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Page 1: HUBBLE SPACE TELESCOPE: Mission Update

H U B B L E S P A C E T E L E S C O P E :

Mission Update

H . J O H N W O O D

Page 2: HUBBLE SPACE TELESCOPE: Mission Update

T he data collected f rom the Hubble Space Tele­scope (HST) today is of such good quality that researchers are s o l v i n g 25- to 30-year -o ld ast rophysica l quest ions w i t h as few as one photograph. For example, Figure 1 shows the HST image of the compact blue cluster at the center of the giant ionized hydrogen region 30

Doradus (the Tarantula nebula). For 25 years, astronomers have written research papers about the star at the center of this gigantic nebula in the Large Magel lanic C loud . Some even said that it could be a single object of 3000 solar mass­es. The first to show that this was possibly a compact clus­ter was G . Weigel t 1 and G . Baier w h o found eight stars wi th in a diameter of one arcsec. They took 8000 holograph­ic speckle interferograms using two telescopes at the Euro­pean Southern Observatory in Chi le and, after considerable computer processing, were able to show a reconstructed image wi th resolution 0.09 arcsec.

The new HST image shows that the cluster actually has over 3000 stars, the largest of wh ich probably have ind iv id­ual masses of 100 solar masses. The ultraviolet radiat ion coming from this massive compact (diameter 1 light-year) object is so strong that it ionizes gas and dust for many hun­dreds of light-years around the cluster. The HST image of the cluster has a resolution of 0.046 arcsec. N o computer processing of the image was necessary aside from combin­ing the three black and white images taken wi th three col­ored filters.

In the epoch 1837-1860, the central object i n the Eta Carinae nebula in our M i l k y Way Galaxy suffered a series of enormous outbursts. The explosions over several years were so strong that several solar masses of materi­a l were p u m p e d in to twin mushroom clouds expand ing at 200-300 km/sec . The new H S T p i c tu re o n page 10 shows the expand ing c l o u d s i n d r a m a t i c de ta i l . If Eta C a r i n a e was b o r n as a s ing le object, the behavior of the m a t e r i a l v i s i b l e nearby tells us that i t w o u l d have to be an object w i t h a mass of 200 s o l a r m a s s e s . Ast rophys ica l calcula­tions, however, tel l us that s u c h a l a rge object i s u n s t a b l e . Speck le in te r fe rome­try observat ions have ind ica ted that it m a y be mul t ip le objects, i n wh ich case the p r ima­ry object w o u l d have 100 and the three com­panions 60 solar mass­es each.

R E C A P OF T H E H U B B L E S P A C E T E L E S C O P E

P R O J E C T

The HST is a 2.4-m aperture Ritchey-Cretién reflector in a circular orbit of 615 km. First proposed by Lyman Spitzer in 1946, HST is the pinnacle of telescopes. Turbulence in the Earth's atmosphere means that even in the best mountain sites, large 8 to 10 m reflectors rarely achieve 1 to 2 arcsec­ond resolution. This is the equivalent to the diffraction-lim­ited performance of an 8- to 10-in (20-25 cm) telescope. Wi th adaptive optics as described by R. Fugate, 2 sub-arcsecond resolution can be achieved from the ground, but the field of v iew is restricted in size and the resolution degrades as the 3 /5 power of the cosine of the zenith angle. By comparison, HST can deliver star images wi th 0.007 arcsec jitter and bet­ter than 0.02 arcsec diameter throughout an observat ion period.

Another major advantage of HST is access to the vacu­u m ultraviolet and certain infrared bands of the spectrum that are blocked by molecules in the Earth's atmosphere.

Ready for a planned launch in 1986, H S T had to wait un t i l 1990 to f ina l ly be dep loyed i n orbit because of the Challenger tragedy. The mission was planned from incep­tion for 15 years w i th servicing visits every three years.

The First Servicing Miss ion to H S T occurred December 2-13, 1993 and was an exciting, albeit exhausting, television event for those who stayed up five nights w i th the astro­nauts dur ing their spacewalks. In fact, many people com­pare the mission to the first manned landing on the moon. The sequence of events was carried out just as Joe Rothen-berg 3 described i n his report in Optics & Photonics News' spe­cia l H S T issue (November 1993). A l l miss ion goals were

a c h i e v e d w i t h one exception: One of the o ld solar arrays had d a m a g e o n one bi­stem rod , cou ld not be rolled up, and had to be je t t i soned b y as t ronau t K a t h y Thornton.

R E S U L T S OF T H E

FIRST SERVICING

MISSION

The resu l t s of the S e r v i c i n g M i s s i o n O rb i t a l Ver i f i ca t ion (SMOV) testing were d i s c u s s e d at a c l o ­sure meeting chaired b y G e r a l d R e p a s s (HST Deputy Opera­tions Serv ic ing M i s ­s i o n M a n a g e r for N A S A ) M a y 9 of this year at G o d d a r d Space Fl ight Center. The optical results of the r e p a i r m i s s i o n can n o w be g i v e n . The corrective optics

Figure 1. The giant ionized hydrogen region 30 Doradus in the Large Magellanic Cloud 170,000 light years distant. The compact blue cluster was once thought to be a possible single supermassive star of 3000 solar masses. HST has shown it to be a compact cluster of hot blue stars of tens to hundreds of solar masses liv­ing in a volume of space 1 light-year in diameter.

1047-6938/94/7/08/06-$06.00 © Optical Society of America OPTICS & PHOTONICS N E W S / A U G U S T 1994 9

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H U B B L E S P A C E T E L E S C O P E

are per fo rm ing as des igned a n d have r e q u i r e d l i t t l e ad jus tment to ach ieve the specified optical performance.

WFPC II F i r s t l i gh t images s h o w e d s l i gh t de focus a n d s m a l l amounts of coma i n the four cameras . H o w e v e r , i t w a s clear to the scientists watch­ing that al l was okay wi th the W i d e F i e l d a n d P l a n e t a r y C a m e r a ( W F P C ) II. F i n a l focus was a c o m p r o m i s e between the best focus for the three wide fields (WF) ( f /12.9 and each 15-μm p i x e l sub ­tend ing 0.1 arcsec) and the planetary camera (PC). H igh ­er weight in focus was given to the P C at f/28.3, since it is near ly cr i t ica l ly sampled at 0.046 arcsec per pixel. A t the S M O V meet ing , C h r i s Bu r ­r o w s gave the f o l l o w i n g resu l ts for the w a v e f r o n t errors of the cameras as aligned: PC1=λ/18.8, WF2=λ/14.7, WF3=λ /9 .8 , and WF4=λ/11.9 . The speci f ied wavef ron t errors were λ /12.3 for P C and λ /12 .6 for the W F s . The sl ight out-of-spec performance of W F 3 and W F 4 results f r om the focus comprom ise w i t h the P C ; a l l c o u l d be brought into spec if the Optical Telescope Assembly (OTA) was refocused.

Because W F P C II has no focus m e c h a ­n i s m , the O T A sec­o n d a r y m i r r o r wa s m o v e d to b r i n g the W F P C II images in to focus. The f ina l move brought the instrument to a focus +19 μm rela­t ive to l a u n c h , w e l l w i t h i n the requ i red range for the F i n e G u i d a n c e Sensors to retain their pre-launch performance. The star images are so s m a l l that they are one pixel w ide for the WFs and several pixels wide for the PC.

Exposures of 3,000 seconds s h o w stars w i t h a v i s u a l m a g n i ­tude range 20.3 - 28.5 for the WFs and 19.7 -28.2 for the P C . The shortest exposure, 0.11

s e c o n d s , s h o w s stars between v isual magnitudes 9.2 - 17.9 for the W F s and 8.6 - 17.3 for the P C . The f ie lds of v i e w are 2.5x2.5 arcmin for the three WFs in an L-shape and 35x35 arc­sec for the lone P C . A l l four images are obtained at once for each exposure. For fur­ther d e t a i l of the i n s t r u ­ment's performance see the Instrument Handbook b y Chr is Burrows, 4 available on request from the Space Tele­scope Science Inst i tute i n Baltimore, M d .

A team c h a i r e d b y Charles Townes (UC-Berke­ley) rev iewed progress on the W F P C II as it was being bui l t and had the power to i n f l u e n c e the pro ject to revise the instrument design i n severa l va luab le w a y s . Notable among the contr i ­butions of the "Sage Pane l "

was the actuated fold mirrors that were adopted by the pro­ject in addit ion to a tip of the P C ' s C C D head by 3° to pre­vent ghosting. The performance of W F P C II had been pre­d i c t e d based o n t h e r m a l v a c u u m tes t ing ex tens i ve l y discussed in the Instrument Definit ion Team's Science Ca l i ­brat ion Report edi ted by the p r inc ipa l investigator John

Traugher (Jet P r o p u l ­sion Lab [JPL]).5

Opt i cs for the far u l t rav io let part of the spectrum are extremely sensit ive to sources of contaminat ion. Before launch, the team mem­bers h a d expec ted a slow U V obscuration of the w i n d o w s o n the four coo led-CCD chips. The c o n t a m i n a t i o n effect s h o w e d up as expected. Fortunately, by heating the chips to room temperature, the contaminant bo i l s off and the fu l l U V sensi­t iv i ty is restored. This p lanned decontamina­t ion cycle is current ly carried out on a month­ly basis. It is gratifying to note that the rate of contamination is s low­i n g d o w n g r a d u a l l y . This contaminant does

The great explosion in the center of the Eta Carinae nebula in our galaxy at a distance of 7000 light years. It blew up between 1837 -I860. We now see the twin mushroom clouds comprised of several solar masses of material expanding away at 200 - 300 km/sec.

Astronauts work to install COSTAR into NASA's Hubble Space Telescope (HST) during an extravehicular activity on STS-61, the first HST servicing mission.

10 OPTICS & PHOTONICS N E W S / A U G U S T 1994

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not affect the visible and red perfor­mance of WFPC II.

A linear 10% sensitivity grada­tion of the charge transfer efficiency over the C C D chips has been mea­sured on WFPC II. That is, stars tend to appear less than or equal to 10% brighter at one side of the chip than they do at the opposite side. A study is underway as of this writing to see if the variation can be reduced. The WFPC II science team is not con­cerned that the effect wil l cause loss of data, it simply must be understood and calibrated out since the goal is to be able to measure star brightnesses to 1% accuracy. Recent reduction of the operating temperature of the CCDs has virtually eliminated this effect.

Even before launch, some of the C C D pixels were known as "warm pixels." These are not useless parts of the CCD array—they are merely pix­els with elevated dark current (they perform as though they are warmer than they are). The warm pixels may be caused by cosmic ray damage to the silicon lattice. Warming up the chips and re-cooling does seem to improve their performance and prob­ably has an annealing effect on the lattice structure. Running the chips at a cooler temperature also improves their performance. Currently only one-tenth of one percent of the pixels are affected. The effect increases at a rate of 1% per year at a temperature of -78°C. The project has already cooled the chips to -88°C in an attempt to reduce this effect. Warm pixels can be calibrated out using lamps on board the instrument. As the contaminant is gradually removed, pro­ject members can gradually turn down the temperature thereby keeping the performance of WFPC II at an optimum level for years.

Confocality of WFPC II and COSTAR The Corrective Optics Space Telescope Axial Replacement (COSTAR) has a focus mechanism, so once the OTA was refocused for the WFPC II, the COSTAR deployable optical bench (DOB) was moved to bring the Faint Object Camera (FOC) images into their best focus. The final focus moves of the DOB by John Troeltszch (Ball Aerospace) showed the best images at 0 mm, the center of the ±10 mm range of motion of the DOB. Figure 2 shows the images of a star with the FOC before and after the COSTAR was deployed. This excellent result had been predicted from measurements using the Ball Image Analyzer by the Ball optics team including Joe Sullivan and Mike Kaplan.6 The NASA engi­neering directorate at Goddard provided an Independent Verification Team consisting of Wil l iam Eichhorn, Pam

Davila, 7 and Mark Wilson of the Goddard Optics Branch. They developed and used a Hartmann wavefront analyzer to verify the aberration correction and focus locations of COSTAR.

Confocality and straylight tests for the project were designed by the writer assisted by Sanford Hinkal and Carolyn Krebs (NASA) and were implemented by a team from Swales and Associates includ­ing Richard Harner, Wi l l iam Northcutt, and Qian Gong. The tests were carried out at Goddard when both instruments were mounted in a high fidelity mechan­ical simulator (HFMS) of the aft end of HST.

The "Gl in t Survey" was designed by the writer and imple­mented by Tim Zukowski and the glint team at Swales. It showed that optical interaction of reflected light from the mechanisms of COSTAR and the WFPC II pick off mirror mechanism contributed little to the flux at the OTA's exit pupil. There is no indication that COSTAR or WFPC II have any straylight prob­lems on orbit, since tests of faint star spectra with bright binary com­panions show no mixing of the light of the nearby companion. The HST specification on straylight is 23rd magnitude per square arcsec.

Confocal i ty testing of the COSTAR was aided by the FOC Structural and Thermal Model pro­

vided by the European Space Agency (ESA). Testing in the HFMS went smoothly and showed that, when latched into the telescope, the COSTAR instrument would function as designed. To measure the focus position of the WFPC II instrument, a telescope was developed to image an optical element called the "pyramid," originally designed to be at the focus of the OTA. The part of the test on WFPC II required imaging of the back-illuminated calibration spots called "K-spots" on the WFPC II pyramid by a CCD camera on the telescope. The K-spots were fainter than expected and an ambiguity in the focus calibration of the confocality telescope caused a 6-mm error in the first reported focus for WFPC II. Had this result been correct, it would have put WFPC II close to the range limit for the COSTAR DOB focus mechanism. There followed an intense period of activity to understand the 6-mm difference in the measured focus posi­tion of the WFPC II.

When the ambiguity in the confocality telescope (it has three foci 6 mm apart due to the use of a Fresnel mask over the objective) was discovered and understood by the God­dard, JPL, and contractor teams, it was clear that WFPC II was going to focus near the center of the DOB range. A l l

Figure 2. A comparison of star images taken with the ESA Faint Object Camera. The aberrated star image (top) has most of the light in a skirt 2-4 arcseconds in diameter. The COSTAR corrected image (bottom) has most of the light within a cir­cle 0.2 arcseconds in diameter.

OPTICS & PHOTONICS N E W S / A U G U S T 1994 11

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H U B B L E S P A C E T E L E S C O P E

agreed to recommend shipment of the WFPC II to Cape Kennedy.

NASA top management, however, insisted on an addi­tional focus check on WFPC II at the cape. A simple visual focus test of WFPC II was devised by Steve Macenka and the JPL team and was carried out in a cleanroom at KSC. It unambiguously verified that WFPC II would be in the mid-range of the DOB focus capability.

FOC + COSTAR The first light image with the FOC + COSTAR was within 2 arc seconds of where it was expected on the sky. George Hartig and Robert Jedrzejewski of the STScI found that only small adjustments were required of the built-in alignment actuators. The corresponding pupil shear is a fraction of 1 percent of the pupil diameter—enough to make the actua­tors necessary but small enough to tell us that the alignment work carried out on the ground at Ball (Ball Aerospace Sys­tems Division) was excellent.

A team from the ESA including Peter Jakobsen, Hein­rich Schroeter, M. Saisse, and B. Calvel also provided verifi­cation that COSTAR would adequately correct the HST spherical aberration using a structural/thermal model of the FOC.

At the SMOV closure meeting, Hartig reported that the flux in the central pixel of the f/96 camera is 4.3% of the total at 633 nm, which satisfies the requirement that it be ≥3%. The f/48 camera has been not fully functional for the

last year (it suffers from erratic high detector background), so its COSTAR-corrected performance could not be evaluat­ed. Jedrzejewski reported that pre- and post-COSTAR observations confirm that the reflectivities of the mirrors are as predicted to within ±5%.

COSTAR + Spectrographs Aperture transmittance is the criterion for good optical alignment for the spectrographs. For the Goddard High Resolution Spectrograph, the requirement was that the ratio of the 0.25 arcsec small science aperture flux to the 2.0 arcsec large science aperture flux shall be ≥60% for a point source at 633 nm. The ratio measured was 70%.

A similar result was achieved for the Faint Object Spec­trograph (FOS). The requirement was 0.3/4.3 (arcsec) aper­ture transmittance for a point source at 633 nm ≥60%. For the FOS, the ratio measured was 80% for the blue channel and 82% for the red channel.

INNOVATIVE ENGINEERING DEVELOPED FOR THE HST MISSION

Perhaps the most innovative effort for the servicing mission was developing phase retrieval methods based on images from HST's two on-board cameras to determine the HST's prescription. Aden and Marjorie Meinel (JPL) developed ways to measure spherical aberration using the size changes of inner and outer diameters of the pupil images in a series of through-focus images. Arthur Vaughan (JPL) used radial measurements of the "Arago spots" produced by the pads covering the three bolts on the front of the primary mirror as a measure of spherical aberration and focus. In addition, he produced an Arago mask for monitoring the JPL aberrat­ed beam projector called the "stimulus," which was used in

the thermal vacuum testing of the WFPC II. Daniel Schroed­er (Beloit College) developed ways to estimate coma and astigmatism in addition to spherical and focus from Arago spot measurements. Anthony Grusczak (Hughes Danbury Optical Systems—HDOS) produced movies of synthetic aberrated Hubble images showing the distortion of the aberrated images due to pure coma and astigmatism changes.

The corrective optics were manufactured by Tinsley Laboratories Inc. using computer-generated holograms (CGH) to produce the complex aspheric shapes required (see article, page 25). Robert Kestner (Tinsley) worked with Ray Cahill (Ball) and Jim McGuire (JPL) on the independent testing techniques. Three different tests assured the HST optics team that the optics were correct: precision profilom­etry, C G H interferometry, and conventional commercial interferometry with a refractive nul l corrector. Kevin Thompson (Optical Research Associates) developed soft­ware nulls to further correct the interferograms derived from the refractive null tests since there was not time in the schedule to design, build, and verify exact optical nulls for each of the optics.

A virtual prototype model was developed by Dennis Hancock8 (Lockheed) and the writer for the new optics to assure that the COSTAR and WFPC II would complement each other on board HST. Both mechanical and optical clear­ances were checked in advance of the build as soon as the computer-aided design drawings were available. This saved the cost and schedule slips associated with problems that might occur in actual hardware testing. The virtual proto­type was viewed in three dimensions, enabling project members to "fly" around inside and out of the COSTAR, WFPC II, and OTA riding on starlight beams.

The spherical aberration caused a much more stringent requirement on pupil alignment than was the case in the original OTA design. A one percent shear between the pupil diameter and the corrective optic gives more wavefront error than we had originally in spherical aberration. Thus, JPL was forced to add four new actuators to the WFPC II. The electrostrictive devices were developed by Jim Fanson9

and his team at JPL in a remarkably short time. COSTAR was also designed with actuators from the start since Murk Bottema10 recognized the need from the beginning.

HST'S SUCCESS STORY Agreement by teams of competent engineers and scientists after full and open discussion of the various test results was essential. The principal of independent optical testing and review worked quite well. It has resulted in restoration of the optical performance of HST. It was the lack of this type of open discussion of test results that caused the flaw in the reflective null corrector to go undetected—the flaw that caused the HST primary mirror to be manufactured incor­rectly.

A series of precision measurements of the optical and mechanical parameters of the reflective null corrector were devised for the Allen Commission by Dave Hansen, Joan Schwartz, and Tom Dubos (HDOS). When these measure­ments were put into the design software by Laurie Furey, it became clear that most of the error had been caused by the mislocation of the field lens by about 1.3 mm. This error

12 OPTICS & PHOTONICS N E W S / A U G U S T 1994

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M I S S I O N U P D A T E

caused the pr imary mirror to be pol ished too flat (about 2.2 μm at the edge).

Phase retrieval from eight independent teams was car­r ied out on special images taken w i th both the F O C and W F P C I. The most useful images were those taken far from focus on either side of focus.

The H u b b l e I n d e p e n d e n t O p t i c a l R e v i e w P a n e l (HIORP) oversaw the data from the measurements of the f l awed n u l l corrector at D a n b u r y and f r o m the phase retrieval analyses by half a dozen groups work ing on the aberrated images f rom orbit. N A S A and the astronomical commun i t y owe a debt of grat i tude to D u n c a n M o o r e , chairman, and George Lawrence, Diet r ich Korsch , A d e n and Marjor ie M e i n e l , and D a n Schul te, w h o formed the HIORP. This panel reviewed the data f rom the measure­ments of the f lawed nu l l corrector at Danbury and from the phase retrieval analyses by eight groups wo rk i ng on the aberrated images. They had a total of 10 meetings at month­ly intervals and announced the prescription for the the H S T exactly a year after they had first convened. The prescrip­tion value for the pr imary mirror conic constant measured from the new images is wi th in the error bar publ ished by Moore, 1 1 namely K = -1.0139 ± 0.0003 (1 sigma).

Henry Sampler and Bi l l Eichhorn of the Goddard Opt i ­cal Test Section worked wi th a team from NSI Technology Services Corp. including Kev in Redman, Dan Mus insk i , and Tom French to character ize the H F M S and the p r ima ry metrology calibration standards for the opto-mechanical sys­tems on the spacecraft. Their work led directly to the excellent precis ion w i t h which the new instruments were verified to be in correct alignment in the HST. The NSI team worked we l l at JPL and Ba l l throughout the b u i l d of W F P C II and C O S T A R to he lp and assure that the metrology was right. Then at Godda rd they p rov ided the ver i f icat ion that the H F M S was a true standard representation of the orbiting aft shroud of the HST.

H S T ' S F U T U R E

H S T ' s f i rst Serv i c ing M i s s i o n has not only repaired the f lawed optics and the other subsystems that needed work , it has validated the concept of servicing a scientific satellite on orbit. After a large ini t ia l investment i n the telescope and spacecraft, the payoff i n scientif ic pro­duct iv i ty w i l l be real ized. HST, un l ike single-mission spacecraft, has an infra­s t ruc tu re of g u i d a n c e , p o w e r , a n d telemetry that is reused on each succes­sive mission. Thus, resources can be con­centrated on b u i l d i n g science ins t ru ­ments and science data analysis.

M i n i m a l deg rada t i on of the tele­scope during the first years in orbit, and p lanned future ins t rument upgrades , w i l l enable H S T to cont inue to re turn outstanding results in astronomy. HST is expected to pe r fo rm w e l l b e y o n d the

planned 15-year mission in the most cost-effective manner ever attained in the history of space astronomy.

R E F E R E N C E S 1. G. Weigelt and G. Baier, "R136a in the 30 Doradus nebula resolved by

holographic speckle interferometry," Astron. Astrophys. 150, L18-L20 (1985).

2. R.Q. Fugate, "Laser Beacon Adaptive Optics," Opt. & Phot. News 6, 14 -19 (1993).

3. J. Rothenberg, "Hubble Space Telescope: Restoring the Image" Opt. & Phot. News 11, 10-16 (1993).

4. C. Burrows, "Hubble Space Telescope Wide Field and Planetary Camera 2 Instrument Handbook," Version 1.0 dated March 1993.

5. J. Traugher, "WFPC2 Science Calibration Report," Prelaunch Version 1.2, 2 November 1993.

6. M. Kaplan and J. Sullivan "COSTAR Pre-Ship Review," Presentation Package by Ball dated June 29, 1993.

7. P. Davila et al., "Final Report on the Aberrated Beam Analyzer Test of the COSTAR Optics," Hubble Space Telescope Independent Verification Team report dated November 18, 1993.

8. D. Hancock, "'Prototyping' the Hubble Fix," IEEE Spectrum 30:10, 34-39 (1993).

9. J. Fanson and M. Ealy, "Articulating Fold Mirror for the Wide Field/ Planetary Camera II," SPIE 1920 -37, Albuquerque, N.M. 1993.

10. M. Bottema, "Reflective correctors for the Hubble Space Telescope axial instruments," Appl. Opt. 32, 1768-1774 (1993).

11. D. Moore, "Final Report of the Hubble Independent Optical Review Panel," see official prescription on page 34 dated HJW:92.12.14 and asso­ciated discussions of the statistics, N A S A publication P-442-0078.

H . J O H N WOOD is HST Optics Lead Engineer at NASA God­dard Space Flight Center, Greenbelt, Md.