Astronomical Fourier spectrometer Pierre Connes and Guy Michel A high resolution near ir Fourier spectrometer with the same general design as previously described labora- tory instruments has been built for astronomical observations at a coude focus. Present spectral range is 0.8-3.5 Am with PbS and Ge detectors and maximum path difference 1 m. The servo system can accom- modate various recording modes: stepping or continuous scan, path difference modulation, sky chopping. A real time computer is incorporated into the system, which has been set up at the Hale 500-cm telescope on Mount Palomar. Samples of the results are given. 1. Introduction The system to be described here is one in a set of several near ir Fourier interferometers built during the last 10 years. The first (interferometer I) con- structed at the Jet Propulsion Laboratory and later modified at Laboratoire Aim6 Cotton", 2 had been in- tended for use at a telescope coud6 focus. Limita- tions were 0.1-cm'1 resolution and 5-samples/sec op- eration; with the recording and computing facilities available, the practical maximum number of samples was about 50,000. Many results have been pub- lished, in particular a Near Infrared Planetary Atlas, and are reviewed in Ref. 3; a more recent result is de- scribed in Ref. 4. Interferometer II, built by Pinard at LAC, 5 extend- ed the resolution to 0.005 cm-' (2-m maximum path difference), while keeping the same speed and capaci- ty limitations. Maximum resolution could only be achieved across a narrow spectral region ( 125 cm-'); the instrument was used on laboratory emis- sion or absorption sources. 6 The next generation"" 2 consisted of three nearly identical systems (interferometers IIIABc) built to take advantage of the improved techniques for com- puting Fourier transforms.' 3 "1 4 The same Amax 2 m was kept, but one could record up to 2 X 106 sam- ples at speeds up to 100 samples/sec. These were ba- sically laboratory instruments, and two of them are now routinely used for recording molecular absorp- tion' 5 "1 6 or atomic emission spectra.1"1 7 Neverthe- less IIIC has been carried to the coud6 focus of the Haute Provence 193-cm telescope by Maillard with The authors are with the Centre National de la Recherche Scientifique and Laboratoire Aim6 Cotton, CNRS, Facult6 des Sciences d'Orsay, France. . Received 3 February 1975. good results,1 8 but the size and weight of the instru- ment made the whole operation rather cumbersome. Interferometer IV (to be described here) has again been built for the coud6 focus of a large telescope, thus made somewhat more portable and easier to set up. The complete optical system, including guiding and chopping facilities, is located on a single plat- form and enclosed in a 170 X 70 X 70-cm insulating box, which also serves as a carrying case; total weight is about 200 kg. This may still appear much; how- ever, the performance of the system should be com- pared with that of the largest existing coud6 spectro- graphs, which are not exactly portable instruments. While Amax has been limited to 1 m, which seemed adequate for astronomical problems, the speed of op- eration can now reach 2000 steps/sec. The servo sys- tem has been made simpler and more versatile than in the previous version. The operator can select sev- eral different recording modes; the normal one for faint sources is still the already described", 2 associa- tion of relatively slow stepping and fast internal modulation (i.e., modulation of path difference inside the interferometer). A sky chopping technique com- patible with the stepping has also been included. A real time special purpose digital computer' 9 20 is systematically used during recordings; it displays a 4096-sample slice of the spectrum which is adequate for checking resolution and SNR and even for detect- ing interesting features. This relatively cheap and simple device has been to a large extent responsible for the success of the first observations. The size of the interferograms (routinely 106 samples) makes them too large to transmit over regular telephone lines, as was done with interferometer I, while small general purpose computers are far too slow. The complete interferogram is simultaneously recorded on digital magnetic tape, and the complete spectrum is produced after the end of a run by a general pur- September 1975 / Vol. 14, No. 9 / APPLIED OPTICS 2067
Transcript
Astronomical Fourier spectrometer
Pierre Connes and Guy Michel
A high resolution near ir Fourier spectrometer with the same general design as previously described labora-tory instruments has been built for astronomical observations at a coude focus. Present spectral range is0.8-3.5 Am with PbS and Ge detectors and maximum path difference 1 m. The servo system can accom-
modate various recording modes: stepping or continuous scan, path difference modulation, sky chopping.A real time computer is incorporated into the system, which has been set up at the Hale 500-cm telescopeon Mount Palomar. Samples of the results are given.
1. Introduction
The system to be described here is one in a set ofseveral near ir Fourier interferometers built duringthe last 10 years. The first (interferometer I) con-structed at the Jet Propulsion Laboratory and latermodified at Laboratoire Aim6 Cotton",2 had been in-tended for use at a telescope coud6 focus. Limita-tions were 0.1-cm'1 resolution and 5-samples/sec op-eration; with the recording and computing facilitiesavailable, the practical maximum number of sampleswas about 50,000. Many results have been pub-lished, in particular a Near Infrared Planetary Atlas,and are reviewed in Ref. 3; a more recent result is de-scribed in Ref. 4.
Interferometer II, built by Pinard at LAC,5 extend-ed the resolution to 0.005 cm-' (2-m maximum pathdifference), while keeping the same speed and capaci-ty limitations. Maximum resolution could only beachieved across a narrow spectral region ( 125cm-'); the instrument was used on laboratory emis-sion or absorption sources.6
The next generation"" 2 consisted of three nearlyidentical systems (interferometers IIIABc) built totake advantage of the improved techniques for com-puting Fourier transforms.' 3"14 The same Amax 2m was kept, but one could record up to 2 X 106 sam-ples at speeds up to 100 samples/sec. These were ba-sically laboratory instruments, and two of them arenow routinely used for recording molecular absorp-tion'5"16 or atomic emission spectra.1"17 Neverthe-less IIIC has been carried to the coud6 focus of theHaute Provence 193-cm telescope by Maillard with
The authors are with the Centre National de la RechercheScientifique and Laboratoire Aim6 Cotton, CNRS, Facult6 desSciences d'Orsay, France.. Received 3 February 1975.
good results,18 but the size and weight of the instru-ment made the whole operation rather cumbersome.
Interferometer IV (to be described here) has againbeen built for the coud6 focus of a large telescope,thus made somewhat more portable and easier to setup. The complete optical system, including guidingand chopping facilities, is located on a single plat-form and enclosed in a 170 X 70 X 70-cm insulatingbox, which also serves as a carrying case; total weightis about 200 kg. This may still appear much; how-ever, the performance of the system should be com-pared with that of the largest existing coud6 spectro-graphs, which are not exactly portable instruments.
While Amax has been limited to 1 m, which seemedadequate for astronomical problems, the speed of op-eration can now reach 2000 steps/sec. The servo sys-tem has been made simpler and more versatile thanin the previous version. The operator can select sev-eral different recording modes; the normal one forfaint sources is still the already described",2 associa-tion of relatively slow stepping and fast internalmodulation (i.e., modulation of path difference insidethe interferometer). A sky chopping technique com-patible with the stepping has also been included.
A real time special purpose digital computer' 9 20 issystematically used during recordings; it displays a4096-sample slice of the spectrum which is adequatefor checking resolution and SNR and even for detect-ing interesting features. This relatively cheap andsimple device has been to a large extent responsiblefor the success of the first observations. The size ofthe interferograms (routinely 106 samples) makesthem too large to transmit over regular telephonelines, as was done with interferometer I, while smallgeneral purpose computers are far too slow. Thecomplete interferogram is simultaneously recordedon digital magnetic tape, and the complete spectrumis produced after the end of a run by a general pur-
pose computer. Present transform time on the 370/168 IBM we use is 3 min for 106 samples, which issmaller than the time normally spent later treatingthe spectra (preparing suitable tracings, detectingand measuring lines, looking for differences, etc.).
The system has been set up since October 1973 atthe 500-cm Hale Telescope of Palomar Observatory;sample results have been published already.2 Wewill try here to give a complete technical description;this will include some features that are not new(present in the previous types) and some that havenot been tested on the sky during the Palomar runs(not yet available). We will also refer in passing toelements in the design of interferometer V (a visibleand near uv 2-m path difference instrument present-ly operating at LAC, built in collaboration with P.Luc) and interferometer VI (a far ir 20-m path differ-ence instrument under construction at Laboratoirede Spectroscopie Moleculaire of Professor Henry);both will be fully described elsewhere. These two in-struments use the same electronics and servo systemsas IV, which illustrates the degree of versatility of ourlatest design and shows it not limited to astronomicaluse.
1. Optical System
The complete optical system is supported on awelded aluminum alloy 150 X 50-cm base plate; thethree-dimensional optical path occupies 60 cm ofheight. This volume contains all facilities, i.e., inter-ferometer, guiding and chopping systems, laser andtest sources except for the two detectors that projectoutside the thermally insulating case. The systemcan in principle be used as it stands at any f/30 coud6focus; this is in contrast with our previous designs(interferometers I to III) for which two separate plat-forms were needed.
A. Astronomical Light Path
The coud6 focus image of the star S is formed onthe CaF2 field lens LF (Fig. 1), which also acts as anentrance window for the case. Dimensions are 80 X150 mm (110 X 210-sec of arc field with the 5-m tele-scope). LF images the telescope aperture on the50-mm diam light weight servo controlled mirror MA;and MA can be directed to project an image of theobject on either of the two interferometer entranceapertures D, and D2 . The two off axis CaF2 lensesLI, L2 serve two purposes. First, the front surfacereflects the visible part of the beam (X < 0.8 ,im)toward the guiding photomultiplier (Fig. 2); the coat-ing has better than 85% transmission throughout thenear ir. Second, they act as prisms (mean angle 50), and the two ir beams diverge toward the colli-mating mirrors MB1, MB2; at the same time the cur-vature of both surfaces is computed in order to imagethe telescope aperture on the beam splitter Bs.Chromatic aberration is negligible through the 1-6-,m range.
MB1, MB2 produce two parallel 8-cm diam beamsfor the interferometer. They are used slightly offaxis (like MA), so they are made rather thin (L5
mm), and a suitable pressure on the back produces aslight toroidal distortion that compensates the totalsystem astigmatism. The plane mirrors Mc,, Mc2send the two beams horizontally through the inter-ferometer.
Interferometer design (Fig. 1, levels 3 and 4) isidentical to the one of types II and III. The originalreason for the use of cat's eyes was the cancellation oftilt, which seems less important today as a plane mir-ror with tilt servo control might be preferred. How-ever, the additional advantages are (1) clear separa-,tion of entrance and output, leading to separate sym-metrically used coatings on beam splitter and beammixer, thus to perfectly achromatic interferograms;(2) availability of a small light weight mirror withinthe interferometric path that can be used for pathdifference modulation (§IV,A,1) and field compensa-tion (§II,F). The main disadvantage of cat's eyescompared with corner cubes is their great length,which is not an important factor when the carriagemotion is itself very large.
Beam splitter Bs and beam mixer BM are two iden-tical Infrasil plates, 90 mm in diameter and 10 mmthick; the plates are plane, parallel, and of equalthickness within about one quarter of a visible wave-length. Coating is silicon, quarter wave at 1.6 Atm;reflection R and transmission T are such that the4RT efficiency is greater than 0.9 from 0.95 gm to 2.5Am and greater than 0.8 from 0.9 gm to 3.5 Aim. Re-flecting surfaces are on opposing sides of BM and Bsto ensure complete optical symmetry (equality of thereflection phase shifts). Second surfaces have iden-tical antireflection coatings.
The two plates are not in the same vertical plane(Fig. 3), but offset by [(n - 1)/n]t - 0.3 cm, where-tis their common thickness. This is necessary toeliminate the slight lateral displacement of the raysdue to refraction in the plates. Since the large mir-rors ME1, ME2 of the cat's eyes are spherical, the twooutgoing wavefronts are distorted by spherical aber-
Fig. 2. Visible light path to finder eyepiece E and guiding photo-multiplier PM (vertical plane).
ME SM Bs
Fig. 3. Left: cross section of beam within interferometer. Pro-
jected light path of a principal ray is shown. Right: top view of
beam splitter and beam mixer. Sideways displacement added forclarity.
'H 4Fig. 4. Reference light path. Simplified diagram; actually thepath is three dimensional, and the ingoing and outgoing rays passthrough the centers of BM, Bs, thus they should appear on top of
each other.
ration.22 Compensation is complete-at zero pathdifference-only if they superpose exactly; hence theBS,BM system must introduce no lateral shear. Athigh path difference compensation is not exact, butthe residual term is negligible.
Cat's eyes diameter is 18 cm, focal length 70 cm;the small mirrors are 2 cm in diameter, slightly con-vex, and their radius is chosen so that Bs (and thetelescope aperture) are imaged on BM; the usablebeam diameter is 8 cm.
The two output beams are sent to the two detec-tors. Most commonly used are two pairs of squarePbS cells, of sizes 0.1 X 0.1 mm and 0.25 X 0.25 mm.Two f/1 parabolic mirrors MG1 and MG2 act as con-densers; image spread from a point input is about 0.1mm. However, the cells are immersed in optical con-tact with hyperhemispheric strontium titanate lensesof index n = 2.2, which give an additional reductionfactor of n2 _ 5; thus the paraboloid errors are negli-gible. The two pairs of cells cover 15 sec of arc and38 sec of arc on the sky respectively (with the 500-cmtelescope). The vacuum windows WiW2 of the cellenclosures act at the same time as output windowsfor the interferometer box. An intrinsic germaniumdetector2 3 has lately become available to cover the0.9-1.7-um range with greatly increased detectivity.Since the sensitive area is about 5 mm in diameter, asimple glass lens acts as condenser. Only one detec-tor was used because of cost. The use of InSb detec-tors is planned.B. Test Facilities
The plane mirrors MH, Ml (which can be flippedout of the beams) permit illumination of the entranceapertures by various test sources and visual examina-tion of the fringe pattern. Light paths are not shownon Fig. 1 for clarity. The test sources are: (1) Afraction of the reference laser light beam that hasbeen passed through a scatterer to provide crudelyuniform illumination of both DI (or D2) and Bs. Itis used for checking interferometer adjustment andfor recording interferograms (and spectra) of a singlemonochromatic line. (The absorption of silicon at X= 6328 A is still reasonably low.) (2) A set of spec-tral lamps giving various emission spectra. (3) A
double passed, 1 m long sealed tube filled with N20at about 8-cm Hg pressure, illuminated by a tungstenlamp. This provides a stable absorption spectrumwith sharp lines; when used with the real time com-puter it provides an excellent test of complete systemperformance (see Fig. 21).
Finally a small variable speed chopper C modu-lates the beams from the various sources, includingthe astronomical ones; the waveform is a roughly tri-angular, since beam diameter is equal to blade width.It is used to check detector adjustment and perfor-mance at various frequencies (Sec. IV.B.1).
C. Reference Beam (Fig. 4)
The reference source is a homemade, single mode,Lamb dip stabilized helium-neon laser. The suc-cessfully used 3.5-Am superradiant line from xenon1
was abandoned for two reasons: first the long tube iscumbersome for a portable instrument; second wewanted to develop a servo system applicable to a uvinterferometer, and the 3.5-,gm wavelength is ratherlarge for that purpose.
The laser output is plane polarized. Half-waveplate H adjusts the polarization azimuth. Withinthe two interferometer beams, quarter-wave platesQ1Q2 produce right and left circular polarizations.Adding the two beams gives a linearly polarized out-put of constant intensity, the azimuth of whichmakes half of a revolution for a path difference
M
I
SLITMOUNT
A
I
I
I
I
I
L F
Fig. 5. Relay system used for Hale telescope (vertical plane).M 3, M 2 form a unit magnification system. LF is the field lens and
Fig. 6. Guiding mirror MA. T, Tv:torque motors; FF,: magnesium al-loy forks (not touching each other). X1 ,X 2 , Y1 , Y2 magnesium alloy cubes fixedwith epoxy to mirror back. Four flatsprings S permit about 5 of rotation.On the right: guiding photomultipliercathode and scan pattern for a centeredstar (giving zero demodulated output).Star follows ABCDEFGH path; actuallydistances such as HB, BD, etc. are zero.Half of the time is spent within the
square aperature.DDE
change of one wavelength. A small beam splitter Ssends the light to photomultipliers PM1, PM2through the Polaroid analyzers P1, P2, which made a450 angle with each other.24 Thus when the pathdifference varies, the photomultipliers see two si-nusoidally modulated signals in quadrature; one peri-od corresponds to one wavelength of path difference.The second interferometer output might be used andS eliminated, which would double the light output,but this did not seem necessary.
D. Palomar Relay Optics
No space being available close to the Hale tele-scope coud6 focus, it was necessary to relay the lightto the interferometer input about 8 m away on alower level. The beam must pass through two differ-ent apertures that cannot be enlarged. The relaysystem (Fig. 5) implies four additional reflections,and the no vignetting field is barely 25 sec of arc indiameter. A larger field could only have been real-ized with a far more complex system. Mirrors M1 ,M 3 can be removed quickly to permit use of the spec-trograph.
E. Guiding and Chopping (Fig. 2)
The coated front surfaces of field lenses L1, L2 areadjusted to fall on the same sphere and so act as afield mirror for the reflected visible light. Lens L4collimates the beam; the small beam splitter b, sendshalf of the light to an external finder eyepiece. Theother half is focused onto the guiding photomultipliercathode by system S, which is either a converginglens or a telephotolike association of converging anddiverging lenses. Six different focal lengths, eachapproximately double the preceding one, can be real-ized. The field in the sky increases from 2 sec of arcto 60 sec of arc. The small fields are used for stars,particularly in daytime, the others for planets. Thesmallest cannot be used under conditions of high tur-bulence, and the largest was not usable at Palomarbecause of the relay system field. The practical day-time limit has been 6th magnitude, on red stars, witha clear sky.
The guiding photomultiplier is a S20 cathode, ITTtype FW 130. A 5 X 5-mm aperture is electrostati-cally imaged on the 19-mm diami cathode and can be
made to scan the optical image by two rectangulardeflection coils. A standard, star-shaped sweep pat-tern is used at 1 kHz (Fig. 6). After demodulation Xand Y linear error signals are produced; they are in-dependent of object brightness and also of the imagediameter as long as it is smaller than the aperture.
The servo controlled mirror MA can be tilted in Xand Y by two MFE, type T4-150 torque motors (Fig.7). Each has two coils; one is used as a motor and re-ceives the error signal through a suitable phase loopand power amplifier. Gain can be increased by fac-tors of 2 according to the selected field. The secondcoil generates a voltage proportional to angular veloc-ity and is used for damping. Figures 8(a) and 8(b) il-lustrate the response time of the system.
Two different operating modes are available. Forsimple guiding, the coud6 focus image is left free towander over LF, and a stabilized image is producedon either D1 or D2. The unused aperture is illumi-
a
b
C
dV- 5.10 3s
Fig. 7. Guider performance. (a) Guiding mode. The residualerror trace illustrates response to 20-Hz square wave perturbation.Transient error is corrected after 5 msec. (b) Slight cross couplingon perpendicular channel. (c) Chopping mode. The mirror ismade to oscillate in X, and the beam goes from D1 to D2 (1 min ofarc on the sky). (d) Cross coupling on Y channel. The small rip-
Fig. 8. Deformation of flexible mirror shown with a simple Mi-chelson interferometer at X = 6328 A: in the second arm a highaccuracy concave, constant curvature (R = 740 mm) mirror is
placed. Jupiter image diameter would be 10 mm.
nated by sky light, provided the telescope covers thefull 2-min of arc field.
The system does compensate for the lowerfrequencies of air turbulence by stabilizing the imageCG. However, our experience has been that when ahigh enough (700 Hz) internal modulation frequencyis used in the interferometer, fast guiding does littleto improve the final spectrum, and our guiding sys-tem is normally used with a reduced bandpass (0.2-sec response time). Nevertheless, it is very conve-nient to eliminate completely the effect of telescopedrift, which is checked with a monitoring oscilloscopeon which the two coil currents are shown in X and Ycoordinates. Thus the operator knows when theimage has drifted to the edge of LF.
For guiding plus sky chopping the mirror is madeto oscillate with a nearly square waveform by feedingan additional square wave to the X coils [Figs. 7(c)and 7(d)]. The image jumps from D to D2; theshortest response time of the servo is used. Maxi-mum frequency is about 50 Hz, although 130 Hz canbe reached with a different damping adjustment, if asine wave modulation (of lower efficiency) is tolerat-ed. Interface with the recording system is describedin Sec. IV,A,1. The chopping mode has performedwell in the laboratory, but has not been tried at Palo-mar: The complex nature of the relay system-inparticular of the central obstruction-and the insuf-ficient field of view mean perfect background com-pensation could not be achieved in the thermal ir.Thus our recordings have been limited so far to the0.9-2.5-,um region, for which sky chopping is notneeded. The difficulty could be overcome by an in-dependent chopper placed in front of the relay sys-tem.
F. Field Compensation
The principle of field compensation for a cat's eyeinterferometer has already been discussed.25 While
system construction has not been completed, it seemsworthwhile to describe it in full.
Any interferometer of the Michelson type (i.e.,with plane mirrors, corner cubes, or cat's eyes) suf-fers from variation of path difference with ray obliq-uity, and Newton ring type fringes appear at infinity.For an axial path difference AO the actual path differ-ence at incidence 0 is A = AO cos0, and the variation A- Ao = A002/2. If the center of the field is bright, thefirst bright ring angular radius is 01 = (2X/AO)1/2.When working on an extended source, one generallyselects a circular. aperture such as A - A = X for themarginal ray. The angular diameter is then 201, andthe solid angle Q1 = 7r012 = 27rX/Ao = 2/Ro, where Ro= Ao/X is the theoretical resolving power for wave-length X. The instrumental line shape of the finalspectrum, expressed in wavenumbers, will be convo-luted by a rectangular function of width 1/Ao and theactual resolving power reduced to approximately R =0.8Ro.
While other compensation schemes have been de-scribed,26 the cat's eyes system is remarkable becausecompensation can be achieved without adding anyoptical element; only the small cat's eye mirror MFmust be made flexible. This is because the fringes atinfinity are localized on MF. If we can deform MFwhile keeping it spherical, we can increase or de-crease at will the number of circular fringes for anypath difference.
Let F be the focal length of the large mirror, A thesmall mirror diameter, M = A/2F the maximum rayangle, AM the corresponding path difference. Wewill achieve compensation if the displacement of theMF edge with respect to its center is d = (AM - Ao)/2= Ao0M2 /4 = (Ao/16) (A/F)2 . This result is indepen-dent of wavelength, and compensation is fully achro-matic. Fringes have disappeared and been replacedby a uniform interference field.27
The compensated solid angle Qc is in principle lim-ited by A and can be written c = r/4.(A/F . How-ever, we also have Qc = 4rd/Ao; thus it is limited inpractice by the maximum edge deformation dmax onecan produce while still keeping the mirror spherical.The light gain compared to the uncompensated fieldQ% will be G Qc/Q = 2dmax/X; it is also equal to thenumber of fringes that have disappeared. The corre-sponding path difference and resolving power will beA = 16dmax(F/A) 2 , and R = Ao/X; thus for a givendmax, G and R are inversely proportional to X.
To summarize, in order to achieve a certain gain Gat wavelength X, the edge deformation must be GX/2and the mirror must be kept spherical, a reasonabletolerance being X/4. This means in particular themagnitude of the deformation must be accurate towithin 1/2G relative error.
The most difficult part of the system is obviouslythe flexible mirror itself. A prototype has been de-signed and built of stainless steel by Lemaitre at Ob-servatoire de Marseille.28 29 The accuracy test per-formed by P. Luc at LAC are shown in Fig. 8. Themirror diameter (20 mm) is somewhat larger than
Fig. 9. Left: front view of planetary image projected onto small
cat's eye mirror. Ox and Oy are directed along the equatorial and
polar diameters, respectively. OX and OY are the lateral transla-tion axes for the fixed cat's eye. Right: side view showing angle
of tilt F.
necessary: Let DT and DB be the telescope andbeam splitter diameters and a the source apparentdiameter. Then the image diameter on MUV will bes = Fa.DT/DB. With Jupiter (a = 2.25 10-4), theHale telescope (DT = 500 cm), and our present sys-tem (F = 70 cm, DB = 8 cm), one gets s = 10 mm. Atthe edge of this image we find that the maximum us-able deformation corresponds to eight fringes at0.6328 ,gm, i.e., G = 8 (or 2G = 16 with two identicalmirrors on both cat's eyes) for this particular wave-length. We also find that the accuracy would bemore than adequate (within this diameter) for opera-tion down to X = 0.4 gAm, in which case one would getG = 12.5, 2G = 25, Ao = 40 cm, and Ro = 106. With-out compensation this result would require an in-crease by (25)1/2 in DB, i.e., a 40-cm beam splitter and80-cm cat's eye.
While the nearly full diameter of this particular
mirror would be usable for X > 2 in on largersources, it would still only give 2 = 5 and R =200,000 on Jupiter at 2 gim. From the experiencegained a smaller, more flexible, lower accuracy mirrorshould be built that would be better suited (i.e., pro-vide larger G and R) for ir work on Jupiter. Alter-nately the same mirror could be kept and a shorterfocal length cat's eye used.
G. Planetary Rotation Compensation
For rapidly rotating planets, another limitation toresolving power lies in the Doppler widening of thelines if light from the entire disk is used. For Jupiterequatorial speed is ve = 12.6 km/sec, and the ex-tremities of the equatorial diameter move with radialspeed ve for an earth observer. A sharp line ofwavenumber ao is Doppler stretched over 2cove/c =
8.4 10-5ao. Maximum resolving power is of the orderof 12.000.
To avoid the large loss of light implied by the re-striction to a narrow strip in the image, compensationschemes have been devised for grating spectrome-ters32 and Fabry Perot etalons,33 which are simplyused off axis. We will show that the cat's eye Mi-chelson interferometer may easily be modified for ad-ditional compensation of planetary rotation and thatthe solution is rigorous, unlike the one for the Fabry-Perot, which is not field compensated to start with.
In the planetary image on MF (Fig. 9) the radialspeed vr is proportional to distance x from the polardiameter: r = 2vex/s. This result is independent ofellipticity; simply, s must be taken equal to the equa-torial image diameter. At any point x, o is shifted tod = a4l + 2(velc)-(x/s)]. If at the same point thepath difference can be modified by the same amount,i.e., AO be replaced by Ao' = Ao[1 + (2ve/c)(x/s)], theratio a'/Ao' will be constant everywhere and Dopplerbroadening eliminated.
Since we have field compensation Ao is so far con-stant, i.e., independent of 0 or x over the wholeimage. However, providing the wanted variationA0' - AO = 2A(v/c), (xis) is a simple matter: Thesmall mirror may for instance be tilted by an angle f
Fig. 10. Proposed control system for compensation ofinterferometer field and of planetary rotation.
Fig. 11. General view of servo system. Operation of slave carriage involves a completely independent servo loop (upper left). The inter-ferometric signal from the photomultipliers first produces a SSB modulated carrier (Fig. 13), then a dc error signal (Fig. 17), which is sepa-rated into high and low frequency components. These go to the servoing linear motor and piezoelectric ceramic. The HP and LP filtersare actually complex, empirically adjusted phase correcting loops that incorporate signal differentiation for damping.
= (e/c) (Ao/s), with the tilt axis coinciding with thepolar diameter; 3 must be made to increase linearlywith AO. An alternative technique calls for a verysmall lateral translation of the fixed cat's eye as awhole, perpendicular to the polar axis. In both casesthe two interfering wavefronts are sheared by 21. Bytaking = F = (vec)(Fis)Ao, one will obtain thesame variation A' - AO as above, and with mono-chromatic light straight equidistant fringes will ap-pear across MF just as if it had been tilted.34
The second technique is distinctly simpler becausethe fixed cat's eye is already provided with two preci-sion motor operated controls for rectangular transla-tions, which are needed anyway for adjustment atzero path difference. For Jupiter and the Hale tele-scope again, we get 1 = 3 X 10-3 Ao or 0.3 mm atA = 10 cm. This shear is too small to introduce anerror from the cat's eyes spherical aberration.
Clearly a stable and sharp image is needed; goodseeing and guiding are essential, otherwise the spec-tral resolution will be degraded. However, for agiven resolving power the requirements are not dif-ferent if the image is projected onto a slit and a smallfraction of the planetary light used.
The control system for both field and rotationcompensations will now be described (Fig. 10); how-ever, it has not been built or tested because the flexi-ble mirror was available too late for the Palomarruns..
The carriage potentiometer (Sec. III.A.2) providesvoltage VA proportional to A0 with better than 1% ac-curacy. A pressure transducer with output V mea-sures the oil pressure behind M. One wants to
servo Vp to VA, thus their difference is amplified andenergizes a small dc motor that compresses oil filledbellows connected to MF. A large reduction geartrain is used, and only very low frequency servo re-sponse is required.
Similarly two linear transducers produce voltagesVx and Vy proportional to the fixed cat's eye lateraltranslation. Since operation is at a coud6 focus thepolar diameter in the image rotates during the night.While a classical image rotator might be used, a pure-ly electronic solution involving no additional optics ispreferred. A synchronous motor rotates two sine co-sine potentiometers at a 1-turn/24 h rate, and theseprovide V cosHA and Vy sinHA voltages, whereHA. is the hour angle. The differences VA - VxcosHA and VA - V sinHA are fed to the controlmotors. In this manner the x direction is made torotate relative to X and Y.
111. Error Signal Generation and Servo SystemA general diagram is found in Fig. 11; the different
parts will be described separately.
A. Interferometer Carriage
Two different carriages have been built, both with50-cm displacement (1-m path difference). Only thefirst was available during the Palomar runs.
1. Air Bearing and Linear Motor SystemAn air bearing was preferred to the previously used
oil bearing (interferometers II and III) for the sake ofgreater simplicity since vacuum operation of interfer-ometer IV was not contemplated. Five small stain-
Fig. 12. Slave carriage system. Top: horizontal view. Rv: three vertical ball bearings. M: two magnets providing horizontal restor-
ing force. B: two flexible belts pulling the carriage. Bottom: side view (linear and rotary motors omitted for clarity). F: four flexure
hinges, each incorporating two pairs of crossed flat springs and giving a well defined rotation axis. Angle of tilt is greatly exaggerated com-
pared to actual maximum.
less steel polished pads running on glass plates definethe translation; air film thickness is a few microns.A servo controlling force is applied through two sym-metrical dc linear motors; these are homemade butanalog in principle to the ones used on some fast penrecorders. A row of identical parallel bar magnetsproduces a constant field along the displacementpath within a gap in which the coil moves. For50-cm displacement the weight of each unit is about15 kg.3 5
While the mechanical performance of the systemhas been adequate, the use of an air bearing in alarge, lightly built interferometer, incorporating lib-eral use of long and thin members of light alloys, hasnevertheless been a clear mistake. Whenever thesystem is operated, a small thermal gradient appearsafter a few hours and is disturbed every time the in-sulating box is opened. Small interferometer misad-justments are induced, and the instrument is dismal-ly less stable than its predecessors. Air turbulenceby itself has not been a problem after proper placingof a few paper shields.
Clearly air bearings work well in existing small lowresolution interferometers. In a large instrument wethink a more massive construction, with better inter-nal heat conduction, and/or low expansion materials,or again controlled heating, should eliminate the dif-ficulty; however, a different and more original solu-tion was preferred.
2. Slave CarriageThe principle of the system (which we sometimes
call the airless air bearing) is, in a modest way, analo-gous to the one of the dragfree satellite. The cat'seye is supported by the carriage through a parallelo-gram suspension with flexure hinges (Fig. 12).Weight and spring rigidity are adjusted to make thesystem astatic, i.e., for the maximum displacement ofcat's eye with respect to carriage ( :L3 mm), the re-storing force is hardly measurable. Relative motionis measured with about 1-gm accuracy by a differen-tial transformer type transducer.
The carriage moves along the 50-cm track on fiveordinary ball bearings; rolling surfaces are aluminumalloy worked on a milling machine; no attempt ismade at improving them by lapping. A small dcprinted circuit motor pulls the carriage through twoparallel belts; a tachometer and ten-turn potentiome-ter provide speed and position indications. In opera-tion the dc transducer error signal is fed to the motorthrough a suitable servo loop, and the tachometerprovides damping. Thus the carriage is slaved to fol-low the cat's eye, the position of which is not (so far)defined. If a very slight force (air motion, tilt, orcurrent through the linear motor-to be describedlater) acts on the cat's eye, the carriage starts drift-ing. A frictionless support is simulated; the cat's eyebehaves as if supported on air bearings.
Motion of the carriage is far from frictionless; it
SHIFTER Fig. 13. Single sideband signal generationwith two photomultipliers and high frequency
path difference modulation.
OSCILLATOR PIEZOCERAMIC 1
follows the cat's eye with static errors of the order ofL5 gim; response time is not very fast (0.1 sec).
Thus the servo loop is easy to adjust-compared withthe interferometric servo loop. However, such resid-ual errors are low enough to ensure negligible forceon the cat's eye because of the astatic suspension.By contrast large forces may act on the carriage with-out perturbing the drift; the roller bearings climbeasily over pieces of cardboard placed on the tracks,and the carriage pulls effortlessly the necessary elec-trical cable.
We now have to move the cat's eye, which is donein the usual way by applying force through a linearmotor. However, this can now be a small, simple,and efficient loudspeaker type device; the magnet isplaced on the carriage and the coil on the cat's eye.Thus, for normal operation the interferometric errorsignal is sent to the linear motor coil while the trans-ducer output goes to the rotary motor. There is alsoa slewing mode: The cat's eye is slaved to the car-riage, which can be moved in either direction; speedis controlled by the tachometer. In a third mode thecat's eye is servoed from the output of a second posi-tion transducer placed at the zero path differencepoint. The accuracy ( 1 gim) is sufficient for imme-diate locating of the central interferogram fringes.
The particular carriage built for interferometer IVwas perfected too late for the Palomar observations;however, an identical one has been incorporated intointerferometer V and routinely used with such goodresults that we now consider air or oil bearings as ob-solete for this particular purpose. A larger carriagewith a 25-cm diam cat's eye and 10-i displacementhas also been built and tested for interferometer VI.In this case belts are eliminated; the rotary motor isplaced on the carriage and acts directly on the rollers.
B. Interferometric Error Signal
The signal from photomultipliers PM,, PM2 (Fig.4) can be written as S = 1, cos27rA/Xo and S 2 = 12cos27r(A + o/4)/Xo, where Xo = 1/co is the laser wave-length. Ideally we should produce from these inputsa voltage proportional to A - Ar, where A is the actu-al and At the wanted path difference; At is a function
of t and will be programmed according to the desiredinterferogram scanning pattern.
The technique for producing the error signal to bedescribed here involves slightly more complex elec-tronics than the previous ones1"1l; however, it is me-chanically simpler (no rotating half-wave plate need-ed) and has much greater bandwidth and linearrange. This makes servo operation faster and morereliable.
1. Production of Single Sideband ModulatedSignal
One of the two small cat's eyes mirrors MF is sup-ported by a small piezoelectric tube (10 X 10 mm)(Piezo 1) driven at the resonant frequency No = 150kHz by a sine wave voltage (a few volts) (Fig. 13).The peak-to-peak amplitude of path difference mod-ulation is about '/oth of a reference fringe, or 600A. Thus, loss of efficiency for ir signals is negligible.We can now write A = AM + EXO sinwt, where =2-7rNo, AM is the mean path difference, and E << 1.The photomultiplier output contains terms atfrequencies 0, No, 2NO, etc. After passage through abroadband filter centered on No and giving large at-tenuation at 2No we are left with ' = 7rIlEsin27raoAM-sinwt and S2' = 7rI2E cos27raoAM-sinwt.
We equalize the two amplitudes (I1 = 12 = I) withan attenuator and pass one of the two signals througha fixed phase shifter giving exactly 900 phase shift at150 kHz. Then
Si" = 7TIE sin2rcroAm sinwt,
Si" = 7TIE cos27TcrOAAfcoswot,
and a summing amplifier will give
S = S" + S2" = 7r1E cos27T(Not - aoA).
This signal has constant amplitude (independent ofAM), but the phase is proportional to AM. We cansay it is phase modulated by the path difference, avariation of A equal to o giving 27r phase shift. Ifthe carriage moves at constant speed AM = Vt and S= rIe cos27rNT with N' = No - o V. We have a fre-quency shift proportional to speed or, in other terms,a single sideband modulated carrier. Frequency isincreased or decreased according to direction of dis-placement.
Fig. 14. Alternativescheme for single sidebandsignal generation involv-ing three photomultipli-ers, dc amplification, and
linear multipliers.
The bandpass of the system is limited by the phaseshift variations in the filter and the phase shifter.No detailed study has been made; in practice theerror signal remains usable for N = 150 + 30 kHz, orV = L2 cm/sec. The adjustments, which prove verystable in practice, are: (1) Azimuth of Ql, Q2: Thecarriage moves at constant speed, and one searchesfor a zero modulated signal without an output polar-izer; then the two interfering vibrations are circularand their sum of constant intensity. (2) Azimuth ofpolarizers: one of them is rotated until Si, S2 give acircle on a scope. (3) Electrical attenuation of SI:The high frequency path difference modulation(whose amplitude is uncritical) is introduced; onelooks at the sum signal S and tries for pure phase,zero amplitude modulation.
A different system involving no path differencemodulation and no filtering of signal, thus havinglarger bandpass, was also tested (Fig. 14). Directcurrent amplification is used everywhere, and a thirdphotomultiplier PM3 measures laser intensity; theoutput S3 goes through attenuators A,, A2 and issubstracted from S, S2 in order to produce balancedoutputs I1 cos2wra0A and 12 sin2-ra 0A. These are dcsignals, the average level of which is zero and inde-
pendent of laser intensity. After passing S2 througha third attenuator we have again two signals of equalintensity S' = I cos2raA and S2"' = I sin27rrA.Next S,"' and S2"' are used, in a classical scheme, toproduce SSB modulation of a carrier: two linearmultipliers Ml, M2 give the products S'. coswt andS2 "' sinwt, where coswt and sinwt are derived from ahigh frequency oscillator output. The sum of prod-ucts is again S = I cos(wt - 2raoA).
Carrier frequency is limited purely by multiplierresponse; no piezoelectric device is needed. With adc to 1-MHz bandpass we selected a 200-kHz carrierfrequency. Since neither filtering nor phase shiftingof the signals takes place, A can be allowed to shiftthe carrier from 0 MHz to 1 MHz, i.e., by -200 kHzor +800 kHz, and we have -13 cm/sec < V < + 53cm/sec.
Unlike the preceding system this one is very sensi-tive to any small signal level variation. Electroniczero and gain stabilities were adequate; however, wecould not eliminate a small optical intensity variationwith carriage position, and tedious readjustments ofbalance were necessary. Since system 1 had suffi-cient bandpass, work was discontinued on system 2;however, we think it could be perfected and shouldprove valuable if very high scan speeds were needed.
150 KHZ SOUARE
WAVE OUTPUT
TO B COUNTER
MANUAL 150KHZ SINEPHASESHIFTER WAVE OUTPUT
TO PIEZO TUBE I
Fig. 15. Digital programmable phase shifter with square wave output and manual analog phase shifter (similar to the hour angle phaseshifter in Fig. 10).
Fig. 16. Waveforms in digital phase shifter. (a) Master oscillatoroutput at 1.2 MHz. (b) After clipping. (c) After differentiating.(d) Square wave output at 1.2 MHz/8 = 150 kHz when the plusgate is opened and minus is closed. (e) Increased frequency out-put if one minus pulse out of eight is added. (f) Reduced frequen-
cy if one plus pulse out of four is subtracted.
2. Production of Reference Signal SOWhile S is being produced by the interferometer
and circuitry of Fig. 13 a reference signal So with fre-quency No and adjustable phase is generated electri-cally. This is done in the following manner: The os-cillator operates at frequency 8No (= 1.2 MHz); theoutput goes through a digital programmable phaseshifter, and frequency is divided by 8 (Fig. 15). Thefinal result is So, a square wave at frequency N, thephase of which can be increased or decreased by anymultiple of 2r/8 (corresponding to steps in path dif-ference multiples of Xo/8). The oscillator output isclipped and differentiated; two trains of 8No pulses/sec each result and are separated into two differentchannels by two rectifiers and go to gates G andG -, which can be separately opened or closed by aprogram generator. The pulses are then summedand sent to a three-stage binary counter that per-forms the division by 8; the output rectangular waveis So. If G+ is permanently opened and G- perma-nently closed, output frequency is No. By closingG+ or opening G-, pulses can be added and sub-tracted. Maximum output frequency variation is ob-viously No (or 0 to 2NO), which is more than actual-ly needed. Waveforms are shown in Fig. 16.
Phase may also be varied slowly and continuouslyby a manual analog phase shifter, which could beidentical to the one in Fig. 10 (involving two sine-co-
PHASE SHIFTER
sine potentiometers). To reduce cost only one linearpotentiometer with four 900 inputs has been used;this solution is approximate since output intensity is,not constant, and phase shift is not linear with han-dle rotation. Since the device needs a sine waveformit operates on the signal fed to the piezoelectric tube;the resulting phase shift thus affects not So but S,which is strictly equivalent.
3. Direct Current Error Signal GenerationWe now have to build up a dc signal proportional
to the phase difference of S and So. These signalsare first clipped and differentiated to produce twotrains of N and No pulses/sec, respectively. Thepulses accumulate in two separate n stage binarycounters A and B (Fig. 17). A n bits arithmeticadder is wired to give the difference between the twocounts, and a n bits digital-to-analog converter pro-duces a voltage output V proportional to this differ-ence. Waveshape is shown by Fig. 18. The differ-ence increases (or decreases) by one unit after everypulse entering A (or B); thus V is modulated by arectangular wave. A low pass filter eliminates thiscarrier residual, and the resulting voltage VF is ap-proximately the time average of V. VF is the wanteddc error signal. It is a continuous (not quantified)function of path difference error A - At. The rangeof linear variation is 2 fringes; if the interferometermoves at constant speed a sawtooth pattern is re-corded (Fig. 19, lower).
Normal accuracy of a DA converter is only xl/2LSB; the actual sawtooth will be linear only with thesame accuracy. This is of little concern since theservo loop will normally null the error signal. How-ever, the zero drift may introduce a path differencedrift. These errors were reduced by using only 6 bitsout of the available 8 bits. The range was then 64fringes, or 32 fringes = 20 m relative to the zeropoint, which is more than enough in practice. Forlarge rapid errors (strong shocks applied to the inter-ferometer) the limitation lies rather in the limitedbandwidth allocated to frequency N.
An important advantage arising from a wideband,wide linear range error signal (compared with simplerschemes where fringe intensity is taken as the errorsignal) lies in the ease of damping: V is simply dif-ferentiated to produce a velocity proportional volt-age. No oil damper or velocity transducer is needed.
Fig. 17. Direct current
D.C. ERROR error signal generator (seeFILTER ~~~~~~~~text).
Fig. 18. Upper. Waveforms as a function of time in dc error signal generation for N' slightly greater than N. V is the direct voltage out-
put of DA converter and simply reproduces the count difference, and VF is the filtered output. Lower: error signal VF, as a function ofpath difference error.
C. Servo System and Performance
The low and high frequency parts of the error sig-nal are sent through two phase correcting loops to thelinear motor and to piezoelectric ceramic 2 on thesecond cat's eye (Fig. 11). This is a pile of platestype that gives d5 Atm of displacement for -1000 V;it is used in an aperiodic mode below the resonantfrequency (10 kHz). A transformer provides thedriving voltage, with a feedback loop for linearizingthe response. The limited current output (coupledwith large transducer capacity _0.02 gF) is presentlythe limiting factor in servo response speed, and itcould be improved upon.
Figure 19 illustrates the error signal in two differ-ent modes. In (a) the interferometer is stepping at1100 steps/sec with steps equal to Xo = 6328 A (freespectral range 0-7900 cm1; smaller steps are nor-mally used). The transient error is corrected after10-4 sec. This means 80% of the recording time isstill used for integration of the light signal at 2000steps/sec. In (b) we show the error signal when si-multaneously stepping and modulating the path dif-ference, i.e., in the usual recording mode.
This much faster servo system permits stepping athigher speeds and modulating at higher frequenciesthan the previous ones. Low frequency performance,in particular freedom from drift with residual errorsof the order of 1 A, are about the same (Ref. 1, Fig.13).
IV. Measuring, Recording, and Computing
The servo and measuring systems can accommo-date different recording modes, suitable for differentsources or conditions.
A. Recording Modes
1. Stepping plus Internal ModulationThis has been systematically used for astronomical
sources. Original reasons for adoption of the step-ping technique were (1) accurate definition of pathdifference for interferogram samples leading to high-ly accurate instrumental line shapes' and wavenum-bers,5 15 so far unmatched by continuous scanning;(2) near optimum use of recording time together withthe minimum number of recorded samples M even in
a
b
1 2 2.1s1
Fig. 19. Servo performance as illustrated by the error signal. (a)Pure stepping at 1100 steps/sec; unfiltered DA converter outputshowing carrier residuals. Step size is Xo = 0.6328 jim. (b) Step-ping at 100 steps/sec with step size 30/8Xo = 2.37 Aim and modulat-ing at 350 Hz with 15/8Xo = 1.18-jim amplitude (i.e., maximummodulation efficiency at 4.75 gim in the recorded spectrum). Fil-
tered DA output. Step transients indicated by arrows.
the case of narrowband spectra (i.e., coarsely sampledinterferograms); (3) elimination of electrical phaseshifts, which permits recording our perfectly achro-matic interferograms from zero point without anyphase correction procedure.
Today our new servo system could provide equalaccuracy for continuous scanning (Sec. IV,A,3), elim-inating advantage (1); advantage (2) is less importantsince the enormous reductions in computing speed13have made it practical to oversample in most-notquite all-cases. Point (3) is still valid however.
The rapid path difference modulation technique2
is independent of-but compatible with-stepping.It should be understood as a way of increasing the in-terferometric signal frequencies above those of moreor less 1/f noise from troublesome sources, astronom-ical in particular; at the same time the observer is leftfree to select the recording speed at will. The alter-native technique is fast scanning, which gives thesame degree of cancellation for turbulence frequen-cies; however, when used on faint sources, coherentadding of many inferograms (or spectra) is required.This has not yet been done with a large M(_ 106), al-though there is no difficulty in principle, if direct ac-cess to a computer with disk memory is available.
Altogether the combination of stepping and inter-nal modulation still provides the simplest method forhigh resolution wide spectral band (i.e., large M) re-cording on faint sources; it is directly compatiblewith our real time computer without any limitationon M.3 6 A sine transform interferogram is produced;this means the signal level is accurately zero at zeropath difference, and the mean signal level is also zeroat high path difference. There are two useful conse-quences: (1) The interferogram is started exactly atthe A = 0 point which can be found very convenientlyby scanning the central interferogram portion over afew fringes with the manual phase shifter either inthe positive or negative direction; one turn of thehandle corresponds to o in path difference. (2) Avery large dynamic range is accommodated by
switching of the ac amplifier gain before demodula-tion, integration, and recording [(Ref. 2), Fig. 9].
All our stellar and planetary spectra were recordedwith 700-Hz internal modulation; this was the high-est frequency permitted by our (relatively fast)cooled PbS detectors while keeping near optimum D*and gave greatest reduction of turbulence noise. Re-cording time was usually between 1 h and 3 h, andstepping speed varied from 10 samples/sec to 160samples/sec. The highest speed was used on Venus,in which case 106 sample interferograms have beenrecorded in about 100 mn. For still brighter objects-and faster recording speeds-the stepping frequencywould get close to the internal modulation frequency.Clearly in this case internal modulation becomes un-necessary, and pure stepping would be used (Sec.IV,A,2).
When sky chopping is wanted the program genera-tor (Figs. 15 and 20) feeds to the guiding mirror asquare wave at the stepping frequency or a multipleof it. Half of the integration time for each step isspent with source on Di, sky on D2, and half withsource on D2, sky on DI; also the signal is inverted atthe same frequency before integration. The interfer-ogram of a discrete source is unmodified by the chop-ping, while the one of any uniform background iseliminated. This has however not yet been sky test-ed because of purely optical problems (Sec. II.E).
2. Simple SteppingSimple stepping without modulation [see Fig.
19(a)] is intended for sources like the sun with step-ping frequencies of the order of 1 kHz. The interfer-ogram is now a cosine transform with a maximum in-stead of a zero at the center, which means the A = 0point is ill defined from the interferogram. A specialstarting procedure must be used: First internalmodulation is applied, and the A = 0 point is foundin the usual way; then the modulation is cut off, andthe interferometer remains locked to the same posi-tion. If we were to start the interferogram scan now
Fig. 21. Upper: N2 0 test spectrum (RTC output). 512,000 input samples; the full 4096-sample (4692-4772-cm-') output is presented
on two lines. Step size is 6/8Xo = 0.47 jm and Amax = 24 cm. Insert at right shows band head on an expanded scale. Complete 0-10.538-
cm- 1 range is available from magnetic tape recording and general purpose computer output. Center: Venus spectrum, 1048.000 samples,
Amax = 48 cm-', 3-h recording time, PbS cells. A fraction of the RTC output is shown: Left, trace giving base line level and noise; right,
4658-4668-cm'1 range (1/1024 of total range) showing CO2 bands. Below: same, a Sco spectrum; broad modulations are due to stellarCO; sharp lines are telluric.
the sample position would be right, but the intensityof the first ones would be distorted by transients inthe amplifiers that are broadband but do not pass dc.Thus the program generator starts the scan in thenegative direction, and 1024 samples (an arbitraryfigure) are taken; the scan then reverses automatical-ly, and the computer is instructed to ignore all sam-ples before 2047, which corresponds to A = 0. Alltransients have died down before the actual record-ing is started, and again no special correction proce-dures are introduced into the computing, permittingnormal use of the real time computer. While thisspecial scan procedure has been checked, no solarspectra have been taken with it so far, the 500 cmtelescope being somewhat ill suited to the purpose.
3. Continuous ScanThe manual phase shifter does produce a continu-
ous scan, but this is used purely for finding zero pathdifference. A more elaborate system might be built(LF oscillator plus SSB generator of Fig. 14 type),but this is unnecessary. With the smallest steps(Xo/8 = 791 A, free spectral range 0-63,211 cm-'), thescan is, in the ir, practically continuous. A finer sub-division would be easy to obtain; the logic as it standscould accept a 10-MHz master oscillator giving Xo/60
100-A steps. We have no present plans for re-cordings of this type; they would be required only forbright short lived sources and recordings at frequen-cies in the 1-100-kHz range-with suitable detectorsand a faster recording system.
B. Recording and Computing System (Fig. 20)
1. Recording
The signals from the two ir detectors go throughtwo separate preamplifiers, an attenuator and aphase shifter, then are added with opposite phases.Adjustment is made by using the mechanical chopperC (Fig. 1) while the interferometer is at high path dif-ference (i.e., the signal level on both detectors isabout average). Phase and attenuation must be suchthat the chopper gives zero output at the working in-ternal modulation frequency. This means any pureintensity source fluctuation is cancelled out while in-terferometric signals give maximum output. Thesignal goes next to a programmable ac amplifier thegain of which can be made equal to 1, 4, 16, 64, 256,1024. Gain switching is done manually or automati-cally (without stopping the record) from a presetsample counter; indication of gain changes is sent tothe tape recorder and real time computer (RTC).Demodulation is achieved with FET switches. Thedemodulating square wave comes from the programgenerator; it is derived from the master oscillator fre-quency and may be changed by factors of 2 (175 Hz,350 Hz, 700 Hz, and 1400 Hz available).
A J:10-V, 0-1-MHz voltage-to-frequency converterfeeds a reversible 12-bit + sign binary counter. Theoutput is sent in parallel to a Kennedy tape recorder(through two 256-word buffer memories alternatelyused) and to the RTC, which takes only 10 bits. By
Fig. 22. Low resolution Jupiter spectrum with Ge detector, through 5 sec of arc slit (about ½th of planetary light used), 4096 input sam-ples, 5-cm'1 apodized resolution, 2-min recording time.
using the gain changes, a 22-bit dynamic range (20for the RTC) is realized. Recording is on 9-tracktape; three 8-bit characters are devoted to each inter-ferogram point. The sample counter labels the 256word blocks and the samples within the blocks as anadditional precaution. Maximum average recordingspeed is about 3000 samples/sec.
So far the guiding photomultipler gives an intensi-ty reference that is adequate for near ir work. Rath-er than using the dc signal one takes the demodulat-ed ac at twice the 1-kHz scanning frequency, whicheliminates background. Output is integrated up to apreset level, and a pulse giving the end of cycle com-mand is sent to the program generator. In this wayintegration time varies according to sky transmis-sion.' It is also possible to change at will the integra-tion time and stepping speed during the recording.A double track 10-turn potentiometer is used, andthe gain in the signal channel is changed to keep theintegrated output constant. The signal in the inter-ferogram is not changed and the spectrum undistort-ed. However, it becomes possible to spend moretime on the small central fraction of the interfero-gram, which gives the important low resolution infor-mation, thus reducing low frequency noise while add-ing very little to the total recording time. This facili-ty is systematically used for the fast recordings(~ 100 samples/sec) which are started at about 10samples/sec; cruising speed is achieved after the firstfew thousands of samples. It is also possible to in-crease speed in order to terminate a recording soonerthan planned if the sky deteriorates, which is done ofcourse at the cost of increased noise for high resolu-tion information.
2. Real Time ComputerThis device has been fully described' 9 and five
models built for laboratory or astronomical work.The present one has a 4K spectrum memory and isused both for general checking of interferometer per-formance with the laser and N2 0 tube sources andfor a real time look at the astronomical spectra. The4K limitation means that the full spectral range Au(0-10,530 cm' with the generally used 6/8XO step-ping interval) can be seen only if we have M 4096interferogram samples, i.e., at very low resolution.Then we can look at A/2 with M = 8192 etc.; with M= 220 106 we see A/256 40 cm- at the final res-olution (u a 0.015 cm-). Two options are possible:either this range is continuous or it is divided in twoparts, the first (30 cm-l) being located around some
Approximately half of 0-12,692-cm'1 RTC output is shown.
interesting spectral feature and the second (10 cm-')in a region where atmospheric extinction is completeand pure noise is expected. Scope and pen recorderoutputs are available; tracings may be made duringor after the recording. Figs. 21 and 22 give samplesof such tracings.
V. Results and ConclusionComplete system performance is illustrated by
Figs. 21 and 22. Figure 23 shows some of the gaindue to Fourier spectroscopy in near ir work over thelast 10 years. The greatest resolving power so far hasbeen achieved on Mars and Venus with PbS cells (106samples; 50-cm path difference, b = 0.015 cm-l).In retrospect the choice of 100 cm for the maximumpath difference seems pessimistic since with the newGe detector the signal will be adequate for higher res-olution, which the study of line profiles would indeedwarrant. The astronomical results will be reportedelsewhere and also an analysis of planetary radial ve-locities. From a preliminary study it is clear that in-strumental error is comparable to the one of type IIIinterferometers,15 i.e., a line pointing rms error ofabout 10-4 cm-, or 10-8 relative error at 10,000cm, which is equivalent to a 3-m/sec radial velocity.
Altogether we have shown that the kind of accu-rate performance previously demonstrated in ourlaboratory instruments is both feasible and useful fornear ir astronomical observations. In addition thedesign has been simplified and made more versatile;usefulness of the improvements is not limited to as-tronomical problems.
A. Ubelman has collaborated with the whole proj-ect from the origin and actually built most of the in-terferometer while R. Leroux, A. Richard, and J. Du-rand built the electronics and real time computer;their constant help has been invaluable. We thankF. Roesler for discussing compensation schemes; A.Marlot for the guider mirror and J. Friteau for theguider electronics; G. Lemaitre for the flexible mir-ror, R. Pelletier for the visible reflecting coatings,and A. Goldman for various filters. A. Mantz and P.Luc participated in the laboratory testing, M. Cohen,G. Guelachvili, R. Larson, J. P. Maillard in the obser-vations. A. Delouis and R. Gautier performed all thecomputations, with help from F. Mertz. Finally wethank Hale Observatories for providing access to the500-cm telescope.
P. Connes is on leave in the Astronomy Depart-ment of the University of California at Berkeley.
I. MAC DONALD OBSERVATORY, 1962210 CM TELESCOPEGRATING SPECTROMETERRESOLUTION 8CM-1
6
6 500 6505
N - PALOMAR, (500 CM),1973,FOURIER SPECTROMETERRESOLUTION 0,015 CM-1
Fig. 23. Improvements in the near ir Venus spectrum due to Fourier spectroscopy; same type detectors (cooled PbS) with almost thesame NEP used throughout. Curve I by Kuiper,3 7 II from Ref. 1, III from Ref. 3, and IV (this work) by ourselves. Telescope sizes and re-solving power gains are indicated; , II, and III are discussed at greater length in Ref 3. Four strong CO2 Venusian bands are shown in I;the rotational structure is resolved in II; III shows lines from much weaker overlapping bands; IV gives a good approximation of the trueline profile, together with ever fainter lines. Trace IV presents approximately 1/800th of the actual spectral range available from the mag-
netic tape-general purpose computer output (parts of which are obscured by H20).
1. J. Connes and P. Connes, J. Opt. Soc. Am. 56, 896 (1966).2. J. Connes, P. Connes, and J. P. Maillard, J. Phys 28C2, 136
(1967).3. P. Connes, Annu. Rev. Astron. Astrophys. 8, 209 (1970).4. U. Fink, H. P. Larson, and R. F. Poppen, Astrophys. J. 187,
407 (1974); and 171, 291 (1972).5. J. Pinard, J. Phys. 28C2, 136 (1967); Ann. Phys. 4, 147 (1967).6. An astronomical interferometer comparable to our types I and
II has been built independently by Beer et al.7 and has pro-duced many astronomical results; latest reference is Ref. 8.An airborne version has been described by Schindler 9 and usedaboard the Concorde supersonic aircraft to record solar spec-tra.10 Another instrument has been built by Malbrouck (Uni-versity of Liage) and set up at the Jungfraujoch Observatory,mostly for recording ir solar spectra. There is a 2-m path dif-ference laboratory interferometer at Air Force Cambridge Re-search Laboratories (Pritchard, Sakai and Vanasse AFCRLReport TR-73-0223). Lastly, an astronomical interferometerhas been built by Wayte for high resolution work in the visiblewith the Isaac Newton telescope in Herstmonceux. This briefreview mentions only operating systems related to our own bysimilarity in type of construction and mode of operation.
7. R. Beer, R. H. Norton, and C. H. Seaman, Rev. Sci. Instrum.42, 1393 (1971).
8. R. Beer, D. L. Lambert, and C. Sneden, Publ. Astron. Soc. Pac.86, 806 (1974).
9. R. A. Schindler, Appl. Opt. 9, 301 (1970).10. C. B. Farmer, Can. J. Chem. 52, 544 (1974); Proc. CIAP Conf.
(1974) (in print) (1971).11. J. Connes, H. Delouis, P. Connes, G. Guelachvili, J. P. Mail-
lard, and G. Michel, Nouv. Rev. Opt. Appl. 1, 3 (1971).12. G. Guelachvili, Nouv. Rev. Opt. Appl. 3, 317 (1972).13. J. Connes, in Aspen International Conference in Fourier
Spectroscopy, AFCRL Spec. Rep. 114 (1971), p. 83.14. H. Delouis, in Aspen Int. Conf. on Fourier Spectrosc., AFCRL
Spec. Report 114 (1970), p. 145.15. G. Guelachvili, Opt. Commun. 8, 171 (1973).16. C. Amiot, and G. Guelachvili, J. Mol. Spectrosc. 51, 475 (1974).17. E. Luc-Koenig, C. Morillon, and J. Verges, Physica 70, 175
(1973).18. J. P. Maillard, M. Combes, Th. Encrenaz, and J. Lecacheux,
19. P. Connes and G. Michel, in Aspen International Conferenceon Fourier Spectroscopy, AFCRL Spec. Rep. 114 (1971), p.313.
20. G. Michel, Appl. Opt. 11, 2671 (1972).21. P. Connes and G. Michel, Astrophys. J. 190, L29 (1974).22. W. H. Steel, Opt. Acta 21, 599 (1974).23. RCA Corporation, Montreal, Canada.24. Interferometer V uses silicon detectors with builtin preampli-
fiers instead.25. M. Cuisenier and J. Pinard, J. Phys. 28C2, 97 (1967).26. W. H. Steel, Interferometry (Cambridge U.P., Cambridge, En-
gland, 1967), p. 84.27. The compensation process can equally well be understood in
different terms. Each cat's eye is a point-symmetrical retrore-flector, i.e., all rays are returned symmetrical with respect to aparticular point of the axis: the image of the MF center ofcurvature given by ME. Varying the curvature of MF in themanner described above means keeping this center of symme-try stationary while the cat's eye moves. Then the two emerg-ing rays originating from a given incoming ray are coincident, anecessary condition for field compensation.
28. G. Lemaitre, These, Faculte des Sciences de Marseille (1973).29. Two recent references on flexible mirrors are Refs. 30 and 31;
in the second case the design is comparable to our own. Noindication of actual accuracy is given.
30. S. Mikoshiba and B. Ahlborn, Rev. Sci. Instrum. 44, 508(1973).
31. E. Bin-nun and F. Dothan-Deutsch, Rev. Sci. Instrum. 44, 512(1973).
32. T. J. Deeming and L. M. Trafton, Appl. Opt. 10, 382 (1971).33. J. T. Trauger and F. L. Roesler, Appl. Opt. 11, 1964 (1972).34. If field compensation is not available, one can nevertheless re-
alize approximate compensation of planetary rotation byapplying this same shear to the wavefronts. Monochromaticfringes within the planetary image diameter would then ap-pear curved, and the situation would be generally similar tothe Fabry Perot one.33
35. Size and weight of the device tend to grow as the cube of thelength; this is why a more complex, nonuniform field, switchedcurrent type was preferred for interferometers III A, B, C."
36. However, the RTC has also been used with a fast scanning in-terferometer at low resolution. 2 0
37. G. P. Kuiper, Commun. Lunar Planet. Lab. Univ. Ariz. 1, 83(1962).
Synchrotron Radiation FacilitiesAvailable at Stanford and Wisconsin
NSF is now supporting two synchrotron radiationfacilities which provide intense beams of high-energyphotons and which are available to all qualified users.TANTALUS I is an electron storage ring at the Uni-versity of Wisconsin Physical Sciences Laboratorylocated at Stoughton, Wisconsin. It produces radiationin the ultraviolet, with photons up to about 60 ev(about 200 A). For further information, contact EdnorRowe, Physical Sciences Laboratory, P.O. Box 6,Stoughton, Wisconsin 53589. The Stanford Syn-chrotron Radiation Project uses the intense radiationcoming from the SLAC electron-positron storage ring(SPEAR) for a variety of experiments. The facilityprovides a source of X-rays that is unique in the Unit-ed States, allowing reflection and absorption ex-periments, photoelectron spectroscopy, and diffrac-tion from a variety of samples. For further informa-tion, contact S. Doniach, Director, Stanford Syn-chrotron Radiation Project, Hansen Laboratories,Stanford, California 94305.
