Development of imaging x -ray telescopes at Max -Planck- Institut Garching

Joachim Trumper, Bernd Aschenbach and Heinrich BrauningerMax -Planck- Institut fur Physik und Astrophysik

Institut fur Extraterrestrische Physik8046 Garching, W. Germany

Abstract

A few years ago, a program for development of space instrumentation for soft X -rayastronomy has been initiated at MPI. It includes the design and fabrication of imagingX -ray telescopes which is done in close cooperation with Carl Zeiss /W.- Germany. After anextensive survey o-F various materials and polishing techniques applied to flat mirrorsamples, first paraboloidal mirrors were built each having a frontal diameter of 15 cm anda length of 1 m. A bundle of 12 such mirrors was flown twice on Aries rockets. The nextstep was the design and production of a Wolter type I telescope compatible with being usedon a Skylark or Black Brant rocket. Actually, three of these telescopes were built. Themirror material is aluminium, plated with kanigen and coated with a reflecting gold layer.Each 'Tas an aperture diameter of 32 cm and a focal length of 143 cm. They were tested inMPI's short beam X -ray test facility and show nominal reflectivity, on -axis angular reso-lution of 5 arc sec (FWHM), and extraordinary low surface scattering of 6 , at 1 keV.Currently, a nested 80 cm diameter Wolter type I telescope of 240 cm focal length is beingdesigned. The mirrors will be made from zerodur, a glass ceramic which has excellent ther-mal properties, low residual mechanical stresses as opposed to aluminium, and the abilityto be polished to high surface finish. X -ray tests of the telescope will be performed inMPI's long beam (130 m) facility just under construction.

Introduction

The prevailing results of the HEA0-2 mission demonstrate in an impressive way that thefield of X -ray astronomy is undergoing a revolution by the introduction of imaging tele-scopes. Compared with the conventional method of collimated counters, these instrumentsoffer three main advantages:

- the possibility to obtain high resolution images of the X -ray sky- an increase of sensitivity for the detection of point sources by orders of magnitude- the possibility of performing effective spectroscopy of moderate and high resolution.

It is clear that X -ray astronomy is now approaching optical and radio astronomy in termsof resolution and sensitivity, and also of instrument sophistication. It is evident thatthe sky is very rich for powerful X -ray telescopes and consequently, as in the otherbranches of astronomy, several of such instruments with complimentary characteristics andscientific objectives will be needed in order to fully exploit this new and exciting fieldof astronomy. These general considerations led us to begin the development of imagingX -ray telescopes about six years ago.

Our current program concentrates on paraboloidal -hyperboloidal mirror systems as firstconsidered by Hans Wolter in 1952(1). It is being carried out in close cooperation withCarl Zeiss /W.- Germany and can be divided into 4 steps, largely overlapping in time:

1) In 1973 we started an extensive program on flat mirror samples in order to selectsuitable mirror materials and to refine polishing techniques, as well as to developmethods of surface diagnostics by X -ray scattering tests. Some of the results of thisstill on -going program will be summarized below.

2) The next step was to build and test paraboloidal mirrors. Twelve large mirrors of 15 cmdiameter and 1 m mirror length have been built by Zeiss and flown twice on Ariesrockets (1977/78) in order to perform spectrophotometry of soft X -ray sources.

3) In 1976 we began the development of Wolter type I telescopes with 32 cm diameter forrocket experiments. This lead to the production of three complete mirror systems whichhave good angular resolution and extremely small scattering. The first ono has beenflown successfully in February 1979 as part of our national space program. At least,two other flights will follow this year in collaborative programs.

4) The last step was initiated in 1978. It aims at building a large fourfold nestedX -ray telescope with a diameter of 00 cm. This instrument will be ready for launchwith the Space Shuttle in 1983/04.

In this talk we will concentrate on the aspects of mirror development and properties,setting aside the MPI activities on imaging proportional counters, transmission gratings,and other related instruments which form a major part of our overall program.

12 / SPIE Vol. 184 Space Optics - Imaging X -Ray Optics Workshop (1979)

Development of imaging x-ray telescopes at Max-Planck-Institut Garching

Joachim Trumper, Bernd Aschenbach and Heinrich BrauningerMax-Planck-Institut fur Physik und Astrophysik

Institut fur Extraterrestrische Physik8046 Garching, W. Germany

Abstract

A few years ago, a program for development of space instrumentation for soft X-ray astronomy has been initiated at NPI. It includes the design and fabrication of imaging X-ray telescopes which is done in close cooperation with Carl Zeiss/W.-Germany. After an extensive survey of various materials and polishing techniques applied to flat mirror samples, first paraboloidal mirrors were built each having a frontal diameter of 15 cm and a length of 1 m. A bundle of 12 such mirrors was flown twice on Aries rockets. The next step was the design and production of a Wolter type I telescope compatible with being used on a Skylark or Black Brant rocket. Actually, three of these'telescopes were built. The mirror material is aluminium, plated with kanigen and coated with a reflecting gold layer. Each has an aperture diameter of 32 cm and a focal length of 143 cm. They were tested in HPI's short beam X-ray test facility and show nominal reflectivity, on-axis angular reso­ lution of 5 arc sec (FWHN], and extraordinary low surface scattering of 6 % at 1 keV . Currently, a nested 80 cm diameter Wolter type I telescope of 240 cm focal length is being designed. The mirrors will be made from zerodur, a glass ceramic which has excellent ther­ mal properties, low residual mechanical stresses as opposed to aluminium, and the ability to be polished to high surface finish. X-ray tests of the telescope will be performed in NPI's long beam (130 m) facility just under construction.

Introduction

The prevailing results of the HEAO-2 mission demonstrate in an impressive way that the field of X-ray astronomy is undergoing a revolution by the introduction of Imaging tele­ scopes. Compared with the conventional method of collimated counters, these instruments offer three main advantages:

- the possibility to obtain high resolution Images of the X-ray sky- an increase of sensitivity for the detection of point sources by orders of magnitude- the possibility of performing effective spectroscopy of moderate and high resolution.

It is clear that X-ray astronomy is now approaching optical and radio astronomy in terms of resolution and sensitivity, and also of Instrument sophistication. It is evident that the sky is very rich for powerful X-ray telescopes and consequently, as in the other branches of astronomy, several of such instruments with complimentary characteristics and scientific objectives will be needed in order to fully exploit this new and exciting field of astronomy. These general considerations led us to begin the development of imaging X-ray telescopes about six years ago.

Our current program concentrates on paraboloidal-hyperboleida1 mirror systems as first considered by Hans Wolter In 1952^^. It is being carried out in close cooperation with Carl Zeiss/W.-Germany and can be divided into 4 steps, largely overlapping In time:

1) In 1973 we started an extensive program on flat mirror samples in order to select suitable mirror materials and to refine polishing techniques, as well as to develop methods of surface diagnostics by X-ray scattering tests. Some of the results of this still on-going program will be summarized below.

2) The next step was to build and test paraboloidal mirrors. Twelve large mirrors of 15 cm diameter and 1 m mirror length have been built by Zeiss and flown twice on Aries rockets (1977/76} in order to perform spectrophotometry of soft X-ray sources.

3) In 1976 we began the development of Wolter type I telescopes with 32 cm diameter for rocket experiments. This lead to the production of three complete mirror systems which have good angular resolution and extremely small scattering. The first one has been flown successfully in February 1979 as part of our national space program. At least, two other flights will follow this year in collaborative programs.

4) The last step was Initiated in 1978. It aims at building a large fourfold nested X-ray telescope with a diameter of 80 cm. This instrument will be ready for launch with the Space Shuttle in 1983/84.

In this talk we will concentrate on the aspects of mirror development and properties, setting aside the NPI activities on imaging proportional counters, transmission gratings, and other related instruments which form a major part of our overall program.

12 I SPIE Vol. 184 Space Optics-Imaging X-Ray Optics Workshop (1979)

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DEVELOPMENT OF IMAGING X -RAY TELESCOPES AT MAX -PLANCK -INSTITUT GARCHING

Much of the work which we will describe in the following has been carried out by meansof the X -ray test facilities which exist at Mfg Garching. Actually, our whole program ingeneral and the mirror development in particular would not have been possibl withoutthese facilities which will be discussed in some detail by Aschenbach et al.2) at thismeeting.

X -Pay Scattering and Surface Microroughness

X -ray scattering is caused by the surface microroughness, and depends on the grazingangle and the X -ray wavelength. It leads to wings in the point spread function of tele-scopes which limit the resolution and contrast, in particular at short wavelengths. Inorder to select suitable mirror materials and polishing methods, many samples have been

7 . r -. -r-T- T T

KAN1Ci=N

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r

., o4 EARLIER SAMPLE

.5 M1 ... BEST SAMPLE

10

r

L_-.-___03 -

4 0 4 8 12 16

OFF SPECULAR OiRECTICNfurcrninl

Fig. 1. Scattering d=istributionsfor A = 8.3 A, a grazing angle of1.5° for aluminium /kanigen mirror.

1.0. 0 1-8.3 A

- 1=13.3A

+ 1=17.6A

0,8- 4 X.44 .0A

CG 0.2HQ

0 .0

-1

9.0 0.5 1.0 1.5

GRAZING ANGLE (DEG)20

Fig. 2. Dependence of the fractional scatteredflux on the grazing angle and wavelength.

(3)Theoretical curves are from Beckmann's theory (3).

DIRCCT HERR 30 PRO IN 60 WHIN SO Mat IN

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-5

Fig. 3. Scattering distributions at X = 8.3 x and different grazing angles for a recentzerodur sample (ßg " 2 A) ,

SPIE Vol. 184 Space Optics - Imaging X -Ray Optics Workshop (1979) / 13

DEVELOPMENT OF IMAGING X-RAY TELESCOPES AT MAX-PLANCK-INSTITUT GARCHING

Huch of the work which we will describe in the following has been carried out by means of the X-ray test facilities which exist at NPI Garching. Actually, our whole program in general and the mirror development in particular would not have been possible without these facilities which will be discussed in some detail by Aschenbach et al.^) a t this meet i ng.

X-Ray Scattering and Surface Hicroroughness

X-ray scattering is caused by the surface microroughness, and depends on the grazing angle and the X-ray wavelength. It leads to wings in the point spread function of tele­ scopes which limit the resolution and contrast, in particular at short wavelengths. In order to select suitable mirror materials and polishing methods, many samples have been

OFF SPECULAR DIRECTION (arc mm)

0.5 1.0

GRAZING ANGLE (DEG)

Fig. 1. Scattering distributions for A. = 8.3 A, a grazing angle of 1.5° for a lunnin ium/kanigen mirror.

Fig. 2. Dependence of the fractional scattered flux on the grazing angle and wavelength. ,~. Theoretical curves are from Beckmann's theory

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Fig. 3. Scattering distributions at X = 8.3 A and different p-razinp angles for a recent zerodur sample (a B ~ 2 A).

SPIE Vol. 184 Space Optics—Imaging X-Ray Optics Workshop (1979) I 13

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TRUMPER, ASCHENBACH, BRAUNINGER

polished by Zeiss and subjected to X -ray scattering tests by MPI. The materials investi-gated include kanigen on aluminium, quartz and zerodur. The latter is a zero temperatureexpansion glass ceramic, similar, but not identical to cervit. It has been used extensi-vely by Zeiss for building large optical telescopes. Most of the scattering measurementshave been done by Lenzen() and more recently by 0ndrusch(4). Figure 1 illustrates theimprovements on polishing techniques achieved in the early phases of our program (1973 -

1976)(5) resulting in a reduction of X -ray scattering by several orders of magnitude.Further progress has been made since then as evidenced by figure 3 which shows scatteringdistributions from a recent zerodur sample.

In order to characterize the scattering properties of a surface by a single parameter,we usually quote the microroughness derived from scattering distributions using Beck -mann's theory(5). This theory predicts that at grazing incidence reflection the fractionalscattered flux (defined as the ratio of the total scattered flux Iscatt to the total re-flected flux Io) is given by

2

(1) Iscatt/Io 1-exp r4rrßina1

where o is the (rms) microroughness, a is the grazing angle and X is the X -ray wavelength.This relation holds for 'gaussian surfaces', i.e. surfaces where the deviations of'heights' from a plane surface are distributed according to a gaussian function.

The angular distribution of the scattered flux is given by the Fourier transform of theautocorrelation function of the surface microroughness. Lenzen(3) showed that for oursamples exponential autocorrelation functions C (T) = exp ( -T /T) give reasonable fitswhile gaussian and Lorentzian functions can be excluded. The typical correlation lengthsfound are of the order of T ' 1 to 10 1m.

The scattering distributions were generally measured between 1 arc min and 1 degreeoff the direction of specular reflection. An extrapolation to smaller and larger anglesyielded the fractional scattered flux and from this quantity the microroughness was de-termined usingequation (1). We call this parameter 6B (derived via Beckmann's theory).

X -ray scattering tests performed on the same sample at different wavelengths and gra-zing angles yield op's which generally agree within the error limits. This means thatBeckmann's theory describes the scattering behaviour in a consistent way (c.f. figure 2).It also makes it possible to quote a single parameter 0B.

The lowest microroughness levels achieved in our program are of the order GB - 2 A.Such values have been obtained for all three materials studied in detail. It should bementioned though that in case of kanigen the necessary polishing effort was much largerthan for quartz and zerodur. As an illustration of the extremely low scattering levelsachieved, figure 3 depicts the results for a recent zerodur sample which has a GB - 2 A.

Table 1. oB derived from X -ray scattering measurements for different wavelengths andgrazing angles(3).

sample 38 A:

112.0 + 17.3A

sample 39 B: sample 61:electronkanigen on aluminium

perthometer: o =kanigen on aluminiumFECO: O = 15 A

quartz gold coated;reflection interference

oB = 107.2 ± 8.4 A oB = 14.1 ± 6.5 A microscopy: G = 3.2 ± 1.2 AßB = 2.53 ± 0.5 Á

\ ) )

a 8.3 13.3 17.6 44.8 a N5.4 8.3 9.9 13.3 a \\ \\

A.3

0.5 108 0.5 17 33 1 3.20.6 96 111 115 0.6 16 20 1.5 2.70.8 91 102 1.1 11 10 13 15 2.0 2.51.1 98 108 110 1.5 8 10 14 2.5 2.01.3 119 2.0 8 10 12 3.0 2.41.5 113 1052.0 116

It is interesting to compare the microroughness measured this way with those determinedby other methods(3). For qualitative comparisons we have used the Nomarski interferencemicroscopy. Quantitative measurements have been made by means of the standard perthometerand FECO techniques which work at high and medium microroughness levels. Table 1 showsthat a satisfactory agreement has been found for samples with o8 - 107 A and 14 A, respec-

14 / SPIE Vol. 184 Space Optics - Imaging X -Ray Optics Workshop (19791

TRUMPER, ASCHENBACH, BRAUNINGER

polished by Zeiss and subjected to X-ray scattering tests by NPI. The materials investi­ gated include kanigen on aluminium, quartz and zerodur. The latter is a zero temperature expansion glass ceramic, similar, but not identical to cervit. It has been used extensi­ vely by Zeiss for building large optical telescopes. Most of the scattering measurements have been done by Lenzen(3) and more recently by Ondrusch (^) . Figure 1 illustrates the improvements on polishing techniques achieved in the early phases of our program (1973 - 1976) * -' ' resulting in a reduction of X-ray scattering by several orders of magnitude. Further progress has been made since then as evidenced by figure 3 which shows scattering distributions from a recent zerodur sample.

In order to characterize the scattering properties of a surface by a single parameter, we usually quote the microroughness derived from scattering distributions using Beck- mann's theory^). This theory predicts that at grazing incidence reflection the fractional scattered flux (defined as the ratio of the total scattered flux I sca tt to the total re­fleeted flux I Q ) is given by

[ , , /I = 1 -exp -i scatt o Vi no.

X

where o is the (rms) microroughness, a is the grazing angle and A. Is the X-ray wavelength. This relation holds for 'gaussian surfaces', i.e. surfaces where the deviations of 'heights' from a plane surface are distributed according to a gaussian function.

The angular distribution of the scattered flux is given by the Fourier transform of the autocorrelation function of the surface microroughness. Lenzen^) showed that for our samples exponential autocorrelation functions C (T) = exp (-T/T) give reasonable fits while gaussian and Lorentzian functions can be excluded. The typical correlation lengths found are of the order of T ~ 1 to 10 Urn.

The scattering distributions were generally measured between 1 arc min and 1 degree off the direction of specular reflection. An extrapolation to smaller and larger angles yielded the fractional scattered flux and from this quantity the microroughness was de­ termined using equat ion (1). We call this parameter Og (derived via B_eckmann's theory).

X-ray scattering tests performed on the same sample at different wavelengths and gra­ zing angles yield erg's which generally agree within the error limits. This means that Beckmann's theory describes the scattering behaviour in a consistent way (c.f. figure 2). It also makes it possible to quote a single parameter o .

LD

The lowest microroughness levels achieved in our program are of the order On ~ 2 A. Such values have been obtained for all three materials studied in detail. It should be mentioned though that in case of kanigen the necessary polishing effort was much larger than for quartz and zerodur. As an illustration of the extremely low scattering levelsachieved, figure 3 depicts the results for a recent zerodur sample which has a o B 2 A.

Table 1. o derived from X-ray scattering measurements for different wavelengths andgrazing angles^).

sample 38 A :kanigen on aluminium perthometer: a = 112,0 + 17. 3Ao- = 107.2 + 8.4 AD

\. A.

aNv

0.50.60.81 . 11 .31 .52.0

8.3 13.3 17.6 44. 8

10896 111 11691 102

98 108 1101 19

113 105116

sample 39 B:kanigen on aluminium FECO: a -= 15 A°B =

\ X a \

0.50.61 . 11 .52.0

14.1 ±6.5 A

5.4 8.3 9.9 13.3

17 3316 20

11 10 13 168 10 148 10 12

samp le 61:quartz gold coat ed; e lect ron reflection interferencemicroscopy: o = 3.2 ± 1.2 A 0 B - 2.53 i 0.5 AV X aX\

1 .01 .52.02.53.0

8 .3

3.22.72 . 52.02.4

It is interesting to compare the microroughness measured this way with those determined by other methods^) m p or qualitative comparisons we have used the Nomarski interference microscopy. Quantitative measurements have been made by means of the standard perthometer and FECO techniques which work at high and medium microroughness levels. Table 1 shows that a satisfactory agreement has been found for samples with og ~ 107 A and 14 A, respec-

14 I SPIE Vol. 184 Space Optics-Imaging X-Ray Optics Workshop (1979)

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DEVELOPMENT OF IMAGING X -RAY TELESCOPES AT MAX -PLANCK- INSTITUT GARCHING

tively. The FECO method becomes insensitive at very low roughness levels (G s 10 A). Inthis regime a comparison has been made with results from an electron reflection inter-ference microscopy. This novel technique has been developed by Möllenstedt's group atythe University of Tübingen (Lichte 1977(7)) and has an ultimate sensitivity of ' 0.1 A.

As shown in table 1, the agreement is quite good for a sample with 6B .r 2.5 A.

The 32 cm Telescope

This telescope has been designed to meet the boundary conditions of Skylark and BlackBrant rockets5). It has a front diameter of 32 cm and a focal length of 142.7 cm. Themirrors have been optimized with respect to the effective area at 1 keV, leading to alength of the paraboloidal and hyperboloidal sections of 43 and 38 cm, respectively. Eachsection consists of two pieces for reasons of handling and accessibility during the po-lishing process. Three mirrors of this type have been built by Zeiss; figure 4 shows thephotograph of one of them.

Fig. 4. Photograph of the 32 cm tele-scope.

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Fig. 5. Reduction of X -ray scattering measu-red for one paraboloidal section of a 32 cmtelescope after successive polishing steps.

The mirrors are made of aluminium with a wall thickness of...1.4 cm. The aluminium wasfirst turned on a computer controlled lathe. During this process, the mirror was tempe-red several times in order to reduce the build -up of mechanical stresses. The mechanicalsurface was then plated with a 70 Um kanigen layer and subjected to grinding and poli-shing interrupted by roundness and slope measurements. The final polishing approach wasmonitored by X -ray scattering tests. Up to five iterative steps of polishing and measu-ring have been employed in order to achieve the final surface quality of the 32 cm tele-scopes. Figure 5 shows the corresponding improvement of GB for one of the paraboloidalsections. After polishing the shape of the mirror was measured again and then the indi-vidual elements were cut to their final lengths. During this procedure the polished sur-face was protected by a lacquer which was afterwards dissolved. X -ray scattering measu-rements showed that this did not increase the surface microroughness. During the attach-ment of the different mirror sections the co- alignment was verified by microscopic in-spection of the focal image produced by a parallel light beam. The mirror system was thendemounted and evaporatively coated with ' 600 A of gold. The final mounting was performedagain under microscopical control of the focal image.

Table 2. Optical Properties of the Three MPI 32 cm Telescopes

Full width half maximum ofthe central part of thepoint spread function

microroughness GB

(arc sec) (A)

T 1 5.3 13.2

T 2 5.7 6.2

T 3 5.1 7.9

SP /E Vol. 184 Space Optics - Imaging X -Ray Optics Workshop (1979) / 15

DEVELOPMENT OF IMAGING X-RAY TELESCOPES AT MAX-PLANCK-INSTITUT GARCHING

tively. The FECO method becomes insensitive at very low roughness levels (a ^ 10 A).. In this regime a comparison has been made with results from an electron reflection inter­ ference microscopy. This novel technique has been developed by M61lenstedt's group at the University of Tubingen (Lichte 1977(7)) a n d has an ultimate sensitivity of ~ 0.1 A. As shown in table 1, the agreement is quite good for a sample with o~ ~ 2.5 A.

The 32 cm Telescope

This telescope has been designed to meet the boundary conditions of Skylark and Black Brant ro c k e t s ^ j . It has a front diameter of 32 cm and a focal length of 142.7 cm. The mirrors have been optimized with respect to the effective area at 1 keV, leading to a length of the parabo1o i da 1 and hyperbo1oIda 1 sections of 43 and 38 cm, respectively. Each section consists of two pieces for reasons of handling and accessibility during the po­ lishing process. Three mirrors of this type have been built by Zeiss; figure 4 shows the photograph of one of them.

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-20246

OFF SPECULAR DIRECTION (ARCMIN)

Fig. 5. Reduction of X-ray scattering measu­ red for one paraboloidal section of a 32 cm telescope after successive polishing steps.

The mirrors are made of aluminium with a wall thickness of~1.4 cm. The aluminium was first turned on a computer controlled lathe. During this process, the mirror was tempe­ red several times in order to reduce the build-up of mechanical stresses. The mechanical surface was then plated with a 70 Urn kanigen layer and subjected to grinding and poli­ shing Interrupted by roundness and slope measurements. The final polishing approach was monitored by X-ray scattering tests. Up to five Iterative steps of polishing and measu­ ring have been employed In order to achieve the final surface quality of the 32 c mi tele­ scopes. Figure 5 shows the corresponding Improvement of Og for one of the paraboloidal sections. After polishing the shape of the mirror was measured again and then the indi­ vidual elements were cut to their final lengths. During this procedure the polished sur­ face was protected by a lacquer which was afterwards dissolved. X-ray scattering measu­ rements showed that this did not Increase the surface microroughness. During the attach­ ment of the different mirror sections the co-alignment was verified by microscopic In­ spection of the focal Image produced by a parallel light beam. The mirror system was then demounted and evaporatively coated with - 600' A of gold. The final mounting^was performed again under microscopical control of the focal Image.

Table 2. Optical Properties of the Three HP I 32 cm Telescopes

T 1

T 2

T 3

Full w i d t h half maximum of the central part of the point spread function

(arc sec)

5.8

5.7

5.1

micro roughness ou

(A)

13.2

6 .2

7.9

SP/E VoL 184 Space Optics-Imaging X-Ray Optics Workshop 11979) / 15

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TRUMPER, ASCHENBACH, BRAUNINGER

Table 2 summarizes the properties of the three mirror systems which have been built sofar. For this paper we have excluded the presentation of the reflectivity measurements sincethey are discussed ir a separate paper at this symposium by Aschenbach et al.(9). The micro -roughness oB quoted in Table 2 is an average determined by pencil beam scattering tests at8 to 16 positions distributed over the who're aperture. We note that the micruroughness oftelescope 12 with 6.2 A is only a factor of two larger than that of the best flatsamples. The corresponding scattering distributions as given in figure 6 for telescope T2show clearly the dependence of the fractional scattered flux on the wavelength. In figure 7this dependence is summarized for all three telescopes. The different lines which representbest fits to the data points have wavelength dependencies close to A72. This is in accor-dance with the prediction of Beckmann's theory and indicates that the point spread functionat angles > 1 arc min is largely determined by scattering.

I.EI

NHVELENGTH, 9.3 R

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GFF SPECULFR REFLECTION (RRCMIN) OFF SPECOLFR REFLECTION (RRCMIN)

Fig. 6. X -ray scattering distributions for different wavelengths obtained bypencil beam tests of the 32 cm telescope (T 2). The incident beam width is 29 ".

T1

10 100WAVE:I'ct`iüiH íÁ)

The full width half maximum of thecentral part of the point spread func-tion has been determined by opticalparallel beam tests. The result ob-tained (5 - 6 arc sec FWHM) is con-sistent with that derived from the ob-served broadening of specularly re-flected X -ray pencil beams. However,we note that both methods are not veryaccurate and a final assessment of themirror resolution will be made afterlong beam tests at MSFC which will takeplace in summer 1979.

It may be of interest to compare theX -ray optical performance of the 32 cmtelescope with that of HEAD -2 keepingin mind of course that both instrumentsare quite different in size and com-plexity.

As far as the width of the centralpart of the point spread function is

Fig. 7. Depend!:nce of the fractional scattered flux concerned, the 32 cm telescope seemson wavelength for all three 32 cm telescopes. to be a little worse: 5 - 6 arc sec

16 / SPIE Vol. 184 Space Optics - Imaging X -Ray Optics Workshop (1979)

TRUMPER, ASCHENBACH, BRAUNINGER

Table 2 summarizes the properties of the three mirror systems which have been built so far. For this paper we have excluded the presentation of the reflectivity measurements since they are discussed in a separate paper at this symposium by Aschenbach et al.^^. The micro- roughness OQ quoted in Table 2 is an average determined by pencil beam scattering tests at 8 to 16 positions distributed over the whole aperture. We note that the microroughness of telescope T2 with 6.2 A is only a factor of two larger than that of the best flat samples. The corresponding scattering distributions as given in figure 6 for telescope T2 show clearly the dependence of the fractional scattered flux on the wavelength. In figure 7 this dependence is summarized for all three telescopes. The differentlines which represent best fits to the data points have wavelength dependencies close to A. ~ ̂ . This is in accor­ dance with the prediction of Beckmann's theory and indicates that the point spread function at angles > 1 arc min is largely determined by scattering.

I.Elt

WHYELENGTH: e.3 R

-?.B -IB B ID 23 30

CFF SPECULHR REFLECTION (RRCMIN)

I.E5

I.EM

?'°

inzg'.H

WflVELENKTH- 1 3. 3 3

-22 -IB Z .IB 22 32

CFF 5PECULRR REFLECTION CflRCMlN)

HHVELENBTH: I7.fi n

CFF 5PECULRR REFLECT I DM CRRCMiN)

unvfLFTNSTH: MM.a e

OFF 5PECULRR REFLECT I OH (RRCMIfO

Fig. 6. X-ray scattering distributions for different wavelengths obtained by pencil beam tests of the 32 cm telescope (T 2]. The incident beam width is 29".

The full width half maximum of the central part of the point spread func­ tion has been determined by optical parallel beam tests. The result ob­ tained (5 - 6 arc sec F W H M ) is con­ sistent with that derived from the ob­ served broadening of specularly re­ flected X-ray pencil beams. However, we note that both methods are not very accurate and a final assessment of the mirror resolution will be made after long beam tests at MSFC which will take place in su mm e r 1979.

It may be of Interest to compare the X-ray optical performance of the 32 cm telescope with that of HEAQ-2 keeping In mind of course that both instruments are quite different In size and com­ plexity .

As far as the w1dth of the central part of the point spread function Is concerned, the 32 cm telescope seems to be a little worse: 5-6 arc sec

L10

WAVELENGTH <: A)Fig. 7. Dependence of the fractional scattered fli on wavelength for all three 32 cm telescopes.

16 I SPIE Vol. 184 Space Optics-Imaging X-Ray Optics Workshop (1979)

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DEVELOPMENT OF IMAGING X -RAY TELESCOPES AT MAX -PLANCK -INSTITUT GARCHING

compared with ^-3.5 arc sec FWH(`L On the other hand the microroughness seems to be better:oB ^-7 G for the 32 cm tel versus o - 20 - 25 A for HEAD -2L8). However, these latterfigures have been obtained by quite different methods (FECO in the case of HEAD -2) and thecomparison must be considered with care.

Concluding this section we briefly mention that one of our 32 cm telescopes (T 3, c.f.table 2) has been flown on a Skylark rocket (A 4/2 of the German national space program)in February 1978 with an imaging proportional counter developed at MPI. This detector hasa linear resolution of 0.45 mm (FWHM) and an energy resolution of 50 (FWHM), both averagedover the detector area of 31 x 31 mm2 and measured at 0.93 keV. The angular resolutionof the whole telescope was limited by the counter to ^-1.5 arc min. During the flightpictures of the Crab nebula region and the Pup A supernova remnant have been obtained. Theother two 32 cm telescopes will be flown in collaborative programs during the next fewmonths.

The B0 cm Telescope

Based on the experience gained in the 32 cm telescope program we have started the de-velopment of an 80 cm telescope, which will be launched with the Space Shuttle. As shownin figure 8 it consists of a fourfold nested mirror system and a focal plane assemblywhich can accommodate up to three instruments. Behind the mirror system an objective trans-mission grating can be brought into the X -ray light path.

SUNDOOR SHADE

TRANSMISSIONGRATING

'.

7_-;1-

/bl,

i

1--;}I

- -L_ . t ,y-Il--ll-t'I

THERMAL MIRRORPRECOLLIMATOR SYSTEM

FOCAL PLANEASSEMBLY

The mirror system ha a geometrical col-lection area of 1252 cm , neglecting ob-struction by structural parts. Its focallength is 240 cm. The diameters of theparaboloidal mirrorsvary between 488 and800 mm. The section length is 500 mm. Thereflecting surface will be gold or nickel.The effective area of the mirror system ver-sus wavelength for a gold surface and theangular resolution as function of the off -axis angle are shown in figures 9 and 10,respectively. As far as the principal op-tical tolerances are concerned, the basicdesign goal is to achieve angular resolu-tion and scattering characteristics compa-rable with those of the 32 cm telescope.A main difference to the latter, apart fromsize and complexity, is the choice of ma-terial.

Fig. 8. Cross section of the 80 or telescope The prime candidates discussed were alu-package. minium /kanigen, quartz and zerodur. As stated

above, all three materials can be polishedto very low microroughness levels. A phase -A study performed by Dernier System and CarlZeiss Co., showed, however, that the use of aluminium would result in a very complicated andexpensive thermal control system. Furthermore, there were serious doubts about the longterm stability of an aluminium telescope due to residual internal mechanical stresses. Forthese reasons, aluminium was excluded from further considerations.

Both quartz and zerodur have very low thermal expansion coefficients, which eases thethermal control problems. In case of zerodur for example it is not necessary to heat themirror shells but only the flanges. Both materials have little residual mechanical stressesafter machining and, as already mentioned above, are easier polished to low roughnesslevels, compared with kanigen. There are differences in manufacturing and procurement costsof the mirror blanks: (1) Since quartz is not available in large blocks, cylindrical blanksmust be welded out of staves. This is a costly procedure and, in addition, there is an in-creased bubble density along the weldingseams which may result in a larger microroughnessof the polished surface. On the other hand, zerodur can be produced in large blocks fromwhich the cylindrical blanks are cut. (2) The costs for the raw material are considerablyhigher in case of quartz, compared with zerodur. After careful consideration of all aspects,we decided to use zerodur as the mirror material.

The principal mechanical layout of the mirror assembly is simple: The four pairs of para-boloidal and hyperboloidal mirrors will be supported by a thick central flange and thinnerstructures at the ends of the assembly which are made of Invar. The mirror assembly will bemounted with a central flange to an optical bench system which consists of an outer supportcylinder (c.f. fig. 8).

Major problems in manufacturing of large mirror systems are caused by the influence of

SP /E Vol. 184 Space Optics - Imaging X -Ray Optics Workshop (1979) / 17

DEVELOPMENT OF IMAGING X-RAY TELESCOPES AT MAX-PLANCK-INSTITUT GARCHING

compared with ~ 3.5 arc sec FWHM. On the other hand the microroughness seems to be better: Og ~ 7 A for the 32 cm telescope versus o ~ 20 - 25 A for HEAO-2^°J. However, these latter figures have been obtained by quite different methods [FECO in the case of HEAO-2) and the comparison must be considered with care.

Concluding this section we briefly mention that one of our 32 cm telescopes (T 3, c.f. table 2} has been flown on a Skylark rocket (A 4/2 of the German national space program) In February 1978 with an imaging proportional counter developed at HPI. This detector has a linear resolution of 0.45 mm (FWHN) and an energy resolution of 60 % CFWHN), both averaged over the detector area of 31 x 31 mm^ and measured at 0.93 keV. The angular resolution oft he whole telescope was limited by the counter to ~ 1 . 5 arc min. During the flight pictures of the Crab nebula region and the Pup A supernova remnant have been obtained. The other two 32 cm telescopes will be flown In collaborative programs during the next few mo nt hs .

The 80 cm Telescope

Based on the experience gained in the 32 cm telescope program we have started the de­ velopment of an 80 cm telescope, which will be launched with the Space Shuttle. As shown in figure 8 it consists of a fourfold nested mirror system and a focal plane assembly which can accommodate up to three instruments. Behind the mirror system an objective trans­ mission grating can be brought Into the X-ray light path.

The mirror system has a geometrical col­ lection area of 1252 cm , neglecting ob­ struction by structural parts. Its focal length is 240 cm. The diameters of the paraboloidal mirrors vary between 488 and 800 mm. The section length is 500 mm. The reflecting surface will be gold or nickel. The effective area of the mirror system ver­ sus wavelength for a gold surface and the angular resolution as function of the off- axis angle are shown in figures 9 and 10, respectively. As far as the principal op­ tical tolerances are concerned, the basic design goal is to achieve angular resolu­ tion and scattering characteristics compa­ rable with those of the 32 cm telescope. A main difference to the latter, apart from size and complexity, is the choice of m a - t erI a 1.

THERMAL MIRROR PRtCOLLiMATOR SYSTEM

Cross section of the 80 cm telescope The prime candidates discussed were alu­ minium/ kanigen , quartz and zerodur. As stated above, all three materials can be polished

to very low microroughness levels. A phase-A study performed by Dornier System and Carl 7eiss Co., showed, however, that the use of a 1 umini urn wouId result in a very complicated and expensive thermal control system. Furthermore, there were serious doubts about the long term stability of an aluminium telescope due to residual internal mechanical stresses. For these reasons, aluminium was excluded from further considerations.

Both quartz and zerodur have very low thermal expansion coefficients, which eases the thermal control problems. In case of zerodur for example It is not necessary to heat the mirror shells but only the flanges. Both materials have little residual mechanical stresses after machining and, as already mentioned above, are easier polished to low roughness levels, compared with kanigen. There are differences In manufacturing and procurement costs of the mirror blanks: (1) Since quartz is not available in large blocks, cylindrical blanks must be welded out of staves. This is a costly procedure and, in addition, there is an in­ creased bubble density along the weld ing seams which may result in a larger microroughness of the polished surface. On the other hand, zerodur can be produced in large blocks from which the cylindrical blanks are cut. (2) The costs for the raw material are considerably higher in case of quartz, compared with zerodur. After careful consideration of all aspects, we decided to use zerodur as the mirror material.

The principal mechanical layout of the mirror assembly Is simple: The four pairs of para­ boloidal and hyperboloIda 1 mirrors will be supported by a thick central flange and thinner structures at the ends of the assembly which are made of Invar. The mirror assembly will be mounted with a central flange to an optical bench system which consists of an outer support cylinder (c.f. Tig. 8).

Hajor problems in manufacturing of large mirror systems are caused by the influence of

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TRUMPER, ASCHENBACH, BRAUNINGER

the gravitational forces. Although the mirror shells have a thickness of typically 20 mm,they will undergo substantial deformation in a 1 g environment, in particular if the op-tical axis is in a horizontal position. Therefore, several critical operations will bemade with the optical axis vertical. These include the moulding of the mirror shells(except final polishing), and their assembly including the necessary alignment procedures.

Fig. 9. Effective area of the 80 cm mirrorsystem as function of wavelength for a goldcoating.

ziv

OFF AXIS ANGLE t°1

Fig. 10. Angular resolution of the80 cm mirror system as function ofthe off -axis angle.

Status of the 80 cm Telescope Project, Planned Missions andSummary of Scientific Objectives

A phase -B study on the mirror system has just been completed by Zeiss. A prototype ofone of the zerodur mirrors should be built and polished by spring next year. The X-raycalibration will take place in our 130 m test facility.

At present, two missions with the 80 cm telescope are under consideration:

1) SXT a spectroscopic telescope mounted to the IPS on the Space Shuttle. The baselineinstrument consists of the 80 cm mirror system, a high efficiency, coma correct-ed transmission grating, and a position sensitive proportional counter. Optionalimaging devices of higher spatial resolution are under consideration.

The main scientific objectives of the SXT are- medium resolution spectroscopy with high throughput- structural studies of extended sources- restricted sky surveys.Phase B of the SXT project will begin in summer 1979.

2) ROBISAT is a Shuttle launched free flyer, consisting of the 80 cm mirror system andtwo redundant imaging proportional counters. Its main objective is to performa soft X -ray all sky survey with a resolution of ` 1 arc min.Phase A of this project is just completed. Phase B will begin in fall 1979.

Acknowledgement

Many scientists, engineers and technicians at MPI Garching and in industry have con-tributed to these projects. They are financially supported by the Bundesministerium fürForschung and Technologie and managed by OFVLR Porz.

References

1. Wolter, H., Ann. Phys. Vol. 10, 94. 1952.2. Aschenbach, B., Bröuninger, H., Stephan, K.H., Trümper, J., paper no. 184 -28, this symp.3. Lenzen, R., Abbildungseigenschaften eines Röntgenteleskops vom Typ Wolter -I unterbesonderer Berücksichtigung der Kontrastverminderung durch diffuse Reflexion, Ph.D. Disser-tation, Tübingen Univ. 1978.

18 / SPIE Vol. 184 Space Optics - Imaging X -Ray Optics Workshop (1979)

TRUMPER, ASCHENBACH, BRAUNINGER

the gravitational forces. Although the mirror shells have a thickness of typically 20 mm, they will undergo substantial deformation in a 1 g environment, in particular if the op­ tical axis is in a horizontal position. Therefore, several critical operations will be made with the optical axis vertical. These include the moulding of the mirror shells (except final polishing), and their assembly including the necessary alignment procedures,

-1000

10 19 ' 28 37 46 55 64 73 82

WAVELENGTH A ( A ]

Fig. 9. Effective area of the 80 cm mirror system as function of wavelength for a gold coating.

Fig. 10. Angular resolution of the 80 cm mirror system as function of the off-axis angle.

Status of the 60 cm Telescope Project,_P_l^_n_QjBd Hissions and Summary of Scientific Objectives

A phase-B study on the mirror system has just been completed by Zeiss. A prototype of one of the zerodur mirrors should be built and polished by spring next year. The X-ray calibration will take place in our 130 m test facility.

At present, two missions with the cm telescope are under consideration:

1) SXT a spectroscopic telescope mounted to the IPS on the Space Shuttle. The baseline instrument consists of the 80 cm mirror system, a high efficiency, coma correct­ ed transmission grating, and a position sensitive proportional counter. Optional imaging devices of higher spatial resolution are under consideration.

The main scientific objectives of the SXT are- medium resolution spectroscopy with high throughput- structural studies of extended sources- restricted sky surveys.Phase B of the SXT project will begin in summer 1979.

2] ROBISAT is a Shuttle launched free flyer, consisting of the 80 cm mirror system andtwo redundant imaging proportional counters. Its main objective is to performa soft X-ray all sky survey with a resolution of < 1 arc min.Phase A of this project is just completed. Phase B will begin in fall 1979.

Ac knowledgement

N a n y scientists, engineers and technicians at MPI Garching and in industry have con­ tributed to these projects. They are financially supported by the Bundesministerium fur Forschung und Technologie and managed by DFVLR Porz.

References

Ann. Phys. Vol. 10, H . ,

94. 1952. Stephan, K.H.,

1. Wolter, H., __________2. Aschenbach, B., Brauninger, H., Stephan, K.H., Trumper, J . , paper no. 184-28, this symp,3. Lenzen, R., Abbi Idu ngseigensc haf t en eines Rb'ntgen te 1 es kops vom Typ Wolter-I unter besonderer Berucksichtigung der Kontrastverminderung durch diffuse Reflexion, Ph.D. Disser­ tation, Tubingen Oniv. 1978.

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DEVELOPMENT OF IMAGING X -RAY TELESCOPES AT MAX -PLANCK -INSTITUT GARCHING

4. Ondrusch, A., Aschenbach, B., Brüuninger, H., Hasinger, G., Trümper, J., to bepublished.5. Brüuninger, H., Lenzen, R., and Trümper, J., "X -ray Optical Properties of a WolterType -I Telescope for Rocket Application ", in COSPAR: New Instrumentation for SpaceAstronomy, pp. 251 -255, Pergamon Press. 1978.6. Beckmann, P. and Spizzichino, A., The Scattering of Electromagnetic Waves from RoughSurfaces, Pergamon Press. 1963.7. Lichte, H., Ein Huflicht- Interferenzmikrostop für Elektronenwellen, Ph.D. Dissertation,Tübingen Univ. 1977.8. Van Speybroeck, L.P., SPIE Vol. 106, 136. 1977.9. Aschenbach, B., Brüuninger, H., Ondrusch, A., Trümper, J., paper no. 184 -17, this symp.

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DEVELOPMENT OF IMAGING X-RAY TELESCOPES AT MAX-PLANCK-INSTITUT GARCHING

4 . Dndrusch, A., Aschenbach, B. , Brauninger, H . , Hasinger, G . , Trumper, J . , to be published .5. Brauninger, H., Lenzen, R., and Trumper, J. , "X-ray Optical Properties of a Wolter Type-I Telescope for Rocket Application", in CDSPAR: New Instrumentation for Space Astronomy, pp. 251-255, Pergamon Press. 1978.6. Beckmann, P. and Spizzichino, A., The Scattering of Electromagnetic Waves from Rough Surfaces, Pergamon Press. 1963.7 . Lichte, H., Ein Auflicht-Interferenzmikroskop fur Elektronenwe11 en, Ph.D. Dissertation, Tubingen Univ. 1977.8. Van Speybroeck, L.P., SPIE Vol. 1D6, 136. 1977.9. Aschenbach, B., Brauninger, H., Ondrusch, A., Trumper, 3., paper no. 184-17, this symp.

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