RAPID SCANNING FOURIER TRANSFORM SPECTROMETRY

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RAPID SCANNING FOURIER TRANSFORM

SPECTROMETRY

L. Mertz

To cite this version:

JOURNAL DE PHYSIQUE Colloque C 2, supplkment au no 3-4, Tome 28, mars-avril1967, page C 2

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RAPID SCANNING FOURIER TRANSFORM SPECTROMETRY

Block Associates, Inc., Cambridge, Mass., U. S. A.

Rksumk. - Le balayage rapide sert a rkduire les difficultks dues a la scintillation et l'etendue dynamique nkcessaire en spectroscopie de Fourier, et aussi ?i simplifier les mesures. I1 consiste

a balayer a une vitesse telle que les franges d'interference elles-m6mes donnent la modulation, a

une frkquence qui dkpasse celle des frkquences de scintillation et ne peut &re confondue avec elles. Nkcessairement le balayage doit 6tre rkpktitif pour l'ktude de sources faibles.

Pendant l'observation les interfkrogrammes sont enregistrks sur une piste d'un enregistreur magnktique stkreo, et des marques de synchronisation sur l'autre piste. Les interfkrogrammes sont ultkrieurement digitalis& et additionnks (en synchronisme grgce aux marques de rkfkrence) dans une mkmoire magnktique a tores. L'analyse de Fourier peut alors 6tre effectuke par des prockdks analogues ou digitaux.

Les spectres astronomiques obtenus par deux instruments, l'un dans le visible (photocathode S 20) l'autre dans le domaine des rkcepteurs a sulfure de plomb, sont prksentks afin de dkmontrer les qualitks du balayage rapide. Enfin un effet systkmatique de renforcement de raies d'absorption faibles dfi ?i une erreur de digitalisation est prksentk.

Abstract. - Rapid scanning serves to mitigate the problems of scintillation and dynamic range for Fourier spectrometry, as well as to simplify the measurement procedures.

Rapid scanning means scanning at a rate such that the interferometric fringes act as their own chopper. It also means scanning at a rate such that the fringe frequencies exceed the scintillation frequencies, thereby avoiding confusion between the two. Of necessity the scanning becomes repe- titive for the measurement of faint sources.

During observation the repetitive interferogram signals are recorded on one channel of a stereo magnetic tape recorder, while timing information is recorded on the other channel. The interfe- rograms are subsequently digitized and summed (in synchronism according to the timing channel) into a magnetic core memory. Fourier analysis of the resulting interferogram can be finally car- ried out by either analogue techniques or by digital computation.

Astronomical spectra obtained by two instruments, one in the visible (S-20) range, the other in the lead sulphide range, are shown to demonstrate the efficacy of rapid scanning.

Finally a curious systematic effect of erroneous digitizing to enhance weak spectral absorption lines was discovered and will be illustrated.

The random noise which plagues spectral measu- rements divides into three categories ; proportional t o the zeroth, half, and first power of the optical signal level respectively. As Fellgett pointed out in his thesis, Fourier spectrometry observes the various colors simultaneously. There is an increased observa- tion time per wavelength resolution element compared to sequential observation. In winning this observation time the optical signal levels are proportionally increased. Thus for noise proportional to the zeroth power of the signal, as encountered with infrared detectors, the increased observation time is used to advantage, i. e., Fellgett's multiplex advantage. For noise proportional to the square root of the signal, such as photon statistics, increased observation

time is just balanced on the average by increased noise and no advantage (or disadvantage) of the above sort occurs. For noise which is proportional to signal we lose.

Scintillation noise due to atmospheric turbulence can easily relegate astronomical spectrometry to the last category. The crux of this problem is that scintil- lation fluctuations become spuriously interpreted as interferometer fringes.

Limited dynamic range of the recording meter can also be thought of as falling in this last category. One digitizing bit will be a certain fraction of the maximum tolerable signal level. Although various gimmicks such as fringe chirping [l] may be used to

alleviate dynamic range problems, it has always

C 2 - 8 8 L. MERTZ

proven more expedient in practice to spend more money on a more accurate digitizer.

It would be a mistake now to be intimidated by these problems, especially by scintillation noise. After all there are many things we can do about it to suppress it. At least six possibilities exist :

lo Increased total observation time will of course improve signal-to-noise but it is a slow process, generally absurdly slow.

2O Larger telescopes become relatively unaffected by scintillation. This is a highly recommended approach if it is a t all feasible.

3 O The difference of the complementary interfero-

meter outputs can be used as the signal. This procedure discriminates against scintillation, because scintillation causes both outputs to go up and down together whereas fringes cause one to go up when the other goes down.

40 The ratio the fringe signal to a comparison radiometer (which may be the sum of the two outputs) can also be used.

5O Rapid scanning can be employed so that the fringe frequencies exceed scintillation frequencies, thereby avoiding confusion between the two.

60 High frequency differentiation of the interfero- gram can be used. The interferometric path difference can be jittered, with half wavelength jitter amplitude, to provide a chopped signal which is a finite difference derivative of the interferogram.

I highly recommend the fifth possibility and will now elaborate on it. First of all, by rapid scanning the fringes themselves serve to chop the radiation, eliminating the need for an additional mechanical chopper yet maintaining a. c. circuitry. Fringe fre- quencies lying in the audio range are suitable for detectors, are above scintillation frequencies, and are adaptable to standard audio frequency electronic equipment.

In order to adapt rapid scanning to faint sources one onIy need make the scan repetitive and then average many successive scans. The observational procedure is then to record the repetitive signals on one channel of an ordinary stereo tape recorder. Simultaneously timing information is recorded on the second channel, thereby avoiding any problems of tape speed variations. The timing information consists of a bipolar pulse (simple pulses do not record well) at the beginning of each scan and a low level sinusoid of the desired sampling frequency. That is all there is to the observational part of the measurements. In my opinion it offers unexcelled simplicity for

astronomical work ; no great attention to telescope guiding is required.

The data reduction part follows in the warmth and leisure of a laboratory. The tape is played back into a data processor which consists of an A-D converter and core memory among other things. At each impulse from the timing channel the core memory access is returned to the first address. Thereafter at each cycle of the timing sinusoid, the signal is digitized added into the core memory address, and the address advanced one location. In this fashion the repetitive interfero- grams are digitized and synchronously added. Simul- taneous D-A conversion permits monitoring the process with an oscilloscope. When the signal-to-noise is deemed sufficient the process is interupted. At this point the memory contents may be dumped into a digital recorder for subsequent Fourier transform computation on a computer or the contents may be repetitively played back with D-A conversion into a wave analyzer for analog Fourier analysis. The fact that the initial data is on magnetic tape means that fumbling in the data reduction is no calamity. Even reruns isolating various portions of the initial data are readily performed.

Finally it should be noted that the timing channel may be derived directly from the interferometer by monochromatic reference fringes or by moirC fringes, thereby avoiding the need for accurate constant velocity in the interferometer drive.

Another consequence of rapid scanning is that the dynamic range of the digitizer and tape recorder need not be excessive. In fact it is frequently the case that individual interferograms are indiscernible in the noise. The large dynamic range lies only in the size of each word in the core memory.

Two stellar spectrometers have been so built, one for the S-20 photomultiplier region and the other for the lead sulphide region. The motive for building the visible instrument is not particularly to surpass spec- trum scanners but to demonstrate feasibility. Feasi- bility or the slitless interferometer spectrometer is a prerequisite to the construction of large light collec- tors, where the image becomes necessarily too big to focus through a small slit. Feasibility is also a prerequisite for its adoption on orbiting observatories where limited pointing accuracy would give prefe- rence to a slitless spectrometer.

RAPID SCANNING FOURIER TRANSFORM SPECTROMETRY C 2 - 8 9

SOLEIL COMPENSATOR PERTURES WITH POLARIZERS

AT TELESCOPE FOCUS

per minute, each including about 800 samples, and the resolving power is in the neighborhood of 130 cm-l. Figures 2 et 3 show some results made with a 25 cm telescope.

p GEM, T O R I 10 MINUTES EACH

1 ORION NEBULA

I

The configuration of the infrared instrument is shown in figure 4. Repetition rate is variable from

MICHELSON ELECTROMAGNETIC INTERFEROMETER ,MIRROR DRIVE

HELIUM CALIBRA

\

/

50 to 200 scans per minute, and resolution from 30 cm-' (for strong signals) to 120 cm-l. Figure 5 shows a helium calibration spectrum. Figure 6 was

HELIUM CALIBRATE LAMP

o LYR B? ? ? ::? 2 MIN,SEC t = 1 . 2 0

0

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L. MERTZ SATURN DISC 2.7 MIN, SEC t = 1.73 JUPITER 1.1 MIN, SEC t = 1.52

J

J

ABOVE- WITH BIT ERROR BELOW

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2 1.58

FIG. 10.

those I was able to obtain in 1956, and presented at the previous Bellevue Conference ; this is a measure of the progress in the subject in the intervening decade. J. RING. - I hope that no one is confused by your suggestion that observing longer is a good way to eliminate scintillation noise. There is no evidence that the power spectrum of scintillation decreases with frequency - on the contrary, the strongest component is at a period of 24 hrs. I suggest that it is very bad to use the observing time for one interfero- gram and much better to average short runs as you do in the rapid scan instrument.

L. DELBOUILLE. - We have precise indication showing that atmospheric noise is not white, but that its spectrum is very close to a

llf

Rapid scanning with subsequent averaging of many results will probably be more efficient that a single scan of the same total duration.

J. KATZENSTEIN. - If rapid scan is advantageous why not go to megacycle scanning rates using an electrooptic scanning element (Pockels Cell) instead of a mechanically oscillated Soleil compensator ? interferograms looked elegant and genuine, but it is Note. - See L. MERTZ, J. Physique, 1958, 19, 233.

now evident that the weaker fringes were selectively amplified.

Bibliographic INTERVENTIONS

[I] MERTZ, Les Spectres Infrarouges des Astres, Likge

P. FELLGETT. -This is supposed to be a simple 1964, p. 120.