x-ray performance of the lamar protoflight mirror

X-ray performance of the LAMAR protoflight mirror

Daniel G. Fabricant, L. M. Cohen, and Paul Gorenstein

A grazing incidence x-ray mirror in the Kirkpatrick-Baez geometry was constructed and tested. Its geometric

aperture measures 20 by 30 cm and its length is 100 cm. The focal length is 3.4 m measured from the front to

the focal plane. It is the first of eight mirrors to be built for the LAMAR experiment of the Shuttle HighEnergy Astrophysics Laboratory. Its angular resolution was measured at 1.5 and 6.4 keV in a quasiparallel x-

ray beam at the Marshall Space Flight Center. The half-power width (HPW) of the resolution functionprojected along a horizontal axis is 31 sec of arc at both energies and in visible light. With the addition of

small isolation pads the mirror is able to withstand the vibration and acceleration levels of a Space Shuttlelaunch. The resolution remains under 35-sec of arc HPW for changes in temperature of 9.5°C and when a

modest temperature gradient is imposed on the mirror.

1. Introduction

The first of eight x-ray telescopes intended for theLAMAR (large area modular array of reflectors) ex-periment has been constructed. The LAMAR experi-ment has been under development for a number ofyears, most recently as an experiment for the ShuttleHigh Energy Astrophysics Laboratory (SHEAL) to beflown aboard the Space Shuttle. In the wake of theChallenger disaster, the availability of flight opportu-nities remains uncertain. LAMAR consists of an ar-ray of eight identical telescopes that view the sky alongthe same direction. This experiment is a modest im-plementation of an approach to achieving highthroughput with moderately high angular resolutiondescribed by Gorenstein.1

Each telescope consists of two orthogonal sets ofnested confocal 1-D parabolic plates commonly knownas the Kirkpatrick-Baez (K-B) configuration. The K-B module allows a good compromise between highaperture efficiency, good angular resolution, low cost,and ease of fabrication. The principal advantages ofthe K-B geometry are that all the reflectors are identi-cal flats prior to bending and that each reflector func-tions independently. Alignment of the front and rearsections is not critical. The problem of producing agood mirror reduces to two tasks: mass production of

The authors are with Harvard-Smithsonian Center for Astrophys-ics, 60 Garden Street, Cambridge, Massachusetts 02138.

Received 27 October 1987.0003-6935/88/081456-09$2.00/0.©3 1988 Optical Society of America.

a thin, lightweight, and stiff flat, and formation of theappropriate parabolic curve for the given plate. Lowresolution versions of this type of telescope (2-3 min ofarc) were used in sounding rocket studies of clusters ofgalaxies and stars.2 3 The parabolic figure is producedby an automated system that operates under the con-trol of a personal computer.

The automated system facilitates the rapid, rela-tively low-cost production of mirror modules. It isapplicable to the construction of much larger mirrorassemblies with little increase in cost and complexity.

The geometric aperture of the flight mirror moduleis 30 by 20 cm. The distance from the front of themirror to the focal plane is 3.4 m. There are thirty-twoplates in the front set which measure 20 by 50 cm andtwenty-two plates in the rear which measure 30 by 50cm. The theoretical effective area of one module(which includes losses from structural members thatincrease mechanical strength) is 150 cm2 at 0.28 keV,100 cm 2 at 2 keV, and 23 cm 2 at 6 keV.

A protoflight mirror assembly has been constructedand its angular resolution measured at the x-ray testfacility of the Marshall Space Flight Center (MSFC).The mirror module has been subjected to vibrationand thermal tests. The angular resolution, defined asthe 50% power width projected onto an axis, was mea-sured to be 31 sec of arc at both 1.5 and 6.4 keV. Thisexceeds our goal of 35 sec of arc. The mirror assemblysurvived vibration testing without deterioration of an-gular resolution after small isolation pads were addedprior to the final tests. X-ray reflectivity measure-ments of iridium test samples made after the MSFCcalibration demonstrate that the effective area above 6keV can be somewhat larger than the theoretical valuesgiven above.

1456 APPLIED OPTICS / Vol. 27, No. 8 / 15 April 1988

These goals were not attained without overcomingseveral unexpected problems along the way. In allcases the problems have been addressed and have beenrectified following changes to fabrication procedures.

II. Development of the Protoflight Mirror

The front and rear halves of the mirror assemblywere fabricated sequentially. In the Kirkpatrick-Baez geometry each half functions independently andthe alignment of the two is not critical. This provedadvantageous because it gave us the opportunity toimprove the methodology and rectify problems that weencountered in the front module before proceeding tothe rear module. Consequently, it is the rear modulethat is representative of the true performance of thecurrent methodology. We discuss the various steps inthe fabrication of the mirror modules below.

A. Selection of Glass

The reflector material for the LAMAR Space Shut-tle experiment is commercial float glass. Float glasshas excellent x-ray reflectivity and rather low scatter-ing, especially the surface that was in contact with theliquid tin during the manufacturing process. Its flat-ness varies considerably from batch to batch, appar-ently due to random factors in the manufacturing pro-cess. Hence, the process of locating an acceptablebatch of float glass is a matter of persistence and luck.The minimum thickness of commercially availablefloat glass that is sufficiently flat appears to be 1.8 mm.

Early in the mirror development program it becameapparent that we could not simply issue a specificationfor the purchase of float glass of a given flatness andexpect that material sent to us by commercial suppli-ers would meet that specification. Instead we found itnecessary to search for the material and screen it our-selves. Several times more glass sheets than would beneeded were purchased. With the aid of Visidyne ofBurlington, MA, we developed a procedure for screen-ing a sheet of glass for flatness by scanning it along itslength with an autocollimator coupled to a computer.Every sheet of glass was cataloged and classified ac-cording to its mean curvature and deviation from themean. Approximately one-third of the glass sheetspurchased from a batch previously identified as goodby the manufacturer satisfied our criteria for flatnessand small curvature.

B. Titanium Backing

In previous publications we have described thebenefits of precurving a glass sheet to approximatelyits average final radius of curvature by bonding it to a5-mil coiled titanium sheet.4 As a result of this pro-cess, bending forces are applied uniformly over theentire surface and most of the undesirably elastic dis-tortions associated with bending a sheet of glass areavoided. When a mirror is tuned to its final figure, theconcentrated bending forces are only a very small in-crement on the prestress applied by the spring; this canresult in better angular resolution. However, we dis-covered that this process was not required to meet our

resolution goal of 35-sec of arc half-power diameter.Moreover, we experienced problems with the titaniumbacking process in the construction of the front mod-ule. In an effort to facilitate production, we changedthe epoxy resin bonding agent between the titaniumand glass to one with much less viscosity. It proved tohave excessive differential contraction on setting, re-sulting in inferior performance. Differential contrac-tion introduced distortions on a scale of -2.54 cm (1in.) and degraded the angular resolution. In addition,the backing tended to lift slightly off the edges of theglass. Before returning to the original epoxy orsearching for another, we reexamined the need forprecurving for the flight mirrors, which have smallerplate curvatures than earlier prototypes. Experi-ments demonstrated that our resolution goal could bemet without precurving. Consequently we did not usetitanium coils in the fabrication of the rear module anddo not plan to use them in the construction of futuretelescopes for SHEAL. This eliminates the most cost-ly and complex step of the entire mirror constructionprocess and also improves the mirror assembly's ther-mal behavior.

C. Automatic Tuning of the Plate Figure

A computer-controlled system was developed fortuning the figure of a plate to the desired paraboliccurve with little human intervention.5 Eight motor-controlled precise linear translators are attached toclips with pins along the top and bottom edges of theplate. When the mirror module is placed on the opti-cal bench for tuning, the pins of one plate at a time areattached to long slotted bars which are suspendedabove the top and below the bottom of the mirrorboxes. The linear translators perform the tuning op-eration by exerting forces on the pins by way of theslotted bar. This system is illustrated in Fig. 1. Anoperator is needed to couple the pins to the slotted barsand to make initial coarse tuning adjustments. Thefigure of a plate is adjusted by manipulating one lineartranslator at a time as the visible light image fromabout one-quarter of the plate is sensed in the diodearrays. Signals from the diode arrays are read out andprocessed by a personal computer to calculate the sev-en centroids of the line image. The translators aredriven in the direction which minimizes the deviationof the centroids from the desired focus position. Amovable slit which is also under the control of themicrocomputer varies the region that is illuminated.After several iterations through these regions whichinclude coarse and fine movements of the linear trans-lators the figure is optimized. Typically this requiredan hour of unsupervised operation. The automatictuning system functioned rather well. Its intrinsicprecision appears to be much better than our 35-sec ofarc resolution goal, probably closer to 10 sec of arc.

D. Epoxy Bonding

When the mirror plate's figure is optimized by theautomatic tuning system, the plate is permanentlyfixed in position by bonding the pins with an epoxy

15 April 1988 / Vol. 27, No. 8 / APPLIED OPTICS 1457

Fig. 1. Figure adjustment apparatus on an optical bench. The pins bonded to the top and bottom of the reflectors are attached to the slottedbars. Precision linear translators tune the figure as they vary the positions of the slotted bar.

resin. There are several requirements on the epoxy.It obviously has to be strong enough to retain the bentplate and maintain stability over a long period of time.It has to have good flow characteristics so that it willwet the pin completely and leave no voids in the smallvolume which surrounds the pins. Excessive shrink-age would shift the position of the pin from the initialpoint. Setting time is of prime concern because it isthe limiting factor in the time required to tune thefigure of the mirror plate. The motorized linear trans-lators have to remain in place to retain the tuned plateuntil the epoxy is set.

The protoflight mirror fabrication began with thebonding agent that we had used in the prototype mir-ror: Tracon 2016T, a so-called 5-min epoxy that actu-ally attained sufficient strength after 3 h. This al-lowed us to tune two plates within one day. At first,results were satisfactory as they had been two yearsearlier with a prototype mirror. Problems beganabout halfway through the front mirror module whenwe began to use a new batch of Tracon 2016T withsupposedly identical characteristics. Several platesbonded with the new epoxy failed to remain fixed.Within hours of release their angular resolution de-graded from -30 to 60 sec of arc. The damage isirreparable as there is no provision for retuning platesonce they are bonded. At that point our only alterna-tive was to suspend mirror fabrication and investigatethe epoxy problem.

We carried out a study of the strength of severaldifferent epoxies with various setting times. The sam-ples studied included the original Tracon epoxy as wellas the new material which failed. One variable is the

percentage of inert quartz powder filler in the epoxymixture. The quartz fraction affects the shrinkage,stiffness, and viscosity. There is an optimum percent-age which varies from one type of epoxy to another.Measurements did reveal that the new batch of Tracon2016T was indeed less stiff (after 3 h) than the originalmaterial by a factor of 2. The best epoxy mixture interms of strength and viscosity to emerge from thestudies was Hysol EA 9313 with 100% by weight addedquartz powder. We adopted this as the new bondingagent. In addition, a new step was added to the proce-dure whereby hardness tests were performed on a testsample that was set aside each time a new mixture ofepoxy and hardener was made. The motorized trans-lators were not released until the stiffness of the sam-ple material was above a threshold. However, thisepoxy required over 20-h cure time, effectively limitingthe tuning process to one mirror per day. The bondingagent performed quite well from that point on with noproblems.

Ill. Angular Resolution of the LAMAR Mirror

The primary goal of this program was to verify bydirect measurement that the LAMAR protoflight mir-ror did indeed attain the proposed angular resolutionof 35-sec of arc half-power width or better in x-rays.Measurements were made at the Marshall SpaceFlight Center's 305-m (1000-ft) facility at 1.5 and 6.4keV. The 1.5-keV results are applicable to all lowerenergies because the entire mirror aperture is effectiveat 1.5 keV and below. On the other hand, the 6.4-keVdata involve only the inner quarter of the aperture.The two results would differ if (1) the distribution of


slope errors were different over a quarter of the mirrorcompared with over the entire mirror or (2) if therewere significant small scale surface structures thatwould result in more scattering at higher energy. Pri-or to the x-ray measurements, we obtained the visiblelight angular resolution with very little additional ef-fort during the process of tuning the figure. The auto-matic tuning process is based on adjusting the plate sothat the centroids of the visible light images fromseveral portions of the plate are optimally close to thecentral axis. It is a simple matter to scan the entiremirror when the tuning process is complete. We ob-tain the resolution function from the distribution ofthe centroids of the visible light images. This mini-mizes the diffraction problems which are a factor indirect visible light imaging because the mirror containsmany small segments of projected area. If the surfaceis smooth on scales below 5 cm (2 in.), the visible lightimaging properties will duplicate the x ray. Thisturned out to be true, at least for the unbacked platesof the rear mirror module.

The angular resolution was measured one dimensionat a time. This was a necessity for two reasons. Thefirst is that the angular spread of the beam at the 1000-min of arc distance is 3.4 X 2.3 min of arc over the 30- X20-cm (12- X 8-in.) aperture of the mirror. A Kirkpat-rick-Baez mirror made for astronomical measure-ments cannot be refocused on a source at a finite dis-tance. To reduce the angular divergence of the beamto an acceptable level, i.e., below 20 sec of arc, 2.54-cmsegments of the mirror were exposed at a time. A slitwas used whose width was 2.54 cm and whose heightwas the full height of the mirror in the orthogonaldimension. By measuring one dimension at a time,twenty exposures were sufficient to determine the res-olution in two dimensions at any one energy. Thesecond reason is that the plates sag significantly undertheir own weight when they are horizontal. By mea-suring the resolution of one dimension while its platesare vertical and rotating the mirror 900 prior to thesecond measurement, the effects of gravitational de-flection are avoided.

The x-ray tests were carried out using the opticalbench and detectors built for the AXAF TechnologyMirror Assembly (TMA) test program. Angular reso-lution measurements were carried out by translatingthe mirror and detector in synchronism in incrementsof 2.54 cm across a fixed 1-sec of arc vertical slit whichdefines a constant beam direction. The mirror assem-bly was mounted on a precision table; we verified thatthe table was sufficiently free of wobble with autocolli-mator tests. The detectors were translated by theTMA system. The positions of both were read out bydigital magnetic scales to an accuracy of 2 gtm, whichcorresponds to less than a second of arc at our focalplane scale. A photograph of the LAMAR protoflightmirror, the precision translation table, and the slit isshown in Fig. 2.

Two types of TMA detector were used: a channelplate multiplier, virtually identical to the HRI ofHEAO-2, and a scanning proportional counter with an

aperture defined by a fine slit. In each case, the imagethat was analyzed is the superposition of the data fromthe entire mirror. This is equivalent to a single fullaperture exposure to a beam with a half-power width of8 sec of arc.

A. X-Ray Test Results

By superimposing the data from all slit positions, weobtain the angular resolution and point response of themirror modules. Measurements were made at 1.5 keVwith both the HRI and the proportional counter withslit. At 6.4 keV only the proportional counter wasused because the efficiency of the HRI was too low.

We consider results from the rear module, whichrepresents the current mirror fabrication capability.The differential point response functions at 1.5 and 6.4keV are shown in Figs. 3-5. Values of the projected(along the horizontal axis) half-power width (HPW)are summarized in Table I. The typical value is 31 secof arc which is better than our goal of 35 sec of arc.Agreement between the 1.5- and 6.4-keV values is ex-cellent. This indicates the absence of scattering fromsurface irregularities which would lead to a HPW thatis highly energy dependent. Further confirmation isprovided by the visible light results (Fig. 6) which arein excellent agreement with the x-ray results. Theerror in the HPW is between 1 and 2 sec of arc, based onthe reproducibility of the visible light measurement.

As expected, the angular resolution of the frontmodule (which was constructed first) was not nearly asgood as that of the rear. The HPW is 53 sec of arc.The poorer performance is due to factors that wereeliminated before the second module was fabricated,including the failure of the second batch of the 5-minepoxy and the fact that the surface structure of theplates was degraded as a result of excessive differentialshrinkage of epoxy bonding the titanium to the glass.

IV. Effective Area

In principle, the effective area of the LAMAR mirrorcan be determined very accurately, a priori, if thereflectivity of the gold-coated float glass is known as afunction of grazing angle. The reflectivity is easilymeasured on samples of the material used in the tele-scope. Other variables influencing the effective areaare geometric factors such as accuracy of the place-ment of baffles on the backs of the reflecting plates andthe plate thickness. Baffles exclude all nonreflectedon-axis or straight-through rays from the detector andreduce the number of nonreflected off-axis rays. Thebaffles are positioned approximately halfway down thelength of the mirror module. The titanium backing onthe plates of the front mirror module led to a problemin geometric efficiency. Imperfect adhesion of thebacking at the edges of the plate resulted in a reductionof the aperture. We detected such a problem duringthe fabrication of the front mirror module. When theplates were scanned in visible light, a loss of geometricarea was found at both the leading and trailing edges ofthe plates. We were able to attribute this to a combi-


Fig. 2. View of the complete mirror (front and rear modules) in a vacuum chamber at the MSFC X-Ray Calibration Facility. A 2.54-cm slit(which is not opened to its full height) defines a fixed direction for the x-ray beam.

200

I 501-

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z

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100

50

ANGULAR RESOLUTIONOF REAR LAMAR MODULE

I I I . I I~~~~~~~~~~~~~~~~~~~

30 40 50 60 70 80 90 100

ANGULAR BINS ( I Channel '5 arcsec)

Fig. 3. Angular resolution function of the rear section of the mirrorat 1.5 keV as measured with the HRI detector. This is the 1-D image

projected along a horizontal axis.

22000

20000

18000

16000

za 14000

Z 120000I>)

1000c

8000

600C

400C

200C

0 4 8 12 16 20 24 28 32BIN NO.

Fig.4. The 1.5-keV 1-D angular resolution (projected along a hori-zontal axis) of the rear section measured with a scanning small

aperture proportional counter.


I I . I I

1.5 kV X-RAYS -

I -... P~ I .L

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ANGULAR RESOLUTION AT 1.5 eV

REAR MODULEPROP COUNTER S S

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II DOC

1000C

9000

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0

8000

7000

6000

5000

4000

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1000

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Fig. 5. Same as Fig. 4 at 6.4 keV.

nation of the backing lifting at the edges and excessiveprotrusion of the baffles. These problems were cor-rected prior to the fabrication of the rear module. Theelimination of the titanium backing solved the firstproblem. The procedure for fabricating and position-ing the baffles was changed. They are now cut andplaced on the back of the plates with more accuratemetrology. The elimination of the titanium backinghelped in this aspect as well, because the position of thebaffle is more accurately determined when mounteddirectly on the glass. The visible light scans of the rearmodule indicated no loss of geometric area, confirmingthat the problems had indeed been rectified.

We arrived at the MSFC with the knowledge thatthe front mirror module had a geometric area deficien-cy because of the problem described above. To con-firm that the rear module was not affected, we carriedout 1.5- and 6.4-keV exposures of the rear module byitself, as well as the principal measurements with themirror in the normal 2-D configuration at several addi-tional energies. Full aperture exposures (3.4 X 2.3-min of arc beam spread) were made with a 4-mmcircular aperture (4-min of arc diameter) and a 25-mm aperture in the focal plane centered on the image.Results are listed in Table II.

The difference between the 4- and 25-mm aperturemeasurements is -10% of the power and is due togravity sag in the front in combination with the 3.4-min of arc beam spread. The lack of a strong energy

Table 1. Angular Resolution of the Rear Mirror Module

Energy HPWaDetector (keV (sec of arc)

HRI 1.5 30.1Proportional counter/slit 1.5 31.1Proportional counter/slit 6.4 29.2Visible light 31

a Projected along a horizontal axis.

75

I-z

0

co

:2:

zI-4aJa:

50

25

ANGULAR RESOLUTIONOF REAR LAMAR MODULE

100 120 140 160 ISO

ANGULAR BINS ( I Channel 4.1 wrcseC)

Fig. 6. Projected angular resolution function of the rear section invisible light.

dependence in the 25/4-mm ratio from 1.5 to 8.1 keVindicates that there was relatively little scattering.The theoretical effective area is based on a model forthe reflectivity of gold derived from optical constantsextrapolated from measurements at lower energies.6The theoretical effective area is corrected for the angu-lar divergence of the beam and the fact that the mirrorwas a few min of arc off-axis (because of requirementsof the setup) during the measurements. These twofactors results in a reduction of -8% in geometric areafrom a parallel on-axis beam. It is evident that theobserved effective area falls below the theoretical val-ues, with the discrepancy being largest at the highestenergies. Two factors explain the discrepancy: one isthe geometric efficiency problem of the front moduledescribed above and the other is the failure of the goldto reflect efficiently at large grazing angles. As will bedescribed in a later section, an analysis identified sig-nificant hydrocarbon contamination in the gold coat-ing. To help reconcile the difference between thetheoretical and observed areas, we measured the effec-tive area of the rear module independently in a 1-D

Table 11. Effective Area of the LAMAR Protoflight Mirror

Theoretical Measured areaEnergy areaa 25mm aperture 4mm aperture 25/4mm(keV) (cm2 ) (cm2) (cm2)

1.49 104.0 82.8 76.8 1.082.04 94.8 72.5 67.0 1.082.98 43.9 28.3 26.2 1.084.51 44.6 19.1 17.4 1.106.40 17.8 9.22 8.31 1.118.10 9.2 5.04 4.49 1.12

a Includes correction for beam divergence and 3-min of arc off-axisplacement.


I I I I I I I I I I I

ANGULAR RESOLUTION AT 6.4 .ey

.- /1 REAR MODULE _PROP COUNTER SCANS

I 8IN 8.05 arsec

I I I IVISIBLE LIGHT

I I

Table. Ill. Effective Area of the Rear Module In a 1-D Configuration

AreaEnergy Theoretical Measured(keV) (cm2) (cm2)

1.49 247 2436.4 106 82.7

configuration at 1.5 and 6.4 keV. These results may becompared to the theoretical values. The rear moduleis expected to be free from the loss of geometric areafrom the intruding titanium backing and baffles thataffected the front mirror. This comparison is shownin Table III.

Additional relevant information was derived fromthe slit scans across the mirror carried out for theangular resolution measurement. Each slit position isat a different mean grazing angle. From the relativecount rate at each slit position, we are able to constructa four-point table of relative reflectivity vs graze angle.We observe that, at 1.49 keV, the ratios of the countrates of the two outermost positions, mean graze anglesof 0 = 63 and 45 min of arc, to the innermost position at14 min of arc agree with theory. However, at 6.4 keVthe ratio of the 45-14-min of arc reflectivity is low by afactor of 2. This is what is expected to occur if thedensity of electrons in the reflector is too low for rea-sons of purity or metallic density.

We can draw a consistent picture that will accountfor the discrepancy between the theoretical and ob-served effective area. To do so, we concentrate on the1.5- and 6.4-keV measurements. The behavior shouldbe similar at the other energies. The 1-D measure-ment of effective area of the rear module at 1.5 keVagrees with theory. Hence, the geometric efficiency isnear 100%. As the front and rear modules should haveabout the same reflection efficiency, it follows from the2-D measurements of effective area at 1.5 keV (TableII) that the geometric efficiency of the front module issimply the ratio of the observed to theoretical area or80%. The 2-D relative effective area at 6.4 keV shouldthen be the geometric efficiency of the front modulemultiplied by the square of the 1-D reflection efficien-cy of the rear module or 0.80 X (82.7/106)2 = 0.49.This is consistent with the observed 2-D relative effec-tive area of 9.22/17.8 = 0.52. Furthermore, as there isno indication of scattering, the float glass substrate issatisfactory.

After the MSFC measurements, we set out to solvethe problem of the loss of effective area at higherenergy by these three activities:

(1) measure the 6.4- and 8.1-keV reflectivity of mir-ror samples to verify directly that the gold reflectivityat larger angles was the problem,

(2) analyze the chemical composition of the gold tosearch for contaminants, and

(3) obtain new samples of gold-coated float glassfrom a clean facility to establish that mirrors withtheoretical reflectivity at 6.4 and 8.1 keV can indeed beproduced.

U 10%

* 175A IRIDIUM

+ 500 A GOLD COATED x +BY PERISN-ELMER\

500 A GOLD COATEDBY LIBERTY MIRROR

1% I I l l l10 20 30 40 50 60

GRAZING ANGLE (orcmin)

Fig. 7. The 6.4-keV reflectivity of float glass with various metalliccoatings.

. 10%

LL,X:

10 20 30 40 50 61

GRAZING ANGLE (rcmin)

Fig. 8. Same as Fig. 7 at 8.09 keV.

We completed construction of a laboratory appara-tus that allowed a convenient means of measuring thereflectivity as a function of angle in the quasiparallelbeam of a 10-m x-ray system in our laboratory. Wesupplied float glass sample flats from our mirror stockto Perkin-Elmer for gold coating in their evaporationfacility, which is known to have a clean vacuum. Wealso had float glass samples coated with 175 A of iridi-um by the Acton Research Laboratories. Figures 7and 8 illustrate the 6.4- and 8.1-keV x-ray reflectivityof samples of our mirror, float glass coated by Perkin-Elmer with gold, and float glass coated with 175 A ofiridium. The mirror sample falls a factor of 2 belowthe theoretical reflectivity at 6.4 keV and 45 min of arc,


CA-K

8.09 keV

TYEORETICAL"GOLD REFLECTIVITY.

175A IRIDIUM

+ 500 A GOLD COATEDSY PER.IN-ELMER

500 A GOLD COATED

BY LIBERTY MIRROR

I I I0o

exactly as we had surmised from the scans of the mirrorat MSFC. The points from the Perkin-Elmer coatedmirror are much closer to the theoretical values butstill fall below.

To probe the problem of poor reflectivity of themirror gold coatings, samples of the material wereanalyzed by the Department of Applied Sciences at theBrookhaven National Laboratory. Auger electronspectroscopy was used to analyze the coating as a func-tion of depth. It revealed a 5% carbon contaminationby atomic ratio throughout the gold layer. Since thecarbon is likely to be in the form of a hydrocarbonmolecule (hydrogen is not detectable) it suggests thatonly -85 out of 100 atoms were actually gold and thatthe density could be low. This would explain the poorreflectivity of the material at short wavelengths andlarger graze angles. The Perkin-Elmer and Acton Re-search samples were made in a clean vacuum andwould be expected to be relatively free of hydrocarboncontamination.

The iridium points are best, generally exceeding thetheoretical gold curve in the important regime above10% reflectivity. However, they do fall below the theo-retical values of iridium. As there is no reason not touse iridium coatings, all future mirrors will have atleast the effective area calculated using the theoreticalreflectivity of gold. The iridium data at 4.5, 6.4, and8.0 keV over a range of angles were fit to theoreticalcurves of reflectivity with the density as a free parame-ter. A reasonable fit was obtained using a density ofthe evaporated film 15% below that of the bulk materi-al.

V. Vibration and Thermal Testing

The rear module of the mirror was subjected tovibration and thermal testing. Visible light measure-ments were used to assess the effect of these factors onthe angular resolution of the mirror. As describedabove, the angular resolution was measured by scans ofthe distribution of centroids of images formed by smallsections of the mirror. This procedure is valid due tothe similarity of the angular resolution of the rearmodule measured in this way to the angular resolutionin x rays.

The protoflight mirror underwent a series of tests inthree axes to test its ability to withstand the estimatedvibration loads of a Shuttle launch and reentry. Sinesurvey and random vibration tests at half of the esti-mated g levels of a Shuttle launch were carried outwithout incident along the two axes in the plane of theglass plates. The measured strains were well belowthe design levels as expected. The mirror module wasthen tested in the direction perpendicular to the glassplates. The sine survey indicated a prominent reso-nance at 42 Hz and a number of smaller resonances.

The rear mirror module was then returned to thelaboratory and scanned in visible light. The highstresses encountered during the random vibration testdid cause permanent deformation of the titanium pinsholding the mirror plates, with a resultant degradationof the HPW to 38 sec of arc, from the previbration

value of 31 sec of arc. Detailed computer models of themodule were constructed which were able to reproducethe frequency of the resonance and come within afactor of 2 or better of predicting the stress levels.These models were used to study the effect of addingisolators to limit transmission of the troublesome 42-Hz frequency. As a result, commercially available(Barry Control), compact isolation mounts were se-lected and installed between the shaker table and themirror module. The random vibration test was run onthe rear module at the full specified levels. The effica-cy of the isolators was demonstrated with the higheststress in the glass falling to 1750 psi, 60% of the designlimit. Accelerometer readings confirmed the desiredisolation at the 40-Hz region. The mirror module wasthen rescanned optically and found to have a HPW of39 sec of arc, consistent with the previous value. Thus,with the isolators present the full vibration level didnot affect the mirror.

Two types of thermal test were carried out. Thefirst, was a measurement of the angular resolution withthe temperature of the entire mirror raised above thetypical laboratory value. The second type of test con-sisted of measuring the resolution with a temperaturegradient along the length of the mirror. Both mea-surements were carried out on an optical bench in aparallel visible light beam. Only the rear telescopemodule was tested.

The resolution of the rear module at a higher tem-perature was measured prior to the vibration tests.The temperature of the entire room was allowed to riseby turning off the air conditioner and turning on heat-ers. About 24 h were allowed for the optical bench andthe mirror to come to equilibrium. At a temperatureof -33C, the HPW was 35.3 sec of arc. This com-pares to a resolution of 31 sec of arc at the normaloperating temperature of 23.50C. Thus, for AT =9.5°C, the resolution remained within the nominalgoal. It was not possible to lower the temperature ofthe room by a comparable amount. Analysis indicatedthat behavior would be similar for both negative andpositive temperature changes.

The thermal gradient test was carried out after themirror had been subjected to vibration and had de-graded from its previbration performance. However,that did not interfere with the ability to measure theeffect of a temperature gradient. There was a problemfrom air currents flowing through the mirror in thepresence of a thermal gradient. These made it diffi-cult to measure the resolution because the path of lightrays was disturbed. Images were unstable. Hence,the change in resolution that is observed under theinfluence of a temperature gradient is actually only anupper limit. With a gradient of 30C across the 20-secof arc length of the mirror, we scanned the mirror onone side of the optic axis. The resolution of eleven ofthe twenty-two plates was 43.8 sec of arc. This is to becompared to a postvibration resolution of 40.6 sec ofarc for the same eleven plates without a temperaturegradient. Consequently, the effect of the temperaturegradient is to add a perturbation of 16 sec of arc or less,


in quadrature with the gradient-free resolution. Thiswould degrade a resolution of 31 sec of arc to at most 35sec of arc. This is still within our resolution goal.

VI. Summary

We have constructed a 20- by 30-cm Kirkpatrick-Baez mirror module with an angular resolution of 31sec of arc (projected half-power width) at 1.5 and 6.4keV. Thermal tests have been performed successful-ly. The resolution remains better than 35 sec of arc asthe temperature is changed by 90 C. It is also 35 sec ofarc or better with a temperature gradient of up to 60Calong the 100-cm length of the mirror. With the addi-tion of small commercial vibration isolators, the angu-lar resolution did not change when the mirror wassubjected to the full launch loads expected with theSpace Shuttle.

We thank Visidyne Corp. in Burlington, MA, forassistance in screening float glass and experiments onthe titanium backing.

We would like to thank Paul Ouellette of SAO for hisvaluable assistance in preparing for and carrying outthe x-ray tests. The excellent support provided byCarey Reiley and the other people who operate theMSFC X-ray Calibration Facility is very much appre-ciated. This work was carried out under contract

NSA5-26613 from the National Aeronautics and SpaceAdministration.

This material was presented as paper 830-24 at theConference on Grazing Incidence Optics for Astro-nomical and Laboratory Applications, sponsored bySPIE, the International Society for Optical Engineer-ing, 17-19 Aug. 1987, San Diego, CA.

References1. P. Gorenstein, "Modular Focussing Arrays for X-Ray Observa-

tions of Very High Sensitivity from the Space Shuttle," SpecialReport to the Woods Hole Working Group (1973).

2. F. R. Harnden, Jr., D. Fabricant, K. Topka, B. P. Flannery, W. H.Tucker, and P. Gorenstein, "A Soft X-Ray Image of the AlgolRegion," Astrophys. J. 214, 418 (1977).

3. P. Gorenstein, D. Fabricant, K. Topka, F. R. Harnden, Jr., and W.H. Tucker, "Soft X-Ray Structure of the Perseus Cluster ofGalaxies," Astrophys. J. 224, 718 (1978).

4. P. Gorenstein, L. Cohen, and D. Fabricant, "X-Ray TelescopeModule for the LAMAR Space Shuttle Experiment," Proc. Soc.Photo-Opt. Instrum. Eng. 597, 128 (1986).

5. D. Fabricant, M. Conroy, L. Cohen, and P. Gorenstein, "Auto-mated Figure Formation for a Kirkpatrick-Baez Mirror," Proc.Soc. Photo-Opt. Instrum. Eng. 640, 164 (1986).

6. M. Zombeck, "Advanced X-Ray Astrophysics Facility (AXAF)Interim Report, Optical Constants and Reflectivities for Nickel,Gold, and Platinum in the X-Ray Region of the Spectrum (0.1-10keV)," SAO Report SAO-AXAF-83-016.

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x-ray performance of the lamar protoflight mirror

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