Infrared Fourier spectrometer for laboratory use and for astronomical studies from aircraft and ground-based telescopes

Infrared Fourier spectrometer for laboratory use and forastronomical studies from aircraft and ground-basedtelescopes

Harold P. Larson and Uwe Fink

A portable, versatile, ir Fourier spectrometer is described that provides 0.5 cm-1 spectral resolution in the0.87-5.6-gm region. This spectrometer is employed in a varied program of astronomical observations fromground-based telescopes and from the NASA 91.5-cm airborne ir telescope. A number of spectral resultsare presented to illustrate the performance of this spectrometer in astronomical applications.

Introduction

The field of ir Fourier spectroscopy has now ma-tured to the point where scientific results obtainedwith this method are published with little or no refer-ence to the once novel instrumental techniques them-selves. For applications in astronomy, however, theachievements of individual groups have been depen-dent on their own unique adaptations of availabletechnologies. Considerable effort has been expendedin trying to use commercially produced, rapid-scan-ning, Michelson interferometers in ir astronomy pro-grams.'- 4 These instruments offer simple, compactdesign with sufficient spectral resolution, typically0.5 cm-l, corresponding to a 1-cm mirror displace-ment, for a variety of astronomical studies.

For many years this laboratory has operated a lowresolution (5 cm-'), rapid-scanning Fourier spec-trometer adapted by H. Johnson and his colleaguesfor applications in astronomy. The success of thisproject included applications aboard NASA's CV990aircraft5 -7 and at ground-based telescopes.8 In 1970we decided to augment the capabilities of this pro-gram with higher resolution (0.5 cm-') with an Ideal-ab, Inc. model IF-3 interferometer. Design goalsthat have been realized in this new spectrometer in-clude portability and ease of alignment for Casse-grain operation at any telescope, including the NASA91.5-cm airborne telescope. An all digital data pro-cessing system incorporating real-time diagnostic andcomputing capabilities permits reliable operation inthe field and frees the instrument from specific ob-

The authors are with the Lunar & Planetary Laboratory, Uni-versity of Arizona, Tucson, Arizona 85721.

Received 24 April 1975.

servatory or laboratory data processing installations.Finally, by transferring most of the complex opera-tions in acquiring and reducing data to electronic orcomputer program control, the operation of the spec-trometer is simple enough that other astronomersand graduate students can readily participate in itsuse.

The following description emphasizes those practi-cal features of our spectrometer that have provenespecially useful in our astronomy program. Werefer the reader to the many excellent referencesavailable for the more generalized principles of thefield of ir Fourier spectroscopy9 and its applicationsto astronomy,10 including the rapid-scanning methodemployed in our spectrometer.

Optical Configuration

The weak signals and high thermal backgroundlevels characterizing most ir observations strongly af-fect the optical design of all ir instrumentation. In aFourier spectrometer this requires minimizing thenumber of optical elements to maximize its opticalefficiency and using the two complementary inputs ofa two-beam interferometer for thermal backgroundcompensation. For our planetary spectroscopy pro-gram we required in addition a system providing op-tical fields up to 1 min of arc without sacrificing reso-lution, which could be readily adapted to any aper-ture telescope of any f number. Thus, independentof the consideration of optical efficiency, we were ledto Cassegrain rather than coud6 operation. Figure 1illustrates the optical configuration chosen to satisfythese constraints; it is based on the scheme proposedby Mertzl and used successfully in our low resolu-tion spectrometer. An f/45 beam from the telescopeis presumed. If the telescope does not provide one,transfer optics such as a CaF2 lens must precede the

September 1975 / Vol. 14, No. 9 / APPLIED OPTICS 2085

Fig. 1. Schematic illustration of the internal optical configurationof our astronomical Fourier spectrometer. The f45 telescopebeam is incident vertically and deflected into the spectrometer bybending mirror M1. The ray diagram beginning at that mirror il-lustrates the two-beam input/two-beam output geometry within

the spectrometer using on-axis but diverging beams.

spectrometer to convert the beam to f/45. MirrorMl bends the f/145 beam from the telescope into theplane of the figure. In the focal plane of the tele-scope two small mirrors M2 and M2' select those por-tions of the field to be directed into the interferome-ter via bending mirrors M3 and M3'. After recombi-nation on the beam splitter, the output beams fallback on M3 and M 3' and still diverging as f/145 passby the input mirrors, which now present only a smallobscuration (4% loss foi a 15 sec of arc field at a1.6-m telescope). A detector system is provided foreach output beam. Weak CaF2 lenses LI and LI'doubling as windows first collimate the beams, fol-lowed by off-axis paraboloids M 4 and M 4' that imagethe input mirrors M2 and M2 onto detectors D andD', respectively. The field of view of the spectrome-ter can be varied over wide limits (2-60 sec of arc) byinterchangeable input mirrors of different diametersand detector systems containing cells of different di-mensions, with or without immersion optics.

The greatest convenience offered by this opticaldesign is that all optical adjustments except the rota-tion and tilt of mirrors M2 and M2' can be prealignedin the laboratory to minimize time lost setting up thespectrometer at a telescope. The only adjustmentthat must be made at a telescope involves focusing abright star on the input mirrors, adjusting these mir-rors to send the beam properly through the spec-trometer, and then touching up the positions of bothDewars to maximize the signal. These operationsare normally completed in 30 min or less during twi-light of the first night of an observing run. Thisspectrometer has been used in this manner at fourtelescopes so far: the Lunar and Planetary Labora-tory 1.6-m telescope (f/45); the Steward Observatory2.3-m telescope (f/45); the Kitt Peak National Obser-vatory 4.0-m telescope (f/7.8 and transfer lens), andthe NASA 0.91-m airborne ir telescope (f/13.5 andtransfer lens).

Conversion of the spectrometer to operation inother wavelength regions is convenient, even wheninstalled on a telescope. The kinematically mounted

Dewars are replaced by another pair containing dif-ferent detectors but identical optics to remain com-patible with the rest of the spectrometer. Beamsplitter substitution requires only a minute or so.After this simple interchange of components, the op-tical adjustments described in the preceding para-graph are then made to prepare the spectrometer forobservations in the new spectral region. This con-version can be made at a telescope in about an hour ifan observing program requires measurements in dif-ferent spectral regions. Presently we cover the0.87-5.6 gm region with two beam splitters and PbSand InSb detectors.

Two compromises were required in this optical de-sign to permit the simplicity of operation describedabove. First, the use of diverging rather than paral-lel beams of light within the interferometer limits themaximum spectral resolution that can be achieved atshort wavelengths.'2 At f/45, the theoretical maxi-mum resolution of 0.5 cm'1 can be maintained onlyabove about 1.25 gm. Although use of a beam oflarger f number could have avoided this limitation,two of the major telescopes in Tucson at which thisspectrometer is used produce f/45 beams directlythrough interchangeable secondary mirrors, so ouroptics were designed around this value.

The second compromise involves imaging theobject itself (or, equivalently, the mirrors M2 andM2') on the detectors rather than the primary mirror,as is commonly done in photometric systems. Guid-ing errors, effects of seeing, and nonuniform sensitiv-ity across the surfaces of the detectors can combineto produce uncorrelated intensity fluctuations in theinterferogram that are equivalent to a noise source.Imaging the primary mirror onto the detector can, inprinciple, eliminate this effect; but Connes foundthat for high resolution, high SNR applications thisscheme was not perfect either.' 3 Since the image ofthe primary mirror would be located differently ateach of the telescopes used in our research program,this would require either multiple pairs of Dewarsspecifically aligned for each telescope or realignmentof a single pair for service at different telescopes.Neither choice was attractive, so we decided to startwith the simpler scheme of imaging the object on thedetectors, and in practice we have found no compel-ling reason to change. The success of this arrange-ment is probably due to the fact that our observingprogram has emphasized low resolution observationsof weak, often extended, objects where backgroundnoise or detector noise dominates. In the exampleswe present later of such spectra (asteroids and plane-tary satellites), detailed analyses of their SN ratioshave not revealed any noise source that can be associ-ated with our optical configuration. High qualityspectra (SNR - 3.5 X 103) of bright stars at maxi-mum resolution have also been recorded with thisspectrometer. In these spectra, however, we canclearly identify a 11/ noise spectrum beyond 2.7 gimthat is probably due to the above effects, but it is notat all certain that re-imaging the primary mirror of

2086 APPLIED OPTICS / Vol. 14, No. 9 / September 1975

Fig. 2. Visual guiding field of our ir Fourier spectrometer. Thisfigure is a composite prepared from separate photographs of Sat-urn and the guider field, but it closely represents what the observ-er sees during observations. The optical scale is for the Kitt Peak4.0-m telescope. The two input mirrors (M2 and M 2' in Fig. 1)carried on vertical supports present about a 5-mm diam circular

cross section to the ir beams.

the telescope on the detectors would have eliminatedit. All analyses of those data have been performed atmuch lower wavelengths, where this 1/f noise spec-trum has effectively disappeared. Thus, both ofthese compromises have, in practice, had negligibleeffect on the quality of the spectra generated withthis spectrometer.

A unique feature of our spectrometer is its visualguiding system that permits continuous, preciseguiding at no expense to the ir signal, which is espe-cially useful for spatially resolved observations of aplanetary disk. Light not intercepted by the spec-trometer's input mirrors passes between the two De-wars and is collected by an optical guiding assemblyrepresented in Fig. 1 by the series of components L2 ,M5, L3, and eyepiece L4 . Calibrated scales and otherviewing aids such as crosses, globes, etc. are mountedon a slide S. Although a star is normally wholly ob-scured from view, its diffraction cross can be seen ex-tending beyond the input mirror. If a reference crosscarried on slide S is centered in the image of theinput mirror, hence on the detector, and the star'sdiffraction cross superimposed upon it, then the posi-tion of the star on the front surface of the input mir-ror can be accurately maintained without seeing thestar itself. For weak objects, where no diffractioncross is visible, careful monitoring of the strength ofthe ir signal can keep the object centered in the inputmirror. In planetary observations the disk often islarger than the input mirror's diameter, so the lightpassing by the edge can be used for guiding. Forspatially resolved spectroscopy on a planetary diskmore precise control is needed. A computer drawnglobe with longitude and latitude lines is photogra-

phically reduced on a glass slide to the actual dimen-sion and aspect of the planet as seen in the guider.This globe and a reference cross, separately mountedwith independent X-Y adjustments, are positionedover the image of the input mirror. With the crosscentered in that mirror's image, the globe is displacedso the center of the cross marks the area, as small as 2sec of arc in diameter, whose spectrum is sought.The planet is then centered within the globe, and thelimb is used to detect tracking errors. Figure 2 is aphotograph through the guider illustrating the ap-pearance of the optical field during observations ofSaturn at the KPNO 4.0-m telescope. In silhouetteare seen the backs of mirrors M2 and MY and theirmechanical support. A portion of the disk of Saturnis being diverted into the spectrometer via the right-hand mirror. Less obvious is the fact that both inputmirrors are injecting background radiation from ad-jacent patches of sky into the interferometer. To afirst approximation this common background signalcancels, leaving only the planetary signal as output.Since cancellation is not complete, though, a back-ground contribution could build up in time. Toavoid distortion to our spectra, after a certain inter-val of time, typically 10-15 min, we shift the object tothe other input mirror and continue observing.When the resulting blocks of data are properly sub-tracted during co-adding, the residual backgroundcontribution is fully eliminated. Figure 2 containssome of the viewing aids employed in our guider.The vertical scale calibrated in seconds of arc is usedto measure the seeing disk of a star. The globe posi-tioned in Saturn could be used to guide on a specificcoordinate or marking on the planetary disk. Thecross centered in the left-hand input mirror is alignedwith the diffraction cross of a stellar image, otherwisecompletely obscured by the mirror.

DetectorsWe normally use cells with 0.25 mm X 0.25 mm di-

mensions, although other sizes can be substituted forspecial observing requirements such as very largefields for comet observations or very small fields forspatially resolved spectroscopy. Three combinationsof detectors and beam splitters provide the followingspectral coverage and noise characteristics: PbS,0.87-4.0 gim, NEP(2.2 gin) - 4 X 10-14 W/Hzl/2;InSb, 2.7-5.6 Aim, NEP(5.6 gim) - 0-13 W/HzI/ 2 ;cold-filtered InSb, 0.87-2.7 gim, NEP(2.2 Aim) - 4 X10-15 W/Hz1/2. The improved NEP available withInSb detectors has almost eliminated our use of PbScells at telescopes.

Data Recording System

In the rapid-scan mode of operation of our spec-trometer the velocity of the moving mirror encodesthe optical frequencies of the incident radiation as aband of very much lower electrical frequencies. Nor-mally this velocity is chosen as high as possible, con-sistent with detector response, to place this frequen-cy far from 11/f noise sources such as scintillation and


Binary Count toTape Recorder

Fig. 3. Block diagram of the electronic data processing system and its optical inputs. A single START/STOP switch commands opera-tion of this sytem, which electronically processes the analog input signals to produce digital data tapes suitable for immediate processing

under computer program control as well as a real-time spectral display to aid the observer in evaluating the quality of the data.

seeing that accompany astronomical observations.As a consequence of these speeds, however, interfero-grams are generated at a rapid rate but with lowSNR, requiring repeated scans, sometimes tens ofthousands, followed by averaging (co-adding) tobuild up the SNR. A primary goal of our spectrome-ter design was to make processing of this kind of dataas convenient and automatic as possible.

Since Fourier spectroscopy ultimately depends ondigital computation, a digital data processing systemwas a natural choice for our spectrometer. Datatapes are produced during observations that containall the information necessary for subsequent process-ing under computer program control. Pulses derivedfrom the zero crossings of a reference signal com-mand sampling of the ir signal and drive the digitalcircuits. This reference signal is the interferogramproduced by a monochromatic source (Hg, 5461 A or4358 A in our case), which appears electronically asan 850-Hz sinusoid. The interferogram samplestaken from the ir signal are converted to a 12-bit digi-tal format, 10 bits for the absolute value, a sign bit,and a gain bit. A change of gain by 32, if desired, canbe introduced about the center of the interferogramby preset counters commanding electronic switches.This provides an effective dynamic range of 15 bits,which is adequate for both high and low signal levels.A third optical channel illuminated with white lightprovides an interferogram from whose center a pulseis produced. This pulse precedes the center of the irinterferogram by use of offset mirrors within the in-terferometer. It serves as a fiducial mark pointing toa location in the ir interferogram that is invariantwith respect to its center. The sample count at thispulse is written on the magnetic tape at the end ofeach interferogram. During data reduction this

count is used to shift successive interferograms sotheir centers align to add coherently.

Figure 3 is a block diagram of the data recordingsystem. Since the sample pulse clock is asynchron-ous due to jitter in the interferometer's drive system,the tape recorder must be buffered to prevent loss ofdata. We designed a buffer memory using staticshift registers. While one section of 500 words isbeing written at the asynchronous sample pulse rate,the second section is being read onto tape with a fast-er, synchronous clock. When the tape recorder hasreceived the contents of one memory, a special wordis then written repetitively to keep the recorder atwriting speed until the other memory is filled. Thisspecial word is subsequently stripped from the dataduring data reduction. Each interferogram isblocked into records of 4000 samples and terminatedwith an end-of-file mark. During observations, if theobserver wishes to have the interferogram being re-corded rejected from computer processing, a pushbutton on the panel injects a flag that causes that fileto be skipped when read by the computer.

An important consideration for work at telescopesis the efficiency with which available observing timeis devoted to data acquisition. At moderate to highspectral resolution, our spectrometer's efficiency canbe as high as 94%; the major source of lost time is dueto rewinding and reloading magnetic tape. At lowresolution our efficiency drops since tape marks andflyback of the interferometer carriage begin to de-tract significantly from the observing time. Currentmodifications to the logic control circuits, however,will soon make this spectrometer >90% efficient overits entire range of spectral resolution, 100-0.5 cm-.

The reliability of operation of this system con-cerned us greatly during its development. The rigors


b. 400 interferograms co-added

c. Correction function

-1 d hase c rAect-ed n'tferam- a

d. Phase corrected interferogram

4000 5000 6000 4000 5000 6000

e. Spectrum f. Real-time computerdisplay of spectrum

Fig. 4. Data recorded during observations of asteroid 433 Eros isshown in various stages of reduction. The complete spectrum is

included in Fig. 8.

of frequent transportation on rough mountain roadsand the low temperatures found at telescopes areamong the hazards encountered by instrumentationused in observational astronomy. Although we haveexperienced electronic malfunctions, the preserva-tion of the individual interferograms on tape meantthat in most instances the data could be recoveredwith specially written computer programs. For spot-ting electronic errors and failures within the systemwe have established diagnostic procedures using elec-tronic aids and special computer programs thatquickly and conveniently check out the entire systembefore or even during observations. A built-in D/Aconverter, for example, can be attached at variouspoints in the digital data processing circuits. Thecharacter of a reconstructed signal such as a test sin-usoid applied to the analog input quickly verifies theperformance of the circuits under test. Also, a com-puter program plots each bit of the data words writ-ten on magnetic tape. Again using a test waveform,examination of this display constitutes a bit-by-bit

check that has been especially valuable in identifyinglow order bit failures.

A special feature of our spectrometer is its real-time Fourier computer. Its use both for spectral dis-play and as a diagnostic aid with our spectrometerhas already been described.14 We depend greatly onit for evaluating observations at a telescope.

Data Reduction

Raw data tapes are first read by a computer pro-gram that checks their format and prints a summaryof their contents. Interferograms with parity errors,suspicious data, or even no data due perhaps toclouds or guiding problems can be identified and re-jected from processing. The co-adding program isthen instructed by data card input where to begin co-adding, where to end, what to reject, and where to re-verse sign due to switching the object between inputapertures. From then on, co-adding proceeds auto-matically under program control ending with a plotof the co-added results.

Most of the interferograms produced by this spec-trometer are one-sided, that is, the zero path pointoccurs close to one end. This precludes use of thesimplest way to recover the spectrum, a pure powerspectrum Fourier transform. Short interferograms,particularly those of weak objects where no datashould be rejected, are transformed as power spectra,then phase-corrected by procedures described byMertz.11 For transforms of longer interferograms,where computer time becomes a constraint, we firstphase-correct the co-added interferogram as de-scribed by Forman et al.' 5 and then perform a one-sided cosine transform.

Figure 4 illustrates the appearance of astronomicaldata (asteroid 433 Eros) in some of the above opera-tions. A single interferogram with a well-definedcenter but an obvious noise level is plotted in Fig4(a). After averaging 400 such scans [Fig. 4(b)] theSNR has improved by the expected value of 20.Most of the asymmetry in the interferogram at thisstage is due to nonsymmetric sampling with respectto the zero path point. To perform a one-sided co-sine transform on this data, the interferogram mustbe resampled numerically to begin at the zero pathpoint. This procedure also removes asymmetry dueto other effects such as electronic phase shifts andnonuniform optical response of the beam splitter.Figure 4(c) shows the correction function derivedfrom the original interferogram, which when con-volved with that interferogram produces the symmet-rical, on-center sampled result in Fig. 4(d). Thisphase-corrected interferogram is truncated at its cen-ter and then cosine-transformed to recover the spec-trum, a portion of which is included in Fig. 4(e).

During observations only the single interferogramscan be monitored, but by themselves they provide lit-tle help in predicting the ultimate quality of thespectrum. For very weak sources not even an inter-ferogram center may be seen, leaving the observeruncertain whether a useful signal is building up.


a. Single interferogram

/ ~~~~~~0t'<; ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~I

L ! a __

lvo .� oeh�--v ,,,VII ',I -,, _ . -

I

Fig. 5. This NASA-operated telescope facility, dedicated theGerard P. Kuiper Airborne Observatory, exists principally for irobservations. A 91.5-cm telescope views through the open portseen immediately in front of the wings. Observations are normallyconducted between altitudes of 12-15 km. During flights on 22and 24 October 1974, our ir Fourier spectrometer produced spectrawith this facility that led to the discovery of H20 vapor in Jupiter'satmosphere. Photograph courtesy of National Aeronautics and

Space Administration.

Comparison of Fig. 4(e) and 4(f) reveals the greatvalue of real-time spectral display. It provides theobserver with an excellent representation of the finalspectrum when fully transformed in the laboratorysometime later. Figure 4(f) is a photograph of ourreal-time computer's video display during observa-tions of the asteroid Eros. This computer performs asimple power spectrum transform of these interfero-grams and co-adds the spectra. During observationswe watch the SNR build up in a preselected spectralwindow containing known absorption features. Wefrequently use this information to alter our observingprogram to make best use of telescope time. The in-ability of this computer to perform a phase-correc-tion calculation in real time has the effect of loweringthe spectral resolution in its display in a manner al-most indistinguishable from apodization of the dataand is not a serious distortion.

Applications

The following examples drawn from our current re-search program illustrate the varied accomplish-ments of this spectrometer both in the laboratoryand at telescopes.

A. High Altitude Observations of Jupiter

The NASA 91.5-cm airborne ir observatory16 seenin Fig. 5 is a unique facility for conducting astronom-ical research that would be impossible at even thebest mountaintop telescope sites. Spectroscopy ofJupiter at 5 gim was among our first experiments con-ducted from this observatory with our ir spectrome-ter. Water vapor in the earth's atmosphere obscuresmuch of this wavelength region, and strong methane

and ammonia absorptions in Jupiter's atmospherefurther limit the spectral window in which one cansearch for other atmospheric constituents. From ob-servations with aircraft above 12 km, however, morethan 99% of the earth's water vapor is eliminated,thereby opening more of Jupiter's spectrum for anal-ysis. In Fig. 6 we show Jupiter's high altitude spec-trum in the 5-gm region recorded during flights on 22and 24 October 1974. This spectrum led immediate-ly to the discovery of water vapor in Jupiter's atmo-sphere,17 a molecule of great chemical and biologicalimportance wherever it is found in the solar system.

Many interference spectrometers used in astrono-my are restricted to operation at a coud6 focus, a pro-tected, stable environment. At a Cassegrain focus,on the other hand, low temperatures and mechanicalflexure and maintenance of interferometric adjust-ments with changing tilt are among the problemsthat can be encountered. Aboard an aircraft, strongelectromagnetic fields and mechanical vibrations be-come additional problems. Our Jupiter flights alsoserved to evaluate the performance of our spectrome-ter in an aircraft environment. Figure 7 provides aconvincing demonstration that the spectrometerachieved theoretical performance in spite of the envi-ronmental disturbances. The spectrum of the moonrecorded in flight at high resolution (0.6 cm-') iscompared with a synthetic spectrum generated on acomputer assuming a theoretical, unapodized, instru-mental line shape for a Fourier spectrometer (sinx/x). The excellent agreement in all detail betweenthe experimental and synthetic spectra in Fig. 6means that the experimental instrumental line shapewas very close to theoretical. This performance re-quired, however, rejecting data during observationsthat were visibly distorted by aircraft vibrations.This real-time editing reduced our effective observ-ing time, but it guaranteed a spectrum of high quali-ty.

B. Asteroid Spectroscopy

In recent years spectral studies of asteroids havebecome increasingly important in connection withunsolved problems of the origin and evolution of oursolar system. The origin of meteorites and in partic-ular the extraterrestrial origin of organic matterfound in some meteorites are questions that may besettled with the help of new observational data con-cerning asteroid surface compositions. Laboratoryspectral studies of meteorites, lunar rocks, and ter-restrial samples have provided important insightsinto the remote identification of minerals with spec-tral observations made at telescopes.' 8 These stud-ies show that the most diagnostic spectral features ofminerals occur beyond 0.9 gm. Electronic transi-tions in Fe2+, a common constituent of rock-formingminerals, are present out to 2.3 gim, and virtually allvibration bands (OH-, C03=, S0 3=, etc.) lie beyond1.4 gm. Although only low spectral resolution (20cm-l) is required for studies of surface constituentssuch as minerals and ices, the weak ir signals avail-


Fig. 6. High altitude spectrumof Jupiter from the NASA air-borne ir observatory. A lunarcomparison spectrum indicatesresidual telluric absorptions at 5am. A synthetic spectrum ofH20 vapor illustrates the identi-fication of H20 lines in the spec-

trum of Jupiter.

MICRONS

1850 1900 1950 2000 2050

Cm'

Fig. 7. Comparison of high resolution observations of the moon with a computer synthesized spectrum of H2 0 and CO2 for evalua-tion of the in-flight performance of the Lunar and Planetary Laboratory Fourier spectrometer. The intensity mismatch between theobserved and calculated 03'0 CO2 bands' Q branches is a consequence of a Fermi resonance whose effect was not included in the cal-

culated spectrum.


MICRONS

4B

Fig. 8. The ir spectrum of asteroid 433 Eros observed 24 January1975 at the Steward Observatory 2.3-m telescope. Broad absorp-tions at 1.125 m, 1.4 gim, and 1.85 m are due to telluric watervapor; and the narrow bands at 2.0 m are of telluric CO 2. Theratio spectrum reveals two broad, shallow absorptions at about 0.9Am and 1.95 ,um characteristic of rock-forming minerals present on

this asteroid's surface.

Fig. 9. The first ir spectrum of Titan. A comparison spectrum ofSaturn shows similar CH4 absorptions. The lunar comparison in-dicates telluric CO2 and H20 absorptions. Our program of Titanobservations used the Steward Observatory 2.3-m telescope and

the Kitt Peak National Observatory 4.0-m telescope.

able from asteroids at even the largest telescopes pre-clude the use of classical spectroscopic techniques.Our Fourier spectrometer has produced the only nearir spectra of asteroids. As of this writing, spectrahave been obtained of asteroids 4 Vesta,19 433 Eros,2 0

and 511 Davida. Figure 8 illustrates our spectrum ofasteroid 433 Eros recorded 24 January 1975 at theSteward Observatory 2.3-m telescope. Although itsspectrum is apparently indistinguishable from asolar-type stellar comparison, its ratio spectrum atthe top of Fig. 8 reveals two broad, shallow bands atabout 2.0 ,um and near 0.91 um that are due to Fe2+in a pyroxene mineral. Complete analysis of thisspectrum may strengthen links between asteroidsand the parent-body sources of meteorites.

C. Spectroscopy of Planetary Satellites

The physical properties of the natural satellites ofother planets can provide important insights into un-

derstanding our solar system. Titan, a satellite ofSaturn, has intrigued astronomers since it is the onlysatellite, as well as the smallest body, in the solar sys-tem with an atmosphere. Although the study ofplanetary atmospheres has advanced considerablyfrom ir spectra produced almost exclusively by Fouri-er methods, not even these instruments have beensuccessful in producing new spectroscopic data onTitan. Since the discovery of Titan's methane atmo-sphere in 1944 by Kuiper,2' its detailed compositionand structure have remained speculative. Infraredspectral observations of Titan have been a goal of ourobserving program for years. Early this year thecombination of our Fourier spectrometer, improveddetectors, and a large aperture telescope producedthe first ir spectrum of this object.22 Titan's K mag-nitude (2.2 m) is about 8.0, one of the faintestobjects successfully observed in our program. Itsspectrum in Fig. 9 displays the methane absorptionscharacteristic of Saturn and the other outer planets.Our analysis of this spectrum may reveal other atmo-spheric constituents and define physical conditionsin this atmosphere;

The four largest satellites of Jupiter (Io, Europa,Ganymede, and Callisto) constitute part of a minia-ture solar system around Jupiter that might provideclues to the formation of the planets themselves.Low resolution Fourier spectra of their surfaces haverevealed the presence of H20 ice on two of them(Ganymede and Europa).23 24 Some astronomersthink that the innermost Galilean satellite, Io, mustpossess an atmosphere to explain several observa-tional phenomena associated with this satellite in-cluding posteclipse brightening25 and Na emission.26

We have been conducting a spectroscopic searchsince 1972 for evidence of an atmosphere on Io. Fig-ure 10 summarizes our latest observations with ourFourier spectrometer providing new upper limits onthe gases N2 0, NH 3, CH 4 , and H2 S.2 2 This repre-sents a continuing program where both higher resolu-tion and improved SNR are sought in an effort to de-tect this atmosphere if it exists, or at least to reducethe upper limits to one.

D. Laboratory Comparison Spectroscopy

The analysis of spectroscopic data recorded at tele-scopes often requires comparison spectra of gases andsolids. If the molecular parameters are known withsufficient accuracy, comparison spectra such as areincluded in Figs. 6 and 7 can be generated syntheti-cally on a computer assuming a theoretical instru-mental line shape to represent the spectrometer.Most projects, however, require producing compari-son spectra experimentally. It is important to matchboth the spectral resolution and the instrumentalline shape for detailed comparison with observationaldata. This requires using the same instrument in thelaboratory as was employed at the telescope. Our irFourier spectrometer is used extensively in this man-


Fig. 10. Spectral data used to search for an atmosphere on Io. Along with the observational data (Io, solar-type stellar comparison)are portions of laboratory comparison gas spectra whose prominent absorptions were sought in Io's spectrum. Preliminary analysishas yielded only upper limits. Our program of observations of Io used the Steward Observatory 2.3-m telescope and the Kitt Peak

National Observatory 4.0-m telescope.

ner to support our observing program. Figure 11 il-lustrates a portion of the spectrum of silane (SiH4)recorded specifically for comparison with the high al-titude spectrum of Jupiter in Fig. 6. This spectrumis being used to test for the presence of this gas in Ju-piter's atmosphere, or at least to establish an upperlimit on its abundance, a number often useful in con-straining chemical theories of planetary atmospheres.

Many of our observations have dealt with ices onvarious objects in the solar system: Mars polar caps(CO2),27 Saturn's rings (H20), 28,29 and the Galileansatellites (H20). 23 Aided by visiting scientists J.Ferraro (Argonne National Laboratory) and R. Love-joy (Lehigh U.), we have recorded reflection spectraof various ices (H20, C02, NH3, CH4, H2S, NH4SH)in the laboratory to aid in the interpretation of theseobservations. 3 0 Besides providing a library of com-

CM-

Fig. 11. The V3 fundamental of silane produced in a laboratoryabsorption tube for direct comparison with the high altitude spec-

trum of Jupiter in Fig. 6.


microns

cm-'

Fig. 12. Reflection spectra of H2 S frosts showingtemperature dependence of spectral features.

parison spectra for present and future use, thesespectra contain temperature dependent features thatare not only interesting in themselves, but, in thecase of the H20 frosts, they have led to a remote tem-perature sensing technique.3 ' As an illustration ofthis temperature dependence we show in Fig. 12spectra of H2S frosts as a function of temperature.The effects of temperature include both narrowingand splitting of spectral features. The dramaticchanges in the spectrum between 113 K and 125 Kare due to a phase change in solid H2S from a lowtemperature tetragonal unit cell to the face-centeredcubic structure at higher temperatures.3 2

E. High Resolution Stellar Spectroscopy

The maximum resolution of 0.5 cm-' availablewith this spectrometer is very modest compared withwhat has been achieved by Connes on brightobjects.33 Many astrophysical studies can still bepursued without the ultimate in spectral resolution,however. Synthetic spectrum calculations of plane-

tary and stellar atmospheres in particular can fit in anumerical sense parameters related to spectral fea-tures that may not be fully resolved in the observa-tional data. The portion of the spectrum of a Ori inFig. 13 exhibits several stellar CO bands at 0.5 cm-1

resolution. All but the band heads are clearly re-solved. These data were recorded by T. N. Gautierfor his thesis research. He is determining the stellar12C/13C ratio from the CO bands by fitting to the ob-served spectrum model atmosphere calculationsbased on radiative transfer theory.34

F. Absolute Wavelength Measurements

D. S. Davis (Purdue U.) has used this spectrometerto produce atomic emission spectra in the ir for histhesis research project. His preliminary analysis ofthe spectrum of vanadium (VCl3 with Ne carrier gasin an electrodeless discharge tube) at a spectral reso-lution of 0.5 cm-' has produced absolute wavelengthmeasurements reliable to 2% of the width of the in-strumental line shape, or 0.01 cm-' (see Ref. 35).


CM-,

Fig. 13. A portion of the high resolution spectrum of a Ori show-ing some of the regular and isotopic stellar CO bands. This spec-trum was produced at the Steward Observatory 2.3-m telescope on

11 and 13 January 1975 and has a SNR of about 3.5 X 103.

This is surprisingly good for a spectrometer never de-signed for such demanding measurements. Our ref-erence source is an ordinary low pressure mercurylamp not at all suitable as a secondary wavelengthstandard. Davis overcame this problem through aninternal calibration procedure using Ne lines presentin his spectrum that are known to high absolute pre-cision. For a serious program of absolute wavelengthmeasurements an electrodeless 198Hg lamp couldreadily be substituted to eliminate this bootstrap cal-ibration.

Although the above measurements were made in anonastronomical context, there are astrophysicalproblems such as radial velocity measurements thatdepend on a spectrometer's ability to produce goodabsolute wavelengths. Taking the above uncertaintyof 0.01 cm1 as an estimate of what could beachieved in an absolute sense with our spectrometerin astronomical applications, this translates, for ex-ample, into a radial velocity uncertainty of 10.6 km/sec at 2 gim. This compares very favorably with mea-surements now made only on very large immovablecoud6 spectrographs.

Of the many individuals who participated in thedevelopment of this spectrometer we especiallythank S. Macenka (mechanical), R. Poppen (comput-er support), and G. Michel and J. Percy (electronics)for their contributions. R. Treffers has been respon-sible for incorporating the latest developments in de-tector technology into this spectrometer. Invaluable

assistance in operating this spectrometer in the labo-ratory and at telescopes has been provided by T. N.Gautier. This project was supported by NASAGrants NGR 03-002-332, NSG 7070, and NGL 03-002-002.

References

1. S. T. Ridgway, Ph.D. thesis, State University of New York atStony Brook, 1972.

2. H. L. Johnson, F. F. Forbes, R. I. Thompson, D. L. Steinmetz,and 0. Harris, Proc. Astron. Soc. Pacific 85, 458 (1973).

3. R. R. Treffers, Astron. Astrophys. 38, 345 (1975).4. R. Hanel, M. Forman, T. Meilleur, R' Westcott, and J. Prit-

chard, Appl. Opt. 8, 2059 (1969).5. G. P. Kuiper and F. F. Forbes, Commun. Lunar Planet. Lab.

95, 177 (1967).6. G. P. Kuiper, F. F. Forbes, D. L. Steinmetz, and R. I. Mitchell,

Commun. Lunar Planet. Lab. 100, 209 (1969).7. U. Fink, H. P. Larson, G. P. Kuiper, and R. F. Poppen, Icarus

17,617 (1972).8. U. Fink and H. P. Larson, in Aspen International Conference

on Fourier Spectroscopy, G. A. Vanasse, A. T. Stair, Jr., andD. J. Baker, eds., AFCRL Spec. Rep. 114 (1970), p. 453.

9. R. J. Bell, Introductory Fourier Transform Spectroscopy (Ac-ademic Press, New York, 1972).

10. P. Connes, Ann. Rev. Astron. Astrophys. 8, 209 (1970).11. L. Mertz, Transformations in Optics (Wiley, New York, 1965).12. G. A. Vanasse and H. Sakai, Prog. Opt. 6, 261 (1967).13. P. Connes and J. Connes, J. Opt. Soc. Am. 56, 896 (1966).14. G. Michel, Appl. Opt. 11, 2671 (1972).15. M. L. Forman, W. H. Steel, and G. A. Vanasse, J. Opt. Soc.

Am. 56,59 (1966).16. R. M. Cameron, M. Bader, and R. E. Mobley, Appl. Opt. 10,

2011 (1971).17. H. P. Larson, U. Fink, R. R. Treffers, and T. N. Gautier, As-

trophys. J. Lett. 197, L137 (1975).18. J. B. Adams, J. Geophys. Res. 79, 4829 (1974).19. H. P. Larson and U. Fink, paper presented at the annual meet-

ing of the Division for Planetary Sciences, American Astro-nomical Society, Columbia, Maryland, 17-21 February 1975.

20. H. P. Larson, U. Fink, R. R. Treffers, and T. N. Gautier(LPL), to be submitted to Icarus.

21. G. P. Kuiper, Astrophys. J. 100, 378 (1944).22. U. Fink and H. P. Larson, paper presented at the annual meet-

ing of the Division for Planetary Sciences, American Astro-nomical Society, Columbia, Maryland, 17-21 Febrauary 1975.

23. U. Fink, N. H. Dekkers, and H. P. Larson, Astrophys. J. 179,L155 (1973).

24. C. B. Pilcher, S. T. Ridgway, and T. B. McCord, Science 178,1087 (1972).

25. A. B. Binder and D. P. Cruikshank, Icarus 3, 299 (1964).26. R. A. Brown and F. H. Chaffee, Astrophys. J. 187, L125 (1974).27. H. P. Larson and U. Fink, Astrophys. J. 171, L91 (1972).28. G. P. Kuiper, D. P. Cruikshank, and U. Fink, Bull. Am. As-

tron. Soc. 2, 235 (1970).29. C. P. Pilcher, C. R. Chapman, and L. A. Lebofsky, Bull. Am.

Astron. Soc. 2, 239 (1970).30. U. Fink, J. Ferraro, T. N. Gautier, H. P. Larson, G. T. Sill, and

W. Wisniewski (LPL) manuscript in preparation.31. U. Fink and H. P. Larson, Icarus, in press (1975).32. J. Ferraro and U. Fink, (LPL); manuscript in preparation.33. P. Connes and G. Michel, Astrophys. J. 190, L29 (1974).34. T. N. Gautier, M. S. thesis, University of Arizona, in prepara-

tion.35. D. S. Davis, Purdue University; private communication.


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Infrared Fourier spectrometer for laboratory use and for astronomical studies from aircraft and ground-based telescopes