Airborne infrared astronomical observations byFourier transform spectroscopy

Harold P. Larson

Infrared spectroscopic observations from NASA-operated aircraft constitute a rapidly maturing applicationof FTS methods initially developed for ground-based telescopes. Coupled to airborne telescopes up to 36in. in diameter, these experiments are now producing new astronomical results as exciting and unexpectedas those derived from Connes's first high resolution planetary observations at mountain-top observatories.This review examines the special problems of the ir spectral region that led to aircraft observatories and in-cludes a brief survey of the facilities themselves and their modes of operation. The special problems of oper-ating FTS devices on aircraft and the scientific results achieved with current capabilities are discussed. Fi-nally, airborne observations are compared to the ultimate in high-altitude observing platforms: earth-orbit-ing cooled and uncooled telescopes carried by the space shuttle vehicle.

1. Introduction

For most astronomers Fourier transform spectros-copy (FTS) emerged as an exciting new research tool inthe mid-1960s when Pierre Connes and his colleaguespresented their first high resolution planetary spectra.These spectra represented far more than the technicalachievement of a very complex piece of apparatus, sincewith the detection of such unexpected molecules as HFand HCl on Venus the much talked about potential ofthe ir spectral region for astrophysics was now capableof being exploited. In spite of this promise, however,during the next decade the applications of FTS to irastronomy have been pursued by relatively few scien-tists, while the techniques have found numerically muchlarger followings in other disciplines. Included amongthe many reasons explaining this situation are severalnatural obstacles to conducting astronomical observa-tions at ir wavelengths, whatever the type of experi-ment. Overcoming these requires optimization of anexperiment at all levels which leads to special high al-titude observing sites. This paper is primarily con-cerned with an overview of the application of FTS de-vices on NASA-operated airborne observatories, oneclass of high altitude platform characterized by highlevels of technical sophistication and scientificachievement.

The author is with University of Arizona, Lunar & PlanetaryLaboratory, Tucson, Arizona 85721.

Received 28 September-1977.0003-6935/78/0501-1352$0.50/0.©3 1978 Optical Society of America.

II. Natural Limiting Factors to Observational InfraredAstronomy

A. Obscuration by the Earth's Atmosphere

Minor constituents (H 20, C0 2, 03, etc.) of our ownatmosphere have very strong, often totally saturated,absorption bands at infrared wavelengths where theirstrong vibration-rotation bands occur. Astronomicalobservations can only be made through transmissionwindows, illustrated in Fig. 1 (lower curve), which be-come increasingly narrow at longer wavelengths. Fromabout 20 Atm to beyond 1 mm there is little usefultransmission at all. If a spectral region of astrophysicalinterest coincides with a region of terrestrial obscura-tion, there is usually no alternative but to seek an ob-serving site above most, if not all, the earth's atmo-sphere. This is required, for example, to study traceamounts of molecules such as H20 on extraterrestrialobjects that are present in much larger abundances inour own atmosphere. Another important situationarises when a similarly narrow transmission window ofanother planet's atmosphere lies between the windowsin the earth's atmosphere. The infrared spectrum ofJupiter summarized in Fig. 1 (upper curve) illustratesthe particularly unfortunate locations of the Jovianatmospheric windows, especially that at 2.7 Aim, withrespect to those of the earth.

B. Thermal Background Radiation

A second important limitation to infrared astronomyis thermal radiation from the background. The atmo-sphere, the telescope, and the spectrometer itself allradiate as blackbodies approximately at room temper-ature. Emissivities-of the telescope may vary from 0.1

1352 APPLIED OPTICS / Vol. 17, No. 9 / 1 May 1978

for an optimized ir design to nearly unity for moreconventional instruments. The emissivity of the at-mosphere is 0.1 or less which means that it may benegligible compared with other background sources ofthermal radiation. This background flux becomessignificant at about 3 ,gm, and it peaks in the 10-Am re-gion, thereby affecting all of the middle ir region. Thisthermal flux can exceed by orders of magnitude thesignals received from astronomical sources. Figure 2summarizes the source and background flux levels fora small, weak solar system object such as an asteroid.The background flux is that of a 155-cm Cassegraintelescope designed for low emissivity. The source fluxconsists of two components: reflected solar radiation,approximated in Fig. 2 by a blackbody curve at 5900 K;and thermal self-emission, represented by a blackbodycurve at 230 K. At 1.0 gim the object can be directly

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detected with relative ease. The same absolute fluxlevel at about 10 m produced by its own thermalemission, however, cannot be seen directly due to theobscuring terrestrial background radiation. Any ex-periment operating at thermal ir wavelengths musttherefore include a modulation scheme that discrimi-nates against this background. The use of an oscillatingsecondary mirror in the telescope itself is a standardtechnique for modulating the signal incident on pho-tometers, dispersive spectrometers, and other singleoptical input devices. The dual optical inputs of aMichelson interferometer permit effective internalbackground cancellation without the need for externalmodulation (see Ref. 1, for details, p. 2087). There arelimits to any of these schemes, however, so most irspectral observations beyond 3 gm are background noiselimited at conventional observatories. High altitude

microns3 2 1.55 1.2 1.0

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000crifl

Fig. 1. Top spectrum: overview of the near and middle infrared spectrum of Jupiter identifying the various transmission windows in its at-mosphere. Absorption by Jovian CH4 and NH3 is responsible for forming these windows. Bottom spectrum: transmission windows of theterrestrial atmosphere at a mountain-top observatory. Most of the absorption is due to H2 0 except for the strong band at 4.3 pm which is

CO 2 (figure from Ref. 2).

1 May 1978 / Vol. 17, No. 9 / APPLIED OPTICS 1353

:t

VS

2?

1 14 _TERRESTRIAL BACKGROUND

1016

THER/AL EMISSION (T-230K)

SOLAR SYSTEM OBJECT

I0REFLECTED SOLAR RADIATION

1019

"o-20I I I I I2 3 4 5 6 7 8 9 10

MICRONS

Fig. 2. Comparison of the terrestrial thermal background flux withthe two components of flux from an asteroidal-type solar systemobject. Beyond about 3,um, the beginning of the thermal infrared,signals from many interesting infrared sources are weaker thanbackground levels. Using dual optical inputs and beam switchingtechniques an FTS spectrometer can extract from the backgroundthe infrared spectrum of the hypothetical object in this illustration,but the ratio of source to background flux levels of about 10-2 shown

here is close to the limit of experimental techniques.

is again required to eliminate, or at least greatly reduce,atmospheric radiation and to permit cooled operationof the telescope and spectrometer to reduce their ther-mal emission.

Balloons, rockets, earth-orbiting telescopes, andspacecraft are possible high altitude facilities for re-ducing the severity of these two types of terrestrial ob-scuration. Another type of vehicle, conventional air-craft, has become especially useful for ir spectroscopicobservations. Although an aircraft cannot escape ouratmosphere, it can operate for significantly long periodsof time above most of the heavily obscuring molecularabsorbers such as H20 whose abundance at a typicalobserving altitude of 12.5 km is about 10-3 its value ata conventional mountain-top observatory. An aircraftalso provides an approximate laboratory environmentwith reasonable amounts of room, plenty of electricalpower, and other amenities such as vacuum and highpressure air lines. Most important, the astronomeractively conducts his own experiment in flight just asif he were at a conventional telescope on the ground.NASA responded to the potential of this mode of op-eration by creating a series of airborne astronomicalobservatories. The successful application of FTS de-vices on board these aircraft is an important part of theirhistory.

Ill. NASA Airborne ObservatoriesThe following aircraft based at the NASA Ames Re-

search Center are available for astronomical research.Details concerning the design and operation of thesefacilities are described elsewhere.3 4 Only their mostgeneral characteristics are summarized below.

A. Lear Jet

This high performance executive jet permits obser-vations at 13-km altitude for up to 2.5 h from an openport 30-cm telescope. The aircraft is devoted to a singleexperiment at a time, and although space is verycramped two FTS devices have been operated on it.

B. CV-990 (Galileo I)

This converted four engine commercial jetliner car-ried up to three 30-cm gyrostabilized telescopes lookingthrough ir windows installed in the fuselage. In addi-tion to these multiple astronomical experiments (alllooking at the same object, however), other nonastro-nomical experiments requiring high altitude could alsobe accommodated on a flight. Up to 6 h of flight timeat 12.5-km altitude was possible. Each experimenterteam was responsible for finding their object, trackingit by hand control, as well as operating their spectrom-eter. Programs involving at least five FTS experimentswere conducted with this aircraft before its tragic crashin 1972.

C. C-141 (Kuiper Airborne Observatory)

This airborne observatory with a 91.5-cm open porttelescope has been operational for nearly 3 years. Atleast nine FTS spectrometers have been flown on it.The observatory is dedicated to a single experiment ata time, and NASA professional personnel are respon-sible for the operation of the telescope, the acquisitionand tracking of the object, and in-flight computersupport of data acquisition and data processing activi-ties. This frees the astronomer to concentrate whollyon the performance of his own experiment.

Table 1. Typical Time Sequence for NASA Airborne Infrared AstronomyFlight Program

Time Operation

-1 year Obtain funding with proposal describing scientific ob-jectives of flight program.

-180 days Determine dates of flights upon consultation withmission director and navigators.

-3 days Arrive at NASA Ames to install experiment on airbornetelescope.

-1 day Conduct ground tests.-12 h Ground crew prepares aircraft for flight.-1 h Final briefing of mission objectives, weather conditions

en route, and safety procedures.0 Take-off.1-3 h Attain altitude, latitude, and longitude for first

object.1-7 h Time window in which observations can be made.

Acquisition time per object once on track typically 20sec to several minutes.

7 h Terminate observations for descent.7.5 h Land.24 h Preliminary data processing complete, review objectives

of next flight.24-48 h Next flight.1-2 weeks Duration of flight program (2 flights/week maximum

frequency).

1354 APPLIED OPTICS / Vol. 17, No. 9 / 1 May 1978

-v I

Table II. FTS Experiments on NASA Airborne Observatories

Investigator group Instrumental characteristicsResolution Bandpass Typical

PI Organization Aircraft (cm-l) ,um Reference spectral result

Aumann Jet Propulsion Laboratory C-141 4.0 12-25 5 5Baluteau Observatoire de Meudon C-141 0.02 10-1000 6 15Eddy National Center for

Atmospheric Research CV-990 0.25 238-312 7 7Erickson Ames Research Center C-141 1.0 27-220 8 8Farmer Jet Propulsion Laboratory U-2 0.16 2.5-5.5 9 16Gautier Observatoire de Meudon C-141 10.0 7-14 10 10Hanel Goddard Space Flight Center C-141 2.4 5-50Hilgeman Grumman Aerospace Corporation Lear, C-141 2.0 1.5-3.5Johnson University of Arizona CV-990 8.0 1-4 11 17Johnson University of Arizona Lear 0.5 1.0-5.5 12 12Larson University of Arizona CV-990, C-141 0.5 0.8-5.6 2 18Larson University of Arizona C-141 0.02 0.8-5.6Nolt University of Oregon CV-990, C-141 0.1 500-2000 13 13Thekaekara Goddard Space Flight Center CV-990 2.6-15 14 14

D. U-2

This is not, strictly speaking, an astronomical ob-servatory since it does not have a facility telescope.Certain types of atmospheric research employing FTSinstruments are being conducted, however, using thesun as source. Hence large optics are not required.Instrumentation must be operated remotely on thisaircraft since the pilot has no physical access to the in-strumentation compartment.

All these airborne observatories are available to thescientific community through standard proposal pro-cedures. The lead times for planning and conductingan observing program with these aircraft are similar tothose necessary at major ground-based observatories(see summary in Table I). Compared with the highlyregimented space program, mission objectives in theNASA airborne astronomy program can be changedshortly before flights and, even in some situations,during a flight.

IV. Characteristics of Airborne Fourier TransformSpectrometers

Table II lists the FTS experiments that have beenemployed on the NASA airborne observatories de-scribed above. Although not all projects are currentlyactive, the large number of entries in Table II empha-sizes the major role of Fourier spectroscopy in airborneir astronomy. Collectively, these experiments providecoverage of the entire ir spectral region at low to highspectral resolution. There are in addition many otherFTS experiments operated on special-purpose aircraft,balloons, rockets, and spacecraft that are also significantto ir astronomy. A complete review should, of course,present all of them, but in the more limited context ofthis paper a consideration of the NASA airborne as-tronomy program alone will establish a basic under-standing of high altitude spectroscopic observations.Detailed design characteristics of these various spec-trometers are found in the references in Table II. Alsoincluded in Table II are references to scientific resultsthat illustrate the type of spectral data produced witheach instrument. Some experiments involve only solar

observations, for example, and must be evaluated dif-ferently than instrumentation designed for weak as-tronomical sources. In view of this diversity the fol-lowing discussion is restricted to some general consid-erations affecting all applications of FTS experimentson aircraft.

The one characteristic shared by most of the entriesin Table II is uniqueness. Although some of these ex-periments are built around commercially available in-terferometer components, each spectrometer representsa unique response to the scientific program and wave-length region for which it is intended. A requirementcommon to all experiments is that they tolerate thehostile environment of an aircraft, particularly intenseelectromagnetic fields and mechanical vibrations.Figure 3 illustrates the vibration spectrum measuredon the C-141 91.5-cm telescope during nonturbulentflight conditions. Numerous sharp resonances andbroadband features are present which change, but donot disappear, with the degree of mechanical isolationof the telescope from the aircraft. The effects of thesevibrations on an FTS experiment can be to producesampling errors in the recording of interferograms andto introduce microphonic noise into detector systems.Quiet regions in these vibration spectra are also present,such as at 90 Hz in Fig. 3, which can be used to advan-tage in choosing modulation frequencies for signalprocessing.

Another generalization that may seem surprising isthat many of the FTS experiments in Table II are op-erated under background noise limited conditions.Hence there is no multiplex advantage. Althoughconventional dispersive spectrometers can becomecompetitive with a background limited FTS, it seemsfrom the scientific results that are being produced thatover-all system performance is generally higher for FTSdevices. This is due to combinations of the many sec-ondary advantages of this class of spectrometer, in-cluding the linear, absolute wavenumber scale, thepredictable, reproducible instrumental line shape, ac-ceptance of wide fields of view, broad spectral coverage,and the ease of achieving moderate to high spectral

1 May 1978 / Vol. 17, No. 9 / APPLIED OPTICS 1355

Ld

0

Lu

1-JbJ

0 45.5 91.0 136 182

FREQUENCY (HZ)Fig. 3. These vibration spectra were measured on the Lunar and Planetary Laboratory's 0.5-cm-1 FTS experiment in flight on the KuiperAirborne Observatory. Some of the mechanical resonances at low frequencies disappear when the telescope is fully isolated on its air bearing,but the residual vibrations under observing conditions still present a severe environment to electronic components susceptible to microphonic

noise and to moving elements that must be controlled to interferometric tolerances.

Fig. 4. Top spectrum: instrumental line shape (X = 0.6328) ofthe Lunar and Planetary Laboratory's new high resolution (0.02 cm-)FTS produced during its first engineering flight on the Kuiper Air-borne Observatory in June 1977. The resolution achieved in thisspectrum was about 0.5 cm-'. Bottom spectrum: the theoreticalline shape (sinc2x) for the test conditions of the spectrometer. Bothspectra were produced with a real-time Fourier computer' 9 permitting

in-flight evaluation of the servosystem performance.

resolution in limited space. Both rapid-scanning andstepping drives are in use with significant scientificachievement associated with each type. For high res-olution studies in the near infrared, however, it seemsthat a stepping drive with full servocontrol over pathdifference errors can best render a Fourier spectrometerimmune, as opposed to merely tolerant, of its airborneenvironment. This is illustrated with the instrumentalline profile in Fig. 4 (upper curve) recorded in flight ofan emission line in the visible spectrum (X = 0.6328 gim),a severe test of the stability of an infrared spectrometer.The instrument is a new high resolution FTS beingdeveloped by Larson and colleagues at the Universityof Arizona with the collaboration of Michel and Connesat the Observatoire de Meudon in Paris. It producedthe line profile in Fig. 4 during its first engineering flightin the tail section of the C-141 where it experiencedaccelerations up to 0.3 g during mild turbulence. Thelower curve in Fig. 4 is the theoretical line shape (sinc2x)for the real-time computer19 used in these tests. Theindistinguishability of these two curves indicates theperfection that can be achieved in controlling pathdifference errors with optoelectronic servocontrol. Acomplete description of this spectrometer will be pub-lished at a later time.

Critics of FTS methods frequently cite the involvedcomputations in handling interferograms and the sub-sequent delay in evaluating spectral results. With ex-isting computer technology, however, this need not bea negative factor, and several groups using NASA air-borne observatories have set up very effective dataprocessing procedures. On the Kuiper Airborne Ob-

1356 APPLIED OPTICS / Vol. 17, No. 9 / 1 May 1978

Real-time Display of Jovian SpectrumLPL IR Fourier Spectrometer

Kuiper Airborne Observatory, December

Real-timeComputerDisplay

20 scans, 4.4 cm-' resolution48 sec total integration time

50 scans, 0.5cm-' resolution16min total integration time

Final AveragedSpectrum from0ff -Line Data

Processing

1800 2000 2200 2100 2150Fig. 5. Comparison of real-time and off-line high altitude spectra of Jupiter illustrating that FTS experiments are as capable as conventional

dispersive instruments in providing immediate meaningful spectral data to the user.

servatory, for example, two minicomputers with acomplete selection of peripheral devices are availableto the experimenter. In our airborne program we useone of these computers to record our data, and then weprocess it off line during flight on the other with theresult that upon landing data from at least the firsthours of observations can be fully edited, phase-cor-rected, transformed, and plotted. In addition, ourspectrometer includes a hard-wired Fourier transformcomputer19 that provides real-time spectral display.Figure 5 illustrates the value of this device to us in arecent flight program devoted to Jupiter. The two topspectra in Fig. 5 are photographs made in flight ofreal-time spectra displayed on a monitor oscilloscope.The spectrum at top, left shows the whole Jovian at-mospheric transmission window at 5 ,m at low resolu-tion which indicated to us during the first minutes of ourobserving run that the spectrometer was performingcorrectly. A high resolution window at reduced spectral

bandwidth is displayed in Fig. 5 at top, right. From anexamination of earlier data at low resolution we believedthat this region of Jupiter's spectrum might containevidence of new trace atmospheric constituents.During flight a strong spectral feature did develop near2100 cm-' that subsequently has been identified as theV3 Q-branch of GeH 4 with an abundance on Jupiter lessthan 1 ppb.20 Upon seeing this feature during flight,we were able to evaluate the scientific value of the ob-servations and to estimate the total integration timerequired to produce an acceptable averaged spectrum.The two spectra at the bottom of Fig. 5 are the finalaverages from all flight data, plotted on the same scaleas the real-time spectra. These differ only in SNR andan apparent higher resolution in the off-line results dueto an effective apodization in the real-time computa-tions. Thus, by taking full advantage of these readilyavailable computational aids there is no uncertainty ordelay in evaluating the results of an FTS experiment.

1 May 1978 / Vol. 17, No. 9 / APPLIED OPTICS 1357

1975

V. Scientific Objectives

These diverse instruments working in differentwavelength regions provide observational material thatbears on some of the broader, unsolved problems ofmodern astronomy. These include stellar and solarsystem formation and evolution and the question of lifeitself, which is never far from mind when consideringthe detection and explanation of complex organicmolecules in the interstellar medium, or their possiblesynthesis in a planetary atmosphere, or their undeniableproduction somewhere in our solar system besides onthe earth as evidenced by the carbonaceous chondritemineralogies of certain meteorites. The immediateresults of spectroscopic measurements provide infor-mation concerning the composition, dynamics, orchemical kinetics of the objects observed. As enoughof these and other experimental data accumulate, abroader interpretive framework may emerge uponwhich theoretical predictions can be tested and revised.As concrete illustrations of how this is actually hap-pening, the following discussion briefly outlines theresults of several groups operating FTS devices on theKuiper Airborne Observatory that involve two astro-physically significant objects: the Orion nebula and itsvarious embedded objects; and the planet Jupiter.

A. Orion Nebula

The Orion nebula is a region of ionized H2 gas (H I

region) excited by very hot, young stars that presumablycondensed from this molecular cloud. This nebula alsocontains protostars, or infrared hot spots, representingregions of condensation in the very early phase of starformation. Knowledge of as many physical andchemical properties as possible of this cloud is requiredto understand better the role of molecular clouds in starformation. One type of observational data is ir ionicfine structure emission lines such as S III, P III, etc.From the presence, strengths, and widths of these linesone can deduce such properties of the cloud as elementalabundances, electron densities, and velocity structure.Many such lines are predicted in the 20-100-um region,but they cannot be observed from the ground due toterrestrial H20. Using a high resolution (0.02-cm-')FTS on the Kuiper Airborne Observatory, Baluteau etal. 15 have detected in Orion the S III line at 18.7 ,m andthe 0 III line at 88.2 ,um. The observed line strengthswere in good agreement with the most recent theoreticalpredictions of the properties of H II regions.

Another type of measurement that astronomers canmake of H II regions is that of its continuum emission.The Orion nebula contains numerous embeddedobjects, such as the Kleinman-Low nebula, which is aregion of intense ir emission that is not detectable atvisible or radio wavelengths. Since the nature of thissource is unknown, one of its first properties to deter-mine is its far ir thermal spectrum. Erickson et al. 8

recorded this spectrum between 30 gim and 125 ,m fromthe Kuiper Airborne Observatory with a Fourier spec-trometer. By fitting blackbody curves (75-90 K) totheir data, they were able to constrain models of thesource flux region and the dust that obscures it.

B. Jupiter

This planet occupies a special place in astronomysince it is important both to solar system studies and tostellar problems. Jupiter is sometimes described ashaving almost become a star, partly because it is ra-diating twice as much energy as it is receiving from thesun. Through an understanding of Jupiter's observedcomposition and structure, it may be possible to ex-trapolate backward in time to reconstruct some of theproperties of the primordial nebula from which our solarsystem formed. Jupiter's properties may also be similarto predicted, but as yet undetected, classes of stellarobjects sometimes called pink dwarfs that, like Jupiter,are not massive enough to support the nuclear processesactive in larger stellar objects.

Fourier spectroscopy is providing a wealth of com-positional information for Jupiter's atmosphere, someof it quite unexpected. High altitude observations inthe 5-,gm and 10-,gm regions have been especially pro-ductive for studies of Jovian H20, 18 C2H2 ,5 CO,21 PH3 ,2 2

and GeH4,20 and upper limits to numerous other mol-ecules.23 Each of these molecules provides an inde-pendent constraint on models of Jupiter's atmosphericchemistry. Collectively, this work has identified severalnonequilibrium mechanisms that explain the presenceof these particular molecular species and which bear onother more general properties of Jupiter. The detectionof PH3, for example, provided the first direct evidencefor the coloration of the Jovian clouds, particularly thatof the Great Red Spot. These observations have alsoestablished that the spectroscopically observable re-gions of Jupiter's atmosphere are not characterized bythermochemical equilibrium. Thus, the use of Jupiter'sobserved composition in a broader cosmochemicalcontext, such as inferring the elemental abundances inthe primordial solar nebula, depends upon furtherdefinition of Jupiter's dynamic meteorology. Lineprofile analyses of known atmospheric constituents canproduce temperature and pressure measurementswhich, when combined with other evidence, help es-tablish a more realistic model of Jupiter's atmosphere.High altitude, high resolution Fourier methods are anessential part of this activity.

VI. Future Developments

Many of the instruments and observations describedabove are first-time efforts. There remains much workin pursuing all the observational opportunities nowavailable, yet there is already defined a need to continuesome studies initiated on airborne observatories fromsites with still lower background levels and more com-plete reduction of telluric absorption. Earth-orbitingcooled and uncooled telescopes are attractive in thiscontext, and FTS experiments have a potentially im-portant role in their utilization, especially for observa-tions requiring high spectral resolution, wide fields ofview, broad spectral coverage, and quick conversion todifferent needs. Although advances in detector tech-nology, especially cooled arrays, may render FTS de-vices less competitive than dispersive spectrometers in

1358 APPLIED OPTICS / Vol. 17, No. 9 / 1 May 1978

some situations, the present experience in operatingFourier spectrometers without the multiplex advantageon airborne observatories emphasizes the importanceof secondary instrumental characteristics on the ulti-mate scientific worth of a project.

This should not imply a diminished role for aircraftand other high altitude vehicles now in use. By in-creasing the frequency of flights of the Kuiper AirborneObservatory, for example, or by building a larger aper-ture airborne telescope many more ir spectroscopyprojects can be initiated than are now possible.Moreover, secondary advantages of airborne observa-tories, such as their mobility for occultation studies, areresponsible for additional research projects outside thefield of ir astronomy.

This paper was prepared with support from NASAgrant NGR 03-002-332.

References1. H. P. Larson and U. Fink, Appl. Opt. 14, 2085 (1975).2. S. T. Ridgway, H. P. Larson, and U. Fink, in Jupiter, T. Gehrels,

Ed. (U. Arizona Press, Tucson, 1976), p 384.3. M. Bader and C. B. Wagoner, Appl. Opt. 9, 265 (1969).4. R. M. Cameron, M. Bader, and R. E. Mobley, Appl. Opt. 10,2011

(1971).5. H. H. Aumann and G. S. Orton, Science 194, 107 (1976).6. J. P. Baluteau, M. Anderegg, A. F. M. Moorwood, N. Coron, J. E.

Beckman, E. Bussoletti, and H. H. Hippelein, Appl. Opt. 16, 1834(1977).

7. J. A. Eddy, P. J. Lena, and R. M. MacQueen, Solar Phys. 10,330(1969).

8. E. F. Erickson, D. W. Strecker, J. P. Simpson, D. Goorvitch, G.C. Augason, J. D. Scargle, L. J. Caroff, and F. C. Witteborn, As-trophys. J. 212, 696 (1977).

9. R. A. Schindler, Appl. Opt. 9, 301 (1970).10. T. Encrenaz, D. Gautier, G. Michel, Y. Zeau, J. Lecacheux, L.

Vapillon, and M. Combes, Icarus 29, 311 (1976).11. G. P. Kuiper, F. F. Forbes, and H. L. Johnson, Commun. Lunar

Planet. Lab. 93, 155 (1967).12. H. L. Johnson, F. F. Forbes, R. I. Thompson, D. L. Steinmetz, and

0. Harris, Pub. Astron. Soc. Pac. 85, 458 (1973).13. I. G. Nolt, J. V. Radostitz, and R. J. Donnelly, Nature 236, 444

(1972).14. M. P. Thekaekara, R. Kruger, and C. H. Duncan, Appl. Opt. 8,

1713 (1969).15. J. P. Baluteau, E. Bussoletti, M. Anderegg, A. F. M. Moorwood,

and N. Coron, Astrophys. J. Lett. 210, L45 (1976).16. C. B. Farmer, 0. F. Raper, and R. H. Norton, Geophys. Res. Lett.

3, 13 (1976).17. U. Fink, H. P. Larson, G. P. Kuiper, and R. F. Poppen, Icarus 17,

617 (1972).18. H. P. Larson, U. Fink, R. Treffers, and T. N. Gautier, Astrophys.

J. Lett. 197, L137 (1975).19. G. Michel, Appl. Opt. 11, 2671 (1972).20. U. Fink, H. P. Larson, and R. R. Treffers, Icarus, in press

(1978).21. H. P. Larson, U. Fink, and R. R. Treffers, Astrophys. J., 219,1084

(1978).22. H. P. Larson, R. R. Treffers, and U. Fink, Astrophys. J. 211, 972

(1977).23. R. R. Treffers, H. P. Larson, U. Fink, and T. N. Gautier, Icarus,

in press (1978).

Meetings Calendar continued from page 1346

1978August

18-15 Sept.Summer School of Space Physics, Mathematical andPhysical Principles of Remote Sensing, StrasbourgCentre National d'Etudes Spatiales, 18, ave. Ed-ouard-Belin, 31055, Toulouse Cedex, France

21-24 2nd World Hydrogen Energy Conference, Zurich W.Seifritz, Swiss Federal Inst. for Res. CH5303, Wur-enlingen, Switzerland

21-25 Scanning Electron Microscopy course, Chicago N. Daerr,McCrone Res. Inst., 2508 S. Mich. Ave., Chicago, Ill.60616

22-31 Infrared and Raman Spectroscopy of Biological Mole-cules, NATO adv. study inst., Athens T. Theophan-ides, National Hellenic Research Found., 48 BassileosConstantinou Ave., Athens 501/1, Greece

28-31 SPIE, 22nd Internat. Symp. and Instrument Display, SanDiego SPIE, P.O. Box 10, Bellingham, Wash. 98225

28-29 Meteorological Optics, OSA Topical Meeting, Key-stone, Colo. OSA, 2000 L St. N. W., Washington, D.C.20036

30-1 Sept. Atmospheric Spectroscopy, OSA Topical Meeting,Keystone, Colo. OSA, 2000 L St. N. W., Washington,D.C. 20036

30-2 Sept. Conference on the Physics of the Metallic Rare Earth andActinides, Grenoble R. Lemaire, Laboratoire deMagnetisme du C.N.R.S., 166X, 38042 Grenoble,France

September? American Congress of Surveying and Mapping/American

Society of Photogrammetry, fall technical mtg., Al-buquerque W. E. Stephens, 9237 Snowridge Ct. NE,Albuquerque, N.M. 87111

4-7 Physics of Metallic Rare Earths, colloquium, GrenobleR. Lemaire, Laboratoire de Magnetisme du C.N.R.S.,166X, 38042 Grenoble, France

4-8 8th European Microwave Conference, Paris E. Constant,Centre Hyperfrequences et Semiconducteurs, U. desSciences et Techniques, BP 36, 59650 Villeneuved'Ascq, France

4-9 6th International Conference on Raman Spectroscopy,Bangalore, India J. R. Durig, U. of South Carolina,College of Science & Math., Columbia, S.C. 29208

5-7 International Optical Computing Conference, LondonE. C. Landau, Naval Underwater Systems Center,New London, Ct. 06320

6-8 Fiber Optic and Communication Exposition '78, ChicagoP. Polishuk, Information Gatekeepers, Inc., 167 CoreyRd., Brookline, Mass. 02146

10-17 11th Congress of the International Commission for Op-tics, Madrid J. Bescos, Sociedad Espanola de Optica,Serrano, 121, Madrid 6, Spain

11-15 Transmission Electron Microscopy course, Chicago N.Daerr, McCrone Res. Inst., 2508 S. Mich. Ave., Chi-cago, III. 60616

continued on page 1366

1 May 1978 / Vol. 17, No. 9 / APPLIED OPTICS 1359