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Produced by the NASA Center for Aerospace Information (CASI)
(Di,1s,-Ch - li5b2b) lb C W A T I V r f'E S AA h CH I R T H E h85-13461

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SEACE: ASPECTS Cl GIAN! TEI SCCPIS IN SPACE;

Semiannual Status ReEcrts, 14 5ov. 1983 - 14 unclas

Nov. 1984 ( Arizcna Un iv., TUC;cn.) 15 p G3/69 14732

COMBINED SEMIANNUAL STATUS REPORTS

FOR PERIOD 14 NOVEMBER 1983 -- 14 NOVEMBER 1984

PREPARED FOR THE

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

GRANT NUMBER

NAGW-221

INNOVATIVE RESEARCH IN THE DESIGN

AND OPERATION OF LARGE TELESCOPES FOR SPACE:

ASPECTS OF GIANT TELESCOPE: IN SPACE

BY A^^ `^^y

AP^^gg5

J. R. P. ANGEL ° P.ECE,^4

r

Steward Observatory

University of Arizona t

Tucson, Arizona 85721

April 9, 1985
I. SUMMARY

Continued progress in many areas has been made during the second year

of the contract. Perhaps the most striking advances have been made in our

capability and understanding of how to finish the reflector surfaces needed

for large space telescopes. We have also made a big step in the technology

for making very light glass substrates for mirrors.

Other areas of development have been in wide field imaging design for

ve y fast primaries, in data analysis and retrieval methods for

astronomical images, and in methods for making large area closely packed

mosaics of solid state array detectors.

In the Third year we propose to concentrate largely on demonstrating a 

new concept for polishing fast aspherics and on continued development of

innovati-P techniques for automated analysis of very large numbers of CCD 

frames with 16 bit pixel accuracy.

2
I1. PROGRESS REPORT

1. Reflector Surface Finishing (Angel and Parks) . This work divides

into three domains -- producing machined glass surfaces with the highest

possible accuracy, suitable for LDR or l,)nger wavelength as machined;

methods of lapping and polishing to improve surface finish to the accuracy

for diffractir,. lirrited optical performance; still better finish for X-ray

reflectors .

(a) Machining. The most accurate machining is done by making in

situ measurements of the surface, and applying appropriate corrections.

Using a 'swing arm' generator of the type described by Angel and Parks

(1982) we have demonstrated that it is possible to generate off-axis

parabolic segments with a figure accuracy of 2 pm rins. The off-axis

segments were made with one-half meter in overall dimension, segments of an

F/1.2 parent. The surface finish obtained on these segments was

sufficiently gooti that the segments could be tested interferometrically

with a CO2 laser interferometer in a double pass autocollimating

configuration. This meant that immediately after a generating; run it was

possible to detremine the figure accuracy of the segments. If they met the

specification then the next step of lapping was done, if not an error

correction tape was made to the computer controller and a now generating;

run made. All this was done in near real-time. In fact the genernting rain

took only five hours.

3
4

After generating, the segments were lapped with a diamond abrasive

material bound into flexible pads. We found that the lapping material

worked very progressively, did not cause scratching, and did not cause

contamination in going from one grit size to another. We feel that the use

of this type of material is a great step forward over tradiational loose

abrasive lapping. The accuracy already demonstrated could be adequate to

finish LDR panels for diffraction limited operation nt 60j,. A factor two

improvement would reach the goal of diffraction limit at 30p.

In order to make panels of - 2m a larger generator is needed.

Continued work in this area will be conducted with a new large optical

generator by Campbell, now installed in the Optics shop at Optical Sciences

Center (Shannon and Parks 1983). This generator i;; large enough to do an 8

V meter diameter svmmetric mirror or segments of up to a 15 meter dish. It

r

E' appears from initial work that the large optical generator (L )G) should be

k.

able to produce work with ar. accuracy of 2 to 3 jim rms over 2 meter size

f

segments. Work will continue to improve the accuracy of the LOG for doing

contour work. Initial generating runs with the LOG have been very

promising, a plano pass on the back of a 72-inch borosilicate glass mirror

was sufficiently uniform and regular so as to produce a diffraction pattern

from the grinding marks. While this is a common ocourrance in diamond

turned optics it is the first time we have heard of this phenomenon in

glass and it is even more unusual in that it occurred in n generating pass

where 10 h.p. were being applied to the grinding Spindle. 11suelly in

diamong turned optics or diffraction grating ruling the greatest forces

involved are at the most a few grams.

Lam)
5

(b) Lapping and Polishing

Diffraction limited optical ,p erformance in glass requires

polished surfaces. Conventional polishing methods are very good for making

spherical surfaces, but can become extremely difficult and tendious if fast

or off axis aspheres are needed. Following our work on optical designs of

the past two years of this contract, which has yielded good solutions for

wide field imaging from very fast primaries, we have now devised a method

to make them. This is described in the attached pr(rl)rint by Ar:{!cl "A

Method for Polishing Aspheres as Fast a q f/1".

In order to accommodate the varying curvature of an aspheree surface,

current methods use either flexible or small laps. Flexible laps polish

highs and lows alike, and so the pitch distribution mast be varied to

correct figure errors. 'Phis is very tedious. Small laps also have no

tendancy to correct errors, but must be driven by computer to work on highs 1

identified by metrology. The problem is that very fast aspheres require

very small laps, and figuring times which increase as the inverse cube of

focal ratio become very long. Our proposed method is to use a rather stiff

lap, fitted with hvdraulic or motor driven actuators that can change its

cu^vature as it moves over the work. We have demnnstr:+ted a practical

arrangement of actuators that is capable of inducing ar,urately 1hr changes

of shape needed to conform to conic sections of revolution. The huge

advantage of this method is that the dynamically stressed lap will tend to

form the asphere as naturally a y it would a sphere if it wore used

passively. While the principles of tiiis method have been proven by our

static test, the method needs to be tested by actually polishing a fast

asphere.

0
6

The stressed lap method should he particularly valuable for future

space telescopes and optics, allowing the use of compact fast designs that

otherwise would be prohibitively difficult. Thus it should allow optical

polishing of LDR panels, to give the "light bucket" mode at 21,, even if the

reflector is f/0.5. It also will be a powerful method for polishing to the

diffraction limit 8m monoliths that are now under consideration for launch

in the shuttle fuel tank.

A further application we have recently considered is in the polishing

of the AXAF mirrors. Resolution in this telescope is not diffraction

limited, but limited b. , the surface finish. Presently this is expected to

give about 1/2 are second images. Stress lap polishing could improve this

to perhaps 1/10 are second.

2. Mirror Substrates (Angel -- shared funding with NASA .Ames)

Last year we reported the development of a new process to fuse

lightweight honeycomb. Taking manufactured cylindrical glass tubes as a

starting point, these were clo y : packed and filled with free-flowing

refractory sand. On firing, the sand pressure termed the tubes into a 

monolithic hexagonal honeycomh .

This year we have made a very substantial improvement in the process,

and it is now of great practical interest to glass ma nu facto rers. A copy

of an invention disclosure of the new process, attached, is now under

consideration by the Research Corporation. The new idea is to ,use air

pressure instead of sand pressure to press the tubes together.

Le
7

This is done by first heating a c..mplote assembly of tube: between face

sheets until the sheets seal to the tubes top and bottom. Then compressed

air is introduced into each scaled tube from a small hole in the back

plate. The tubes seal together to form an internal monolithic honeycomb,

while the face plate is prevented from bubbling up by a flat refractory

restraining plate. Gaskets of ceramic fiber prevent the glass from

sticking to the restraining; plates.

The new process has the advantages of (1) low thermal inertia, so the

process is fast and the fused glass joints do not fog due to

crystallization, (2) a complete flat sandAich blank is formed in a single

furnace firing, (3) the starting plates and tubes need no elaborate

machining. In fact we simply use the scribe and snap method to cut the

tubes, using a precision jig to get them all the same length.

While the process forms flat blanks directly, curved blanks can be

formed by slumping in a separate heating cycle. As a demonstration we

slumped to f/l.f, and finished a 70 cm honeycomb blank, which was then used

in the CCD transit telescope.

All the large blanks made by this method have been of borosilicate

glass, which softens and .'uses at about 840°C. The thermal properties of

the finished blanks are very good, and they have been finished to

diffraction limited performance quickly. Their thermal time constant is so

rapid, the optician does not have to wait to get a stable figure to test.

We expect that there will be applications in space for borosilicate honeycomb,

p► ,

11
9

particulary if their thermal environment is not too severe, and if optical

diffraction limited optical performance is not required. Nevertheless, it

would clearly be very desirable to show the process could be used for ULE

and silica, for which the softening point is about 1600°C.

We have made a series of experiments with these materials. The mF,in

problem is to find gasket material to eliminate chemical reaction and

sticking of the silica at such high temperatures. 7.irconia fiber Raper

looks promising. The alternative is graphite, bul this requires the use of

inert atmospheres or vacuu n. We have discussed this approach with

Corning, and they have the equipment to do it. At present we are not

planning to use or develor vacuum furnaces, and we viould be happy to see an

outside manufacturer pick it up.

3. optical Design (Epps, Woolf, Angel)

This year has seen a careful exploration by Epps of the limits of high

resolution imaging that can be achieved with refracture correctors used at

Cassegrair or Ritchey-Cretien foci. These may have an important role

in ST successor telescopes. The uncorrected RC focus is quit ,, limited in

field because of uncorrected astigmatism. We earlier explored 3 iirror

systems which can be superbly corrected, but are inconvenient in

inascessible foci. Epps design goal was a 1 1 field, an enormous advance on

the sT field of 2.7 are minutes. The achromatic correction is

extraordinarily good, and the monochromatic images very small. Thus the

i worst RMS image diameter anywhere in the field is 0.12 nre seconds, and the

worst energy concentration is 987, into 0.25 arc seconds.

r .

91
9

Although not reaching the diffraction limit for n large reflector, the

huge field will be a valuable asset for many astronomical problems. For an

8m primary diameter, the field diameter would be 24 inches, and the resolution

about 17 microns. This presents a challenge for CCD deb-ctors which is

addressed below.

4. Phvsical Optics (Woolf)

In the past year we have studied an nnalytic approach to 'tics process

of obtaining high resolution maps from elongated apertures. rho study

shows that there is a modest degree of information loss related to the

highest spatial frequencies, but tnat there is no major loss in 

signal/noise ratio at the resolu tic a of a disk of the same area. In

consequence the concept of sending an 18 x 4m monolithic mirror telescope

into space using the full size of the shuttle bay still seeir.s attruiotive.

As il;e multi-billion dollar cost associated with schemes fur large

telescopes in space becomes apparent, a solution that provides a single

space observatory for UV-O-111-sub-mm astronomy becomes attractive. The

logic for such a scheme is that the high resolution use at any wavelength

requires the use of the optical image forrr g d by the telescopes for

co-alignment and phasing, and therefore even sub-mm telescopes will benefit

from optics of "visible use" quality.

Such an observatory would use a number of monolithic mirrors in

configurations that were chnngeable in orbit. In this way there would be

potential for separate use as well as interferometric use for the complete

range of wavelengths. Schemes for such individual primary mirrors of such

47

--- - -

U
1+ 0

T 

a complete space observatory, include both 7-8m circular mirrors that could

be lofted in the shuttle fuel tank, and also the elongated mirrurs

discussed above. The most interesting techr ical questions posed by such

concept relate to the forms of synthetic aperture and their reiative

merits. Both linear arrays of apertures with the axis rotated in the plane

of the sky, and Golay like minimally redundant 2-dimensional apertures can

be considered . We will explore the possibility of generatin g; some kind of

figure of merit to compare the effects of both additing more apertures, and

placing th--m in different configurations.

Additional work this yenr has explored the possible use of the space

shuttle for a test for interferometric long-baseline space studies of th3

angular structure of objectF. it appeared that the shuttle environment

does not prohibit such an experiment, but that it requires a major effort 

to undertake such a project.

5. Ima ge Analysis (McGraw, Cawson)

Funds from the contract have been used to address aspects of building

and operating the CCD transit telescope that are relevant to space

telescopes. Thus the lightweight secondary and tertiary mirror blanks were 

inside with contract support. The full instrument hnd first light in June,

and now after very careful optical alignment is about to start rt!gular

operation, producing 400 megabytes of pixel-data each clear night.

The challenge now is now shifted to that of hnndling this huge mount

of very high quality data, extracting lists of objects with properties
sup :h as coordinate, colors, variabilit y . shape, etc. The software for

making and handling; of these lists will be an extraordi rill ry powerful t:.ol

not only for astronomical research and statistical studies but fc r the next

generation of automated guide star selection, and the annlysis of data from

wide field space telescopes.

(a) Software Development

With partial support from this contract, software is being

developed as described below. We have benefitted from the experience of

Mike Cawson, A postdoc from Kibblewhite's group tit Cambridge, i;ngland,

which is currently a world leader in the digital analysis of f,+ll Senmidt

plates. The system being developed represents a suhst,+ntial advance both

in software and hardware.

The raw pixel-data is to be arcl,:ved in Such a wny thi+l eneh pix,-1 n,n

be extracted in its original form -- ti,is requires dnta-co y paction to

reduce most 16-bit pixels to R-bit differences (making; use of the fact that

1

most of the sky is realtively flat), and the use of opticnl discs as a

storage medium. The raw data will he analyzed in real-tune to detect new

or changing objects. This analysis wil! produce lists of parameters for

all objects detected each night, and these lists will he u: ed to update

master-list sun,marizirg every measurement of each ohjeel, to produce a

history-list of the time-sequence of brightnesses of every ohiect, and to

compile a list of 'objects of ir,terest' . Periodically, the raw pixel-data

from many different nights' observations will be condded to reduce the
E'-^ C

12

photon and read-out noise, pr•o ,lucinp an ever deeper image of the region of

sky being sur v eyed. rhose ceadded frames of clam will themselves he

analyzed to detect objects fainter than the nightly detection threshold

whose parameters will be measured and stored in anotoer list .

The four CTI lists, together with c ther 'user' lists produced by

further procesFing of CTI or external data, will he accessibl y• through a

single data-channel so that any application program may address any type of

data transparently. In a similar manner the CTI pixel-files (raw and

I

coadded), together with 'user' pixel-files moy he accessed through a second i

single data-channel. This data-management scheme, whilst being; relatively

simple in concept, provides great flexibility for defining; new file-types

which are supported and extreme ease of use for astronomers who wish to

develop new analysis algorithms without having to learn r.bout a complex 

date-management package.

An interrogation program ha,; been written which provides integration

for extracting sub-sets (or super-sets) from the CTI or user lists of

object parameters, and which allows selection criter ; a to be defined

interactively usinl; graphical input and output.

'rhe CTI telescope and its associated data-base- provide nn ideal

test-bed for the development of a data-manngement Scheme kinclurUng

considerations such ns duta-compaction, storage media, f(-lxibility and

usability) which will be required more and mere as other optical

instruments (such as Space Telescope) start to produce extremely large

volumes of data on a regular basis.
13

F

(b) Ilardware 7

A dedicated Dntn General MV 10,000 computer will for• ,n ille

nu-lens of the system. It tins been our intention t(-^ us.-- funds fr„an the

second year contrc+et toward the purchase of peripheral image processing

hardware, capable of carrying out useful functions on full 16-hit CCD

irntiges. A survey of available equipment has revealed that no product

currently on the market has the required nrchitectur,^ or capability.

However, Dipix of Ca p ad+ A is about to release a cow product (the 11I'X-3)

which at Last seems to here the architecture necess p ry for processing, large

volumes of optienl data.

Up to iiow the development of image processing systems tin:; been striven

by Uie seismology, remote sensing and earth resources mtirkets which typi

have 8 or 12-hit pixel dntn. The Dip ; - prodr_•,t will have an extr,r,nely fnst

32-knit ring--bus archetecture (42 111 words/sec) with the capability of hadling

several special purpose arithmetic/logic card,3 for genuine 16-bit image

processing and up to 128 Megabytes of image data. This will be till first

time that it will he taster to extrrtet data froin IC)-!)it optical images ir, a

special purpose image processing unit than in a conventional computer. We

plan to use funds still remaining from the second y ear to buy part of a

Dipix system as soon ns it is nv„ilable.
14

6. CCD's (Leach, Lesser; Angel)

During the past year we have addressed the problem of making very

large focal plane mosaics of CCDs, in the 0.5m class. To obtain the

highest filling factor the connections and local electronics should all

occupy an area no larger than the sensitive area. Our conc e pt is one in

which the individual devices are bump-bonded to lands on a passive silicon

block (to match thermal expansion). Printed leads on this Mock pass

through a central hole to circuits and a connector underneath, all no

larger in area than the CCD area. The devices would be thinned after being

mounted to the blocks, so they would be optically flat and %vith exactly

controlled dimenisons relative to the block. The blocks in turn would be

exactly registered on a large optically flat substrate.

This concept has evolved from the work of graduate student Mike

Lesser, who ha.i made lab tests of many kinds of bonding. We have also

thinned some imaging devices. Largely with other support, Leach has put

into operation TI and GEC CCUs, with extremely flexible clooRinl; to allow

noiseless co-addition of adjacent pixels. We plan to use this set up for

further t,ssts in the coining year.

it
PUBLICATIONS FUNDED BY NGW-121

"Optical Matching `,)r Fiber Optic Spectroscopy", J. If, hill, J. Ii. P.

Angel, and E. If. Richardson, 1983, Proc. SPIT?, 445, 85.

"Steps Toward 8m Honeycomb Mirrors. IV. Some Aspects of Design and

Fabrication", J. R. P. Angel, N. J. Woolf, J. 71. Ifill, and L. Goble,

1983, Proc. SPIE, 444, 327.

"Adaptive Optics", N. J. Woolf, 1984, Proc. I.AU Symposium 079.

"Gone with the Wind, or Sailiag and Seeing with a Giant Teloscope", N. J.

Woolf and B. I— Ulich , 1983.

"Steps Towards 8m Mirrors V. A Method for Polishing Aspheres as Fast as

F/1", J. R. P. Angel, 1984, Proc. IAU Symposium 479, 'Very Large

Telescopes, Their Instrumentation and Programs', Garching, West Germany.

"Advanced Wid,--Field Broad-Passtarid Refracting Field Correetors for I,arge

Telescopes", H. W. Epps, J. :c. P. Angel, and I?. Andcrtiun, Proc. IAII

Svmposium 479,

"The Versatile Array", N. J. Woolf, J. R. P. Angel, and 1). W. McCarthy,

1984, Proc. SPIE, 444, 78.

INVF,N ION DISCLOSURE

"A Process to Make Honeycomb Sandwich Panels of Glass", J. R. I'. Angel and

P. A. A. Wangsness.

A ngel V Research