focal-device exchange mechanism�FDEM�, requirements were imposed on it to ensure the pos-sibility of independently exchanging the focal devices undereach mirror system.
To satisfy these requirements, an FDEM was proposedthat consists of two carriages that move along guides that runparallel to the Z axis of the SRG automatic spacecraft. Thelayout chosen for moving the carriages was determined by
44 J. Opt. Technol. 73 �1�, January 2006
the number of focal devices mounted on the carriages, aswell as by the necessity of placing a passive radiation cryo-genic cooling system of the SIXA Si�Li� spectrometer on oneof the carriages.
The kinematic layout of the FDEM is two independentsystems, each of which consists of a carriage with focal de-vices, moving along a guide. The focal devices are ex-changed and adjusted along the Z axis by moving a carriagealong a guide by means of a longitudinal-displacement driveunit and by a cable system.
Some of the focal devices on the carriages have break-away membranes intended to protect the entrance windowsof the detectors from various external effects. Radioactivesources for calibrating the detectors are located under theplatform. An SETR is also mounted on the carriages, con-sisting of heating tubes running under the focal devices andradiator–coolers on each of the carriages, with a gas-regulated system of heating tubes that automatically keep thetemperature of the focal devices in the range from 5 to 35 °C.
The carriage velocity in the focal-device exchange re-gime is 2 mm/sec. The displacement zone of the carriagesalong the z axis is 900–1040 mm. The positioning error is 0.5mm.
ACCURACY ANALYSIS OF THE DESIGN OF THE SODARTTELESCOPE
The SODART telescope contains a large number of ele-ments, requiring high accuracies during assembly and stabil-ity of these parameters when it operates in orbit.
The accuracy requirements involved in assembling thetelescope are shown in Table I in terms of the interconnec-tion of its structural elements, the optical monitor, and thefocal devices �i.e., the main elements of the telescope�.
After the telescope is put into orbit, the optical bench ofthe telescope may be strained within the limits of error of theweightlessness system used when assembling the telescope.To monitor the possible strains, as well as the temperaturestrains when the orientation of the telescope relative to thesun varies, a laser monitoring system �AAS� is provided forthe telescope, making it possible to set the focal devices atthe focal point to within the given accuracy. All these per-turbing factors are taken into account when choosing therange of motion of the focal devices.
The maximum temperature excursions on the sunlit andshaded sides of the structural elements that form the opticalbench of the telescope were estimated when determining thepossible temperature strains.
As an example, Table II shows how far off the focalpoint is as a function of the orientation of the telescope rela-tive to the sun for mirror system B, under which the polar-imeter �SXRP� that has the highest accuracy requirements isinstalled.
TECHNIQUE FOR ASSEMBLING THE SODART TELESCOPE
The SODART telescope is assembled in several stages inclass-2 and -5 clean rooms. At the first stage, the telescopesupport and FDEM are assembled in a gantry in a class-5clean room. At the same time, assembly of the optical unit is
44V. K. Sysoev
carried out in a class-2 clean room, including the followingtechnological operations: installation of the mirror systems�XSPECT� on a common plate; attachment to the XSPECTplate of a section of the Bragg spectrometer, the upper tube,the upper and lower panels, and the lower tubes; installationof the heat collimators of the mirror systems; mutual align-ment of the mirror systems and the Bragg spectrometer; in-stallation of the optical UV monitor and its adjustment rela-tive to the base mirror system �B�; installation of the mirrorsystems on the upper panel with a cover; installation of thelower covers of the optical unit; and checking that the opticalunit is vacuum-tight.
After the optical unit is assembled in a class-2 cleanroom, the LEPC/HEPC detector units are installed on car-riages A and B, and the x-ray polarimeter is installed oncarriage B.
All the subsequent operations involving the assembly ofthe telescope are carried out in class-5 clean rooms. Theassembled and fully ampulized optical unit is placed on a teststand, where it is attached to the telescope support, and thetelescope-deploying drives and securing mechanisms are in-stalled. The weightlessness system of the telescope, neededfor deploying it and for checking the actuation of the mecha-nisms for securing it, is mounted on the test stand. The ex-
TABLE I. Referencing accuracy of the main optical elements of
ElementA
Mirror system A–mirror system BMirror system B–panel of Bragg spectrometerMirror system B–TAUVEX monitorMirror systems–optical elements of the AAS of the
focal devices �other than the polarimeter�Mirror system B–optical elements of the AAS
of the polarimeterFocal detectors–optical elements of the AAS
on the carriages
TABLE II. Offsets of the focal point of mirror system B as a function ofthe orientation of the telescope with respect to the sun, caused by tempera-ture effects.
45 J. Opt. Technol. 73 �1�, January 2006
isting scatter in position of the optical axis of the telescopewhen it is repeatedly deployed is determined on the same teststand by means of the AAS.
The telescope carriages are installed and the telescope isadjusted in a class-5 clean room in a gantry equipped with aweightlessness system for the optical unit and the FDEM.The upper part of the telescope and the carriages are installedon the platform of the SRG automatic spacecraft.
Since the strictest accuracy requirements are imposed onthe polarimeter apparatus, carriage B is referenced to themirror system in the working position of the polarimeter, andcarriage A is referenced to the corresponding mirror systemin the working position of the farthest-out focal device—theSIXA Si�Li� spectrometer.
After being adjusted, the carriages are secured in thetransport position, and the optical unit is fitted inside themounting and is secured with explosive bolts in the transportposition.
The final operation is to install the protective screen onthe telescope support and to check the opening of the shutterof the protective screen. After all the operations are com-plete, the SODART telescope is attached to the spacecraft.
BRINGING THE TELESCOPE INTO OPERATION AND ITSOPERATING LOGIC
After the SRG automatic spacecraft separates from thebooster unit and the appropriate commands are transmitted,the on-board information control system �OBCS� is switchedon, along with the SSCU of the telescope and through it theSETR of the mirror systems and the Bragg spectrometer.
Before the constant solar orientation regime is set up, oncommand from the control unit, the upper part of the tele-scope and the door of the protective screen are unpinned bymeans of explosive charges. The telescope is deployed bytwo electric drives on command from the SSCU.
When the upper part of the telescope is completely de-ployed �rotated by 180°�, the explosive pin of the axial-securing mechanism is actuated on command. If the com-mand does not reach the securing mechanisms, after all thedata concerning the position of the upper part of the tele-scope are analyzed, this command is issued via the mainradio complex on command from earth. After the telescope is
ODART telescope.
r deviation,c min
Offset from theaxis, mm
X Y Z
0.4 0.05 0.1 0.11 ±1 ±1 ±12 ±1 ±1 ±1
45 ±0.25 ±0.25 ±0.25
1.5 ±1.0 ±1 ±1
1.5 0.05 0.1 0.1
the S
ngulaar
45V. K. Sysoev
secured in the working position, the door of the protectivescreen is shut.
The telescope begins to operate after the screen–vacuumthermal insulation is fully degassed and after the satelliteenters the constant-solar–stellar orientation regime. Once thisis done, the lower covers of the optical unit are opened oncommand from the SSCU. The thermal regime of the mirrorsystems and of the Bragg spectrometer are maintained byoperating the electric heater on the XSPECT plate and thebuilt-in electric heater of the BXSPECT.
GENERAL PRINCIPLES OF THE INTERACTION OF THESCIENCE AND SERVICE COMPLEXES OF THE TELESCOPE
The electronic devices of the SODART telescope can bebroken up into two major subsystems: the science complexand the service complex. The science complex includes allthe scientific devices of the telescope �KFRD, HEPC, LEPC,SIXA, SXRP, TAUVEX�, and the service complex includesall the devices that allow the telescope to operate properlyand that function largely independently of the systems of theautomatic spacecraft proper.
The science complex and the service complex interactindirectly: In normal operation, interaction is via missionprofiles defined on earth for both of the complexes; in non-standard situations, they interact directly on board via themain radio complex. In this case, the mission profiles aremaintained mainly in the control units mentioned above andpartly in the program–time system �PTS�, and the operationsof the science complex and service complex are synchro-nized in unified on-board time. In nonstandard situations,both complexes form emergency signals on the PTS, whichin this case ensure that the various subsystems of the auto-matic spacecraft and the telescope will react adequately onthe basis of the standard control commands and with activeuse of the SSCU and the OBCS.
The service complex of SODART, as indicated earlier, isintended to ensure the operation of the scientific devices ofthe telescope. The service complex includes the following:an SETR; an AAS for accurately adjusting the position of thecarriages that bring the focal devices to the focal point; theFDEM control system, which ensures the motion of the car-riages, as well as deploying the upper part of the telescope;the system for controlling the drive of the panel of the Braggspectrometer; and the unit for controlling the reusable tele-scope covers.
All the devices are divided into two groups �according toposition� and are connected to the SSCU by serial lines, withthe SSCU located next to its own large group of devices �inthe upper part of the telescope�. This, along with the use ofthe serial channel, makes it possible to economize a signifi-cant mass on information cables. The devices of the servicecomplex located below the telescope structure �including thaton the carriages� are also connected to the SSCU by a serialline. In this case, such a line makes it possible to reduce thepressure of the flexible cable on the carriages. The telemetryand program–time systems are connected using a serial chan-nel. Virtually all the computational elements of the servicecomplex can change programs while operating in orbit.
46 J. Opt. Technol. 73 �1�, January 2006
CONCLUSION
The characteristics of the SODART x-ray telescope thathas been created are found in a number of the parameters ofthe best x-ray telescope projects of space observatoriesXMM and AXAF. The successful implementation of the tele-scope project is associated with the implementation of theentire complex of the Spectrum-Röntgen-Gamma space ob-servatory. Fabrication of this telescope has been begun in theform of not only the separate functioning subassemblies andsystems, but also a full-size assembled structure in the formof a functioning prototype, a photograph of which is shownin Fig. 3.
The vibrothermal testing of the classification prototypeof the SODART x-ray telescope that has been carried outshowed successful operation of all the subassemblies and
FIG. 3. Classification prototype of the SODART telescope: �a� deployingthe telescope, �b� deployed telescope.
46V. K. Sysoev
mechanisms and maintenance of the accuracy parameters ofsuch a large structure. The successful assembly of the flyingoptical unit of this telescope makes it possible to expect suc-cessful implementation of the Spectrum-Röntgen-GammaProject.
1“Grazing incidence,” Appl. Opt. 27, 1397 �1988�.2V. Aschenbach, O. Citterie, Y. M. Ellwood, P. Zensen, P. de Korte, A.Paecock, and R. Willingole, “The high-throughput x-ray spectroscopymission,” in Report of the Telescope Working Group, ESA, SP 1084, 1987,
pp. 1–47.
47 J. Opt. Technol. 73 �1�, January 2006
3G. Burbidge and A. Hewitt, eds., Telescopes for the 1980s �Annual Re-views, Palo Alto, Cal., 1981; Moscow, 1984�, p. 312.
4V. M. Moskalenko, Methods of Extraatmospheric Astronomy �Nauka,Moscow, 1984�, p. 350.
5V. M. Kovtunenko, R. S. Kremnev, and V. K. Sysoev, “SpectrumX-gamma-A space observatory of high energy astrophysics,” in Abstractsof the Twenty-Seventh Plenary Meeting of the Committee on Space Re-search, Finland, 1988, p. 182.
6K. B. Byrna, F. E. Christensen, P. Jonansson, M. Madsen, H. V. Nogaard-Nielsen, H. W. Shnopper, N. J. Wesregaard, and P. Orup, “XSPECT anX-ray spectroscopy and timing mission concept,” Preprint, DSRI, 1987.
