extremely achromatic f/10 all-spherical camera constructed for the high-resolution echelle...

Extremely achromatic f/1.0 all-sphericalcamera constructed for the high-resolutionechelle spectrometer of the Keck telescope

Harland W. Epps, and Steven S. Vogt

We present a very largef/1.O prime focus, all-spherical, all-fused-silica catadioptric camera. It contains atwo-element airspaced corrector, anf/0.76 primary mirror, and a singlet final element. It accommodatesa chromatic range from 0.3 to 1.1um or more without refocus. It is optimized with an external entrancepupil but can be reoptimized for other pupil distances. In spite of its 30-in. (76.2-cm) focal length, itdelivers 12.6-pm (rms) average image diameters to a 3.6-in.- (9.1-cm-) diameter flat focal surface. It isthus well matched to the (7-15-ptm) pixels and to the size of a (2 x 2) mosaic of today's largest availableCCD's.

Introduction

The Keck 10-m telescope, a joint venture of theCalifornia Institute of Technology and the Universityof California, is currently nearing completion atopMauna Kea in Hawaii.' An initial complement offive first-light instruments is being built for the Keck.One of these is the high-resolution echelle spectrom-eter (HIRES). The HIRES design concept and pro-posal were developed by one of us (Vogt), the principalscientist on this project. HIRES was designed andconstructed at University of California Observa-tories/Lick Observatory of the University of Califor-nia at Santa Cruz and was commissioned on thetelescope in June 1993.

Because a telescope, even the world's largest, isonly as good as the instrumentation attached to it,considerable thought and effort have gone into HIRES.The larger the telescope, the more difficult a spectrom-eter design becomes, mainly because larger telescopesrequire correspondingly larger instruments. Forhigh-resolution wide-band grating spectrometers,stepping up a factor of 2 or 3 in size from today's 4-mclass telescopes to a 10-m telescope pushes wellbeyond present limits of optical design and into the

The authors are with the University of CaliforniaObservatories/Lick Observatory, and the Board of Studies inAstronomy and Astrophysics, University of California, Santa Cruz,California 95064.

Received 8 March 1993.0003-6935/93/316270-10$06.00/0.© 1993 Optical Society of America.

domain of large ultrafast cameras. The design prob-lems can be reduced by image slicing or pupil slicing,but these techniques introduce other limitations.The camera for HIRES is itself a large telescope, withoptical lenses and mirrors some 30-44 in. (76.2-111.76 cm) in diameter. Most optical glasses areunavailable in such large sizes, and thus the opticaldesigner, who is trying to bring an enormous range ofcolors down to precisely focused images, has a limitedrange of useful optical materials to work with.

Since the main purpose of this paper is to describethe camera required for HIRES, only a brief descrip-tion of the spectrometer itself is presented here. Arecent description of HIRES was given by Vogt.2A schematic view of the HIRES optical layout isshown in Fig. 1. Basically the instrument is anin-plane echelle spectrometer with grating cross dis-persion provided after the echelle.

Conventionally, in designing a camera, one encoun-ters a fixed entrance pupil located at or near thecamera mouth. Quite a large variety of spherical,aspheric, and hybrid catadioptric systems of this type(Schmidt, Bouwers-Maksutov, Wynne divided menis-cus, etc.) have been invented and developed over theyears (see Ref. 3 for an overview). Most of theseapproaches rely heavily on concentricity and/or asphe-ricity of the elements to control the dominant third-order aberrations, and they generally require theentrance pupil to be at or near a common center ofcurvature. Furthermore these designs have not gen-erally carried aberration balancing much beyond athird-order approximation and have thus been rela-

6270 APPLIED OPTICS / Vol. 32, No. 31 / 1 November 1993

Fig. 1. Top view and side view schematics of the HIRES. Light from the telescope enters from the left.

tively limited in numerical aperture and in colorcorrection.

However, the pupil presented to the camera of theHIRES is quite complex because of anamorphism andvignetting at the gratings and because of the separa-tions between the echelle, the cross disperser, and thecamera. The echelle is a 12 in. x 48 in. (30.5 cm x121.9 cm) mosaic, which accepts a 30.5-cm-diametercollimated beam without vignetting and, being blazedat 69°, produces elliptical monochromatic beams withdimensions of 30.5 cm x 48.26 cm. The spectralformat of the cross-dispersed echelle is trapezoidal,and different positions on the echelle format give riseto different positions for the elliptical beam at thecamera mouth.

Figure 2 shows a map of the echelle format in

second order of the cross disperser. Echelle ordernumbers are listed down the left of center, andcentral blaze wavelengths (angstroms) are listed downthe right of center. The central bold portions of eachorder indicate the echelle-free spectral range in thatorder. The quadruple box patterns illustrate theformat of the CCD detector mosaic (with a fewhypothetical bad pixels and bad columns added forrealism) at two different cross-disperser settings.The detector format shown here is that of a 2 x 2mosaic of Ford 20482 CCD's with 15-lim pixels. ATektronix 20482 CCD with 24-[im pixels is also underdevelopment for HIRES. Eventually, a 2 x 2 mosaicof 40962 CCD's with 7.5-jim pixels may also be used.

The beams that arrive at the four corners of thelower box pattern (corresponding to wavelengths of

1 November 1993 / Vol. 32, No. 31 / APPLIED OPTICS 6271

+ I I I I I I I I I I I I ..I -!- -- --.. .....I ..I ' I. -- --- -- -- ---.. .. . ... .. .

..... .......... 4

f ......0-~~~~~~~~~~ 8 5~~~~~~~~~~~4 t 2 9 4 r~~~~~~...... .............. ..-.4- - .... .......

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86.l7

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. ......--.- I I I I I I III I I I IL-100 -80 -60 -40 -20 0 20 40 60 80 100

[mm]

Fig. 2. HIRES echelle format in second order of the cross disperser. The (2 x 2) Ford COD mosaic detector format is shown by the boxesfor two settings of the cross disperser. Open and filed circles denote the positions of various interesting wavelengths. Filled rectanglesdenote areas of bad pixels on the detector mosaic.

3040, 3100, 4484, and 4570 A) are shown in Fig. 3 asthey enter the camera, illustrating the need for alarge camera aperture. The apparent vignetting iscaused by the cross-disperser. The entrance aper-ture of the camera must be almost 16.2 cm indiameter to accept all the light from various fieldangles over the detector format. The aperture is

20

1 0

x 0

-10

-20

PUPIL AT CAMERA ENTRANCE

-20 -10 0

Y10 20

Fig. 3. Map of the beam entering the camera at wavelengths of3040, 3100, 4484, and 4570 A. These four wavelengths corre-spond to the four corners of the lower CD format in Fig. 2.

underfilled for any monochromatic beam, but thebeam sweeps out a large area as the wavelengthvaries.

Further complicating the pupil situation, the en-trance aperture of the camera must be located some85 in. (215.9 cm) beyond the compound effective pupilformed by the echelle and cross disperser for thecamera to clear the incoming light to the crossdisperser (as can be seen in Fig. 1).

A camera focal length of 76.2 cm is necessary toreimage the entrance slit at the appropriate scale onthe detector. Thus an f1.0 camera of- 76.2-cmaperture is required, fed by a pupil that sits approxi-mately three focal lengths ahead of the camera. Therequired field of view is some 6 to 8 in diameter.Now, despite its rather large physical size and veryfast (f/1.0) optical speed, we also require that theimage quality be such that we can take full advantageof the CCD detector's 15-jim pixel size. In thefuture large-area CCD's with pixel sizes even smaller(7.5 jim) are anticipated, and we would also like to beable to take advantage of these. So very severeconstraints are put on the camera design by way ofthe compound pupil, the large physical size, the fastnumerical aperture, and the required image quality.

There are yet further demands on the cameradesign. To take full scientific advantage of the excel-


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lent UV transmission of the Mauna Kea observingsite and to exploit the broad range of sensitivity ofsolid-state optical and IR array detectors, the cameramust be able to function over an extremely large colorrange, from 0.3 jim in the UV (where the atmospheretransparency becomes the limit) to at least 1.1 jim inthe IR (the response limit of silicon CCD's). In factone would also like to have reasonable imaging perfor-mance all the way out to 2 im (where thermalbackground effects become important) since IR ar-rays sensitive in this region are now becoming avail-able. Furthermore, since all such optical and IRdetectors are generally flat, this camera must have aflat focal surface. Since the echelle format can de-liver an enormous color range of information to thedetector in a single observation, one cannot permitthe focus to shift with wavelength. For example, theformat covered by the detector as shown by the lowerCCD format in Fig. 2 runs from 3000 to 4600 A,corresponding to a chromatic range of four units[where the standard F-C (n4861 - n6563) index gradientequals one unit]. Dealing with this enormous chro-matic range is made especially difficult since mostglasses that might provide the necessary color correc-tion become opaque to light below 0.4 jim and are alsonot available in the large diameters (76.2 cm) neededfor this camera.

Preliminary optical designs for this HIRES camerawere presented by Epps.4 In that paper he describedtwo highly aspheric systems that could meet thespecifications. In this paper we review these designsand then present an all-spherical version that isvastly easier to manufacture, yet achieves a similarimaging performance.

Camera Design Process

At first thought, the optical design requirements forthe HIRES camera appeared to be impossible to meet.The largest, fastest, broad-passband, all-refractingcameras that have ever been designed45 are onlyapproximately one fourth to one third of the requiredsize of the HIRES camera and already push the limitsof manufacturability. Using conventional Schmidtdesigns, which would operate only over a limitedwavelength range because of the excessive opticalspeed required, one needs some three cameras tocover this large color range. Moreover the camera'seffective entrance pupil (as formed by the spectro-graph collimator and the dispersion of the gratings) isconstrained to a location some three focal lengthsahead of the camera's first optical element. There-fore the fundamental principle of the optical correc-tion in a Schmidt camera is strongly violated. At theouter field angles one would not expect excellentimaging from a Schmidt camera at any wavelength.

These limited expectations for Schmidt cameras forHIRES were confirmed through exhaustive study.The best we were able to achieve with a pair ofSchmidt designs covering the 0.31-1.1-jm range wastypically 34-jim (rms) image diameters, with images

growing as bad as 68 jim at full field. Such poorimage quality would have seriously compromised thescientific performance of HIRES at high spectralresolution. This loss of performance is equivalent tohaving a smaller telescope. Indeed, when the instru-ment's performance is poor enough, one can easilyimagine a smaller telescope, equipped with a betterinstrument, outperforming the large telescope withits mediocre instrument.

In January 1987 we discovered that a variation ofthe Schmidt configuration could be designed with therequired entrance pupil size and location, field ofview, and optical speed, which produced outstandingimages over the full 0.31-1.1-jim chromatic intervalwithout refocus. The new configuration required athick (singlet) corrector lens, which was placed well infront of the primary-mirror center of curvature andwhich had high-amplitude aspherics on both sides.The aspheric surfaces were chosen so as to counteractone another so that the net difference between theiraspheric contributions to the aberration correctionwas used to counter the large amount of sphericalaberration from the (spherical) primary mirror. Italso countered the large amounts of coma and astig-matism present by reasons of the displaced entrancepupil and the positive optical power required in thecorrector. A lens group was located near focus whosepurpose was to flatten the field to accommodate theintended flat detector and to complete the higher-order aberration correction. Our initial designs pro-duced typical rms image diameters of < 20 jLm aver-aged over all field angles and colors. However, therequired maximum aspheric deviations (MAD's) onthe corrector lens surfaces exceeded 0.20 in. (0.5 cm),and our attempts to reduce these aspherics to manu-facturable amplitudes were not successful. At thispoint the spectacular image quality obtainable withthis design looked like little more than a numericalcuriosity.

In November 1988, Epps developed a method ofbounding the MAD that can occur on a surface duringthe optical design process without artificially bound-ing any of the degrees of freedom that determine saiddeviation (vertex curvature, aspheric coefficients, sur-face diameter, etc.). This method amounts to deopti-mizing the optical system in such a way that smallpenalties in image quality result in large reductions ofthe maximum aspheric deviations that are present inthe system. The mathematical details of this meth-odology is the subject of a subsequent paper. Thesystem prescriptions presented here use the followingconventional equation for aspheric surface sag (z)versus radial distance (y) from the part center:

1+= [1-+A 4 +A 6Y + A 81 + [1 - C 2y 2(1 + A2)]1/2 Y

where C is the curvature (in inverse inches).Once this new design tool was in place, we reexam-

ined the camera design to see if we could reduce the


EPPS/ASTRONOMY/UCLARUN NO. 6464 (02/08/89)

(3100 A TO 11000 A) WITHOUT REFOCUS

ALL DIMENSIONS IN INCHES

FLAT WINDOW -

FOCAL PLANE(FLAT)

R - 0 61.S612

36.78

PRIMARY MIRROR

-FUSED SIUCA

1 ASPHERIC (MAD = 0.0102)

A2- -1.0000000

ZASPHERIC (MAD - 0.0200)

A2 = 0.0

A4 - 1.4678973 X 10-5 A4 9.2644226 X 1o-6

A 6 -1.4748299 X 10o8 Ag -2.6206082 X 108

A 8 - 4.0543072X 10.11 A8 - 3.4336354X 10.1

KECK TELESCOPE: 30.0 - INCH FOCAL LENGTH CAMERA MODEL 1 FOR HIRESFig. 4. Camera design 6464, showing a 6° diameter field-of-viewf/1.04 system with a highly aspheric singlet corrector, triplet lens group,and Dewar vacuum window.

aspheres to a level at which the camera was buildable.We subsequently obtained a good set of designs forboth 60 and 8 field diameter cameras. The former,Model 6464, is shown in Fig. 4. It is an f/1.04camera optimized for an 8-cm-diameter unvignettedfield at its flat focal surface. The latter, Model 2503,is shown in Fig. 5 and is an f/0.94 system, whichcovers a 10.7-cm-diameter unvignetted field. Bothdesigns use a contact-triplet lens group as the lastoptical element and a flat vacuum window for thedetector Dewar.

The image quality of these designs is shown in Fig.6 and is even better than that of our original discov-ery design studies. More important, we were alsoable to reduce the aspheric amplitudes from 0.5 cm to

0.03 in. (0.0762 cm), which is still quite difficult,but at a level perhaps within range of known opticalfabrication methods.

Since the image quality of these designs was astrong function of the thickness of the corrector, andwe were already up against the thickness limit foravailable large blanks of fused silica [ - 5.5 in. (13.97cm)], we also explored designs in which the corrector

was split into an airspaced doublet. Doing so yieldedspectacular image quality, typically 9.3 jim (rms)averaged over all colors and field angles. Unfortu-nately two aspheric surfaces were still required, andthe MAD's were still quite large [0.020 in. (0.05 cm)].We were not able at that time to reduce themsignificantly or to reduce the steep curvatures.Since the image quality of this design was now muchbetter than required, we moved away from thisairspaced doublet corrector approach.

A further round of design optimization of thesinglet corrector system occurred in June 1990, atwhich time we set the field to 7.20 and eliminated theflat vacuum window by using the last lens groupinstead as the Dewar window. With the addition of aNaCl element, the last lens group became an all-spherical contact quartet. This addition providedadditional color correction and permitted us to reducethe MAD's on the corrector to 0.006 in. (0.015 cm)and 0.012 cm (0.03 cm) on the front and back,respectively. These MA.D's were still quite challeng-ing but even more manufacturable. This design isModel 7840 and is shown in Fig. 7. It features image


ENTRANCE PUPIL85.00(PARALLEL LIGHT)

1 .2

EPPS/ASTRONOMY/UCLARUN NO. 2503 (02/07/89)

* (-) t3.8st2 (3100 X TO 1100 A) WITHOUT REFOCUS

R . (.) 20.6146 ALL DIMENSIONS IN INCHES

R - I 133.2610

_4ASPHERIC (MAD - 0.0028)

A2 - 0.0

An ~Uadls 1 1-

I A 4 A -9.9 683481 X 1icAr = 2.1668342X 106

-FUSED SILICA

1 ASPHERIC (MAD ' 0.0193)

A2= -1.0000000

A4 = 1.2639712 X 10-5

A 6 '-9.6189839X 10-9

A 8- 2.5603906 X 10-1 1

2 ASPHERIC (MAD - 0.0350)

A2 - 0.0

A4 - 8.7250563 X 10.6

A6 - -1.7167959 X 1 8

A 8. 2.2338066 X 10-11

KECK TELESCOPE: 30.0 - INCH FOCAL LENGTH CAMERA MODEL 2 FOR HIRES

Fig. 5. Camera design 2503, showing an 8° diameter field-of-view f/0.94 system with a highly aspheric singlet corrector, triplet last lens

group, and Dewar window.

KECK TELESCOPE: IMAGE QUALITY FOR HIRES CAMERAS WITHOUT REFOCUS

U.I-w

0UJI

CO

r

20

10

3000 4000 5000 6000 7000 8000 9000 10000 11000WAVELENGTH (A)

20

10

3000 4000 5000 6000 7000 8000 9000 10000 11000

WAVELENGTH (A)Fig. 6. Image quality for the camera designs of Fig. 5. FOV, field

of view. Filled points are image diameters calculated from spotdiagrams at selected wavelengths. The dashed curves representimage diameter quality for worst-case images. The solid curves

represent on-axis image diameters.

diameters of 13.5 m (rms) averaged over all colorsand field angles, with worst-case images of 21 m atextreme wavelengths and field angles. We were stillworried about the difficulty of fabricating the steeplycurved, highly aspheric corrector, particularly be-cause these difficult aspherics were also required to behighly coaxial.

Epps installed a new color-correction techniqueinto his optical design code (OARSA), which was thenused for a final round of design optimization inNovember 1990. We returned to the airspaced dou-blet corrector configuration (which had much betterimages than is required for HIRES) and forced theMAD's downward in a balanced fashion while control-ling the image quality degradation uniformly acrossthe camera's full spectral range. In doing so, wewere able to reach a solution in which all aspherics inthe design could be eliminated while retaining imagequality that meets the HIRES specifications. Inaddition, we were able to replace the contact-quartetlens group mentioned above with a single thick fusedsilica meniscus.

The final design selected for construction is Model


FUSED SILICA

FLAT WINDOW

OPTIC

AXIS

ENTRANCE PUPIL85.00(PARALLEL LIGHTI

42.08

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DETAIL AFIELD FLATTENER

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EPPS DESIGN 7840 (6/7/90)7.2° DIAMETER FOV

ALL DIMENSIONS ARE IN INCHES

Fig. 7. Camera design 7840, showing a 7.20 diameter field-of-viewf/0.98 system with a highly aspheric singlet corrector, a contact quartetlast lens group, and no Dewar window.

7465 and is shown in Fig. 8. The overall imagequality for this design is a 12.6-pm (rms) imagediameter, as averaged over all colors from 0.31 to 1.1pm, and over all field angles out to a field diameter of6.70.

ENTRANCE

19.090-

In adopting this final design, we have in effectaccepted two additional dielectric boundaries (withpotential reflective losses) for the luxury of having anall-spherical design and with the added luxury ofhaving a simple singlet final element. This singlet is

ALL DIMENSIONS ARE IN INCHES ALL SPHERICAL SURFACES

EPPS DESIGN 7465 (11/15/90)Fig. 8. Camera design 7465, showing a 7.2° diameter field-of-view f/1.02 all-spherical system with a simple fused silica final element,which also serves as the Dewar vacuum window.


ASPHERIC COEFFICIENTS

SURFACE RADIUS

1 (ENTRANCE P

2 +25.4892

3 +23.2963

4 -62.4262

FIELD FLATTENER QUARTET

SURFACE RADIUS MATERIAL

5 -14.7176 FUSED SILICA6 +30.7056 NaCI7 +30.8976 CaF28 +6.9542 FUSED SILICA9 +27.2671

_

FIELD RADIUS10.67

(15/70)

(12/79)

(9/94)

(9/95)

(9/95)

(11/92)

4,.(12/77)

(15/62)

(17/53)

(18/46)

(21/33)

(39/4)

HIRES SPECTROMETER CAMERA: 30" f.I f/1.0 SYSTEPPS/LICK OBS/UCSC, MODEL RUN NO. 7465 (11/15/90)FIXED FOCUS; LATERAL COLOR TO SCALE

RMS IMAGE DIAMETER (MICROMETERS)/ENERGY IN 15 sm DIAMETER (%)

'EM

15 Em t

NOMINAL DETECTOR PIXEL

Fig. 9. Spot diagrams at a fixed focal plane for camera design 7465 for various combinations of wavelengths and field radius angle. Image

scale is shown by the 15 pum x 15 plm square box, which represents a typical CCD pixel size.


00 20.50 3 035

3000 A

3100 A

3400 A

3838 A

4570 A -

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7500 A -

8500 A -

9500 A -

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11

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also ideally suited for use as a Dewar vacuum windowsince it is quite thick and easy to manufacture.Furthermore the relatively great distance betweenthe rear surface of this final element and the focalplane facilitates the packaging of the detector in acryogenic Dewar. The small amount of additionallight loss in the camera because of reflections at thetwo extra air-glass interfaces is of little consequenceand will be minimized by using broad-passband solgelantireflection coatings on all the camera lenses.6 ,

7

Optical Performance

Spot diagrams at a single fixed focal plane for cameraModel 7465 are shown in Fig. 9. Here the camerawas fed rays from a uniformly illuminated 15.0 in. x19.1 in. (38.1 cm x 48.5 cm) elliptical pupil situated215.9 cm in front of the first corrector element vertex.The image scale is shown by the inset 15 m x 15 mbox, which represents a typical CCD pixel size.Images in the leftmost column are on axis, whilethose in the rightmost column are at full field.Wavelengths run from 0.30 m at the top to 1.8 m atthe bottom. The system was actually optimized foruse only over the 0.31-1.1-jim range, although spotsat 0.30 and 1.8 jim are included here to illustrate theextremely high degree of achromaticity achieved be-yond the primary passband, even without refocus.

Further achromatism is possible beyond both blueand red extremes of the design range with designreoptimization and/or some refocus with little pen-alty in overall image quality. In practice, however, ifan IR array is installed in this camera, it will reside inits own separate Dewar, and we will accordinglyreoptimize its final element (which is also the Dewarwindow) to obtain peak performance in the IR.

Discussion

Optical system designs are often discussed in terms ofhow the dominant third-order aberrations are bal-anced by the various optical elements. Unfortu-nately, it is not possible to explain, in simple third-order aberration terms, how the aberrationcontributions have been balanced to achieve the highdegree of image quality and achromatism in thiscamera. We note that the highly curved sphericalprimary mirror contributes large amounts of spheri-cal aberration, coma, and astigmatism. These arebalanced out to a large extent by correspondingopposite-sign spherical aberration, coma, and astigma-tism at the rear (concave) surface of element 2 (thesecond corrector lens). Beyond that, the other sur-faces contribute less, but each contributes signifi-cantly to the overall aberration balancing. In total,the Seidel aberrations are not particularly well cor-rected in this camera, which is not surprising consid-ering its extreme optical speed.

This camera design can also be reoptimized toaccommodate other pupil distances. For example,15-in. (38.1-cm) focal-length versions of this camera

have been designed that use a 21-in. (53.3-cm) pupildistance.8

While we were unaware of this point during ourdesign development, it was subsequently brought toour attention by a referee that our all-sphericaldesign appears to be similar to a Houghton camera.910

Indeed the Houghton design and our all-sphericalmodel both feature a nearly afocal corrector-lensgroup of a single glass type near the radius ofcurvature of the mirror. However, the representa-tive sampling of Houghton-derivative designs ana-lyzed in Ref. 10 is much slower than ours in opticalspeed ( f/3 to[/10 versus[/1.0). Furthermore, theseHoughton derivatives are corrected only over a lim-ited color range, whereas our design is highly cor-rected from the UV to the IR and without refocus.In addition, our design features a flat field of viewwithout sacrificing field angle or color correction.Finally, we emphasize that the thick element near thefocal surface in our design is essential to all aspects ofthe optical correction.

With due respect to the Houghton patent, we couldnot have arrived at our all-spherical design by balanc-ing third-order aberrations. Instead, in this paperwe describe in some detail how this all-sphericaldesign evolved from (and is thus a derivative of) ourthick-meniscus, double-aspheric, extremely achro-matic prescriptions. We are not aware of any Hough-ton derivative that comes close to the overall perfor-mance of the all-spherical system that we present.

Summary

We have presented a set of designs of very fastcameras that feature an extremely high degree ofachromaticity and image quality. These camerasdeliver spectacular image quality over a 6 to 80diameter field of view and over a spectral range of0.3-1.1 jim or more. They feature a flat focal sur-face and no refocus over this spectral interval.Because they use only fused silica for the transmit-ting elements, these cameras possess very hightransmission throughout the UV, optical, and IRregions. Furthermore, since fused silica can be ob-tained in blanks up to 50 in. (127 cm) in diameter,these cameras can be scaled up and fabricated in quitelarge sizes. Our initial designs used large-amplitudeaspheres to achieve the high degree of aberrationcontrol, but our final design achieved the same levelof image quality using all-spherical surfaces. Thedesigns are optimized for use with pupils that liesome three focal lengths ahead of the camera andthus are nicely suited to cross-dispersed echelle spec-trometer applications or to any other applicationrequiring a displaced camera pupil. Moreover thebasic design can be reoptimized to accommodate quitea range of pupil distances.

We believe that this new family of camera designswill find great use in many applications involvingnext-generation large telescopes and their auxiliaryinstrumentation. Today's most modern solid-statearray detectors are now achieving high responsivity


over a large spectral range from the deep UY all theway into the IR. This family of extremely achro-matic cameras, which can be fabricated easily inalmost any size, seems an almost ideal optical-imaging system for such detectors.

We thank the University of California Office of thePresident for ongoing computer support funds thatmade this and other optical-design research possiblethrough the University of California at Los AngelesOffice of Academic Computing. The optical designwork presented here was done for the HIRES projectunder contract to the California Association for Re-search in Astronomy.

References1. J. E. Nelson, T. S. Mast, and S. M. Faber, "The design of the

Keck Observatory and Telescope," Keck Observatory Rep. 90(Lawrence Berkeley Laboratories, Berkeley, Calif., 1985).

2. S. S. Vogt, "HIRES: a high resolution echelle spectrometerfor the Keck Ten-Meter Telescope," inESO Workshop on HighResolution Spectroscopy with the Very Large Telescope, M.-H.Ulrich, ed. (European Southern Observatory, Garching, Ger-many, 1992)

3. J. Maxwell, Monographs on Applied Optics No. 6: Catadiop-tric Imaging Systems (American Elsevier, New York, 1972).

4. H. W. Epps, "Camera designs for the Keck Observatory LRIS

and HIRES spectrometers," in Instrumentation in AstronomyVII, D. L. Crawford, ed., Proc. Soc. Photo-Opt. Instrum. Eng.

1235,550-561 (1990).5. H. W. Epps, "Fast broad-passband lenses for spectrometers on

large telescopes," in ESO Conference on Very Large Telescopesand Their Instrumentation, M.-H. Ulrich, ed. (European South-ern Observatory, Garching, Germany, 1988), pp. 1157-1165.

6. B. E. Yoldas and D. P. Partlow, "Wide spectrum antireflectioncoating for fused silica and other glasses," Appl. Opt. 23,1418-1424(1984).

7. J. D. Mackenzie and D. R. Ulrich, eds., Sol-Gel Optics, Proc.Soc. Photo-Opt. Instrum. Eng. 1328 (1990).

8. J. P. Brodie and H. W. Epps, "Multi-object spectroscopy atUCO/Lick Observatory," in Fiber Optics in Astronomy II,Astronomical Society of the Pacific Conference Series, P. Gray,ed. (Astronomical Society of the Pacific, San Francisco, Calif.,

1992), pp. 2-9.9. J. L. Houghton, U.S. patent 2,350,112 (30 May 1944).

10. H. Rutten and M. van Venrooij, Telescope Optics: Evaluationand Design (Willmann-Bell, Richmond, Va., 1988), Chap. 13,

pp. 126-129.


extremely achromatic f/10 all-spherical camera constructed for the high-resolution echelle...

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