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Electro-optical Processing in RadioAstronomyT. W. COLE aa Division of Radiophysics, CS IRO, Sydney, AustraliaPublished online: 17 Nov 2010.

To cite this article: T. W. COLE (1975) Electro-optical Processing in Radio Astronomy, Optica Acta:International Journal of Optics, 22:2, 83-92, DOI: 10.1080/713819018

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OPTICA ACTA, 1975, VOL . 22, NO. 2, 83-92

Electro-optical processing in radio astronomy -

T . W. COLEDivision of Radiophysics, CS IRO, Sydney, Australia

(Received 16 September 1974)

Abstract. Electro-optical techniques are beginning to find application inradio astronomy. A brief survey of radio-astronomy instruments and theprocessing of the radio signals is given. It is shown how a comparatively simpleelectro-optical processor can not only perform the functions of current, electronicdevices but can also allow processing not practical by other techniques .

As an illustration an electro-optical spectrograph is described and someobservational results are shown . Possible future applications of such instru-ments are given.

1. IntroductionRadio astronomy is essentially the measurement of the intensity and polariza-

tion of received extra-terrestrial radiation as a function of position in the sky,frequency and time, and quite specialized instrumentation has been developedto make such measurements . In recent years a demand for ever-increasingdata rates has been discerned. For example, when studying bursts of radioenergy from the sun observers require high resolution in position, frequencyand time simultaneously . The data handling and processing problems of suchobservations are enormous ; and electro-optical processing is one way of solvingthem .

Before discussing electro-optical processing a brief description of severaltypes of radio-astronomy instrumentation will be given .

2. ImagingAn image is usually formed with an antenna system which has a narrow,

pencil-beam response . However, it is impossible to build complete aperturesof the kilometric widths needed in radio astronomy for high resolution . Thedilute aperture has therefore been introduced, whereby a circular, cross orT-shaped antenna array is used rather than a complete antenna of the samemaximum dimensions . The cost of diluting the aperture is additional sidelobes(secondary maxima), and in some cases grating responses (higher-order spectra),which must be controlled by processing, preferably in real time . In the con-tinuous cross [1] and grating cross instruments [2] such processing can consistof forming the difference between the received intensities when (a) the twoarms are added in phase and (b) the signals from the two arms are added in anti-phase ; this is illustrated in figures 1 (a) and 1 (b) . The result of this processingis a suppression of the unwanted sidelobe responses . A third dilute aperture

t Paper presented at International Conference on Optical Information Processing,Sydney, Australia, August 1974 .

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Figure 1 . Three dilute-aperture, radio astronomy instruments and their beam-formationmethods are illustrated . The Mills Cross in (a) and Christiansen Cross in (b)produce a low sidelobe beam by forming the difference between the response withthe two arms in phase (centre) and out of phase (right) . The Culgoora ring arrayin (c) uses phasing around the ring to produce two images (illustrated) whosedifference is again a central beam with low sidelobe response .

is the Culgoora ring array [3] in which the sidelobe suppression in use is theso-called J2 synthesis [4], which involves phasing of the elements to form twoantenna responses whose difference once again is the desired result (figure 1 (c)) .Because there is no radio equivalent of the photographic plate an image isgenerated by scanning the beam or using multiple beams, which require con-nection of the elements of the antenna with some pre-defined phase relationships .

Aperture synthesis [5] is a special imaging technique developed in radioastronomy . It relies on the radio sources being constant in position and

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intensity so that points on the two-dimensional Fourier transform of the bright-ness distribution in the region of sky under study can be measured sequentiallyrather than simultaneously . A pair of antennas measure one Fourier componentdefined by the separation of the aerials and their relative orientation . Henceone uses an instrument which can be physically altered between measurements[5] or whose projected orientation is changed by rotation of the earth withrespect to the region of sky under study [6]. The signal processing involvesdelay-path-length compensation to remove inequalities in the signal-path lengths,phase rotation to compensate for relative motion of the two antenna elements inthe direction of the source, and complex correlation of two broadband noisesignals. This final step can be interpreted as taking the difference betweenthe two signal intensities obtained when (a) the two elements are added and(b) they are subtracted .

3. SpectroscopyWhen the spectral feature to be studied is weak, a multichannel spectroscopic

instrument is required which can provide frequency resolution over some totalbandwidth and with a stability which allows long integration times . The weaksignal is swamped by receiver and background noise so that an observation of thebackground alone must be subtracted from one of background plus signal .

Current techniques include the use of a bank of receivers each tuned to adifferent frequency [7], and the autocorrelation spectrograph [8] (Fourierspectrometer), which obtains the power spectrum as the Fourier transform ofthe autocorrelation of the received signal .

After this rather cursory review of radio-astronomy instrumentation, we canconsider the coherent optical processor to see how it might perform the common,basic processing steps of phasing, frequency dispersion and intensity subtraction .

4 . The coherent optical processorIn its most basic form the coherent optical processor [9] uses the Fourier

transform relationship which exists between the distribution of light amplitudeand phase across planes one focal length on either side of a lens . The inputplane is usually illuminated with a collimated laser beam, and an input functioncan be inserted there in the form of a transparency . For example, scaled tem-plates of a radio antenna system can be inserted and the output plane then con-tains the Fourier-related far field, or diffraction, patterns . This approach wasused to produce figure 1 .

McLean and Wild [10] pointed out in 1961 that if, as shown in figure 2,the light passing through each element of the analogue antenna in the inputplane could be modulated by the radio wave being received in the correspondingelement of the radio antenna, then the optical output plane would contain animage of the sky as seen by the radio antenna . The optical arrangement providesthe phasing needed to form all points of the image simultaneously . This simul-taneous nature is the major advantage offered by the optical method, since theresultant image can be obtained much faster than in the sequential, scanningtechnique .

Such an approach to array processing was studied by the Columbia Uni-versity Electronics Research Laboratory over several years and they developed

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4) 4)~~ OPAQUE ~~

SCREEN

PUPILS

Figure 2 . The original suggestion of McLean and Wild [10] of how an optical processorfor a circular array would work .

IMAGE

Figure 3 . The coherent optical processor for the Culgoora array designed by the ColumbiaUniversity Electronics Research Laboratory is shown schematically in section .

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radar array processors which used acousto-optical methods to modulate the light[11] . They also designed a processor for the Culgoora radioheliograph [12],shown in figure 3, which used conical optics to provide the correct geometryfor the acousto-optical light modulators. These modulators, which are uniquelysuited to modulating a light beam with both the amplitude and phase of anelectrical signal, rely on the Bragg or Debye-Sears interaction [13] betweencollimated light and a travelling acoustic wave . In the Bragg effect, lightscattered from successive acoustic wavefronts will reinforce for an incidenceangle defined by the ratio of the light and acoustic wavelengths. In the Debye-Sears interaction the acoustic beam acts as a travelling phase grating of spacingequal to the acoustic wavelength, and light is diffracted into diffraction orderson either side of the main beam.

As stated above, the optical processor is capable of forming the image seenby the radio antenna. Since the radio antenna is dilute, the desired final imagewith low sidelobe level is not obtained until a subtraction of images is made .The image subtraction problem was not solved in the Columbia processor, andthis, as well as cost, prevented its construction. Similar processors can bedesigned for the other imaging radio telescopes, but again they would require anoutput system which performs image subtraction .

No optical processors for antenna arrays are yet in use in radio astronomy .The main development of coherent optical processing for radio astronomy hascentred on the acousto-optical spectrograph .

5. The acousto-optical spectrographIn the Debye-Sears and Bragg interactions the (small) angle between the

deflected (modulated) light and undeflected light is proportional to the frequencyof the electrical signal used to generate the sound beam . For low modulationlevels the intensity of this modulated beam is proportional to the intensity ofthe electrical signal. Therefore, as described by Lambert [14], when a widebandradio signal is applied to the modulator, the diffracted light is spread into a linewhose intensity along that line represents the power spectrum of the radio signal .This is the basis of the acousto-optical spectrograph shown in figure 4 .

The total bandwidth of the device is defined by the centre frequency and thefractional bandwidth of the piezo-electric transducer which converts the radiosignal to an acoustic wave in the medium. With current technology it is com-paratively easy to obtain an octave bandwidth of up to 100 MHz . Vacuum

Laser

Expandinglens and

Lenspinhole

II~Collimating

Lens

Mask

Figure 4 . The basic configuration of the electro-optical or acousto-optical radio spectro-graph is shown. The light modulator is placed in the input plane and the diffractedlight in the output plane is observed .

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deposition of thin-film transducers is capable, with effort, of achieving band-widths of 1 GHz or so and this represents the current state of the art [15] .

The spectral resolution is related to the inverse of the time taken for theacoustic beam to cross the light beam, so that a medium with a low sonic velocityand low acoustic loss is desired . The acoustic loss increases with frequencysquared and this combines with the transducer limitations to restrict frequency

Figure 5 . A photograph of the CSIRO 100 MHz bandwidth, 120 kHz resolution spectro-graph as originally constructed . The output was on to a continuously movingfilm .

T. W. Cole

Figure 6 . The spectrograph of figure 6 was used to obtain these spectral records of solarbursts with a time resolution of 0 . 1 s .

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resolution to less than about 1 part in 4000 of the total bandwidth [15] . Anacousto-optical spectrograph could then be made with a total bandwidth andnumber of channels which would make it uniquely suited for spectroscopicwork in the millimetre-wavelength region where spectral features are broad .

The practical development of the acousto-optical spectrograph began inCSIRO in 1966 and, more recently, a device was used for scientific observationof solar radio bursts [16, 17] . It used a 10 µs Bragg interaction between anapodized, collimated laser beam and a transverse, 100 MHz bandwidth acousticbeam. The resulting spectrum, with 800 resolution intervals covering 100 MHz,was recorded directly onto film . Figure 5 shows the instrument and figure 6some of its results .

The instrument in this form is useful for observations of intense solar radioevents, but in order for it to be useful for observing weak signals it is necessaryto subtract the background intensity .

6. The self-scanned photodiode arrayA recent development in semiconductor technology is the self-scanned

photodiode array [18] . A single semiconductor chip contains a number ofsilicon photodiodes and a circuit to read out sequentially the integrated lightenergy which has been absorbed by each diode since the last sampling . Cur-rently the available geometries range up to a device with 1024 photodiodesspread along a line 26 mm long and to area arrays with up to 100 by 100 points .The geometric accuracy of the arrays, the read-out rates of up to 10 MHz, thehigh photometric sensitivity, and the dynamic range of 30 dB or more, makethese devices ideally suited for sampling an optical image . After processing,their output is a boxcar signal which can be easily sampled, digitized and readinto a digital computer .

The digital computer is introduced because it offers great flexibility inprocessing the optical image read by the photodiodes . It can correct the signalfor diode-to-diode zero offset and gain variations, and for the acoustic trans-ducer and receiver bandpass responses . It can also integrate successive samplesand calibrate the spectrum in some system of units . More important, it canform the difference between spectra to remove the background signal and enableweak signal operation .

The photodiodes introduce noise to the system but it is possible to minimizeits effect by operating at a high enough light level where the final noise level onthe spectrum is controlled solely by the noise introduced by the radio receiver .

An experiment was made at the Parkes radio telescope in which a 256 photo-diode array was used to connect the spectrograph of figure 5 and a computer inorder to observe spectral lines of the hydroxyl radical OH . These initial testsused a transducer system which gave 20 kHz resolution, and the results (seefigure 7) showed that the acousto-optical system was capable of weak-signaloperation with the sensitivity expected from a fully multiplexed, multi-channelsystem .

A redesigned spectrograph is shown in figure 8 with a much more compactdesign for increased stability . Two such devices are being built, one for solarradio-astronomy studies and the other for cosmic spectral line observations .Photodiode arrays with 500 elements are to be used in these instruments to give

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them a combination of features-number of channels, radiometric sensitivity,maximum total bandwidth, and flexibility-unequalled by any other radiospectrograph .

Y

100w

D 80Qw 60CLXLLJ 40

z 20zwNz 0a

FREQUENCY (MHz)

Y 100

FREQUENCY (MHz)

Figure 7 . With a 256 photodiode array used to interface the spectrograph of figure 7 toa computer, a spectral record (a) was obtained for spectral line emission from thehydroxyl radical OH in radio source VY CMa . A comparison is shown (b) with arecord from the autocorrelation spectrograph at the Parkes observatory.

Figure 8 . The rebuilt acousto-optical spectrograph is shown. The base measures90 x 25 cm and prisms are used to fold the light path.

P

(b)80

60W

40wI-< 20zZI 0ZQ

1612.0

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7. Future developmentsSo far the only example of practical application of electro-optical processing

in radio astronomy is the acousto-optical spectrograph . Even as the initialinstruments are being prepared for routine observations it is already becomingclear that they could be improved in a number of ways . One could increase thenumber of photodiodes . One could use the newer materials available as trans-ducers and acoustic media . For example, paratellurite (TeO 2 ) crystals have verylow acoustic velocities for a solid medium and have an anisotropic Bragg inter-action which offers wide bandwidths and high resolution with very low electricaldrive powers . In addition, vacuum-deposited CdS or ZnO can be used astransducers well into the microwave region .

The acousto-optical spectrograph can also be used for the broad-bandwidthcross-correlation of two radio signals . Basically, two spectra would be produced,one of the two signals added in-phase at the input to the spectrograph and theother with them added in anti-phase . The difference of these spectra representsthe cross-correlation as a function of frequency . Any path difference betweenthe two signals introduces a series of fringes as a function of frequency . Thecomputer can easily remove this effect of path difference as well as correctingfor instrumental phase errors across the frequency range .

The acousto-optical spectrograph described above is a single channel devicebut it is comparatively simple to have a number of transducers and acousticbeams and to use cylindrical optics to produce spectra in parallel . This wouldobviously be useful as a basis for a processor for an aperture synthesis array .

As already indicated, processors for imaging radio telescopes have alreadybeen designed . By using the new devices available for image sensing andsampling, image subtraction is now no longer a problem . However, an opticalprocessor for an imaging instrument is still complicated and expensive . Fortwo-dimensional arrays the modulator geometry is difficult and the necessity touse only 1 or 2 per cent modulation of the laser light is extremely inefficient. Itis the expectation that the processors for the imaging arrays are more simplyformed using acoustic waves alone . It is then simple to arrange a two-dimen-sional array of transducer elements in the form of the antenna and all the acousticenergy is used in the image . This was thought of in 1961 as a possible pro-cessor for the Culgoora heliograph [10] and it is now in use in an array for iono-spheric studies [20] . However, then as now, the limiting factor is the detectorsystem where no equivalent to the scanned photodiode array is yet available .Optical processing appears then to be best suited to the frequency dispersionrole of the acousto-optical spectrograph and its applications in correlation .

8. ConclusionsWe have seen that most radio-astronomy instruments process the received

signals by phasing and image subtraction . The phasing for imaging and fre-quency dispersion are ideally carried out by optical, analogue means, while theuse of photodiode arrays and a computer introduces flexibility and permitsimage subtraction . This hybrid approach has developed the acousto-opticalspectrograph as a practical scientific instrument . It should also be able toperform other functions with a combination of total bandwidth, simplicity, andflexibility which is difficult to achieve by other techniques.

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Electro-optical processing in radio astronomy

Les techniques electro-optiques commencent a trouver des applications en radio-astronomie . On presente une breve revue des instruments de radioastronomie et dutraitement des signaux radio . On montre comment un systeme electro-optique de traite-ment des donnees peut effectuer non seulement les fonctions des systemes electroniquescourants mais permet aussi un traitement qui n'est pas realisable par d'autres techniques .

A titre d'exemple, on decrit un spectrographe electro-optique et 1'on presente quelquesresultats d'observations . On indique des applications futures possibles de tels instruments.

Elektro-optische Techniken beginnen Anwendung in der Astronomie zu finden . Eswird eine kurze Vbersicht der radioastronomischen Instrumente and der Verarbeitung vonRadiosignalen gegeben . Es wird gezeigt, wie ein verhaltnismaBig einfaches elektro-optisches Verarbeitungsgerat nicht nur die Aufgaben ublicher elektronischer Geri toausfuhren kann, sondern auch Verarbeitungsaufgaben iibernehmen kann, die mit anderenTechniken nicht praktikabel sind .

Als Beispiel wird ein elektro-optischer Spektrograph beschrieben and einige Beobacht-ungsergebnisse mitgeteilt. Mogliche zukunftige Anwendungen eines solchen Instrumentswerden angegeben .

REFERENCES

MILLS, B. Y., and LITTLE, A. G., 1953, Aust . Y. Phys ., 6, 272 .CHRISTIANSEN, W. N., and MATHEWSON, D. S ., 1958, Proc. Inst. Radio Engrs, 46, 127 .See collected papers, The Culgoora Radioheliograph, 1967, Proc . Inst . Radio Electron .Engrs, Aust., 9, 277 .

WILD, J. P ., 1965, Proc. R. Soc. A, 286, 499 .RYLE, M., and HEWISH, A ., 1960, Mon. Not. R. astr . Soc ., 120, 220 .RYLE, M., 1962, Nature, Lond ., 194, 517 .MCGEE, R. X., and MURRAY, J. D ., 1963, Proc. Inst . Radio Engrs Aust., 24, 191 .WEINREB, S., 1963, MIT Res. Lab. Electron . Tech . Rep. 412 .See, for example, CUTRONA, L . J ., 1965, Optical and Electro-Optical InformationProcessing (Cambridge, Mass . : MIT Press), Chap . 6, p . 83 .

McLEAN, D . J., and WILD, J. P ., 1961, Aust. Y. Phys ., 14, 489 .LAMBERT, L. B., ARM, M., and AIMETTE, A., 1965, Optical and Electro-Optical Informa-

tion Processing (Cambridge, Mass . : MIT Press), Chap . 38, p . 715 .McLEAN, D . J ., LAMBERT, L. B., ARM, M., and STARK, H., 1967, Proc . Inst . RadioElectron . Engrs, Aust ., 9, 375 .

QUATE, C. F., WILKINSON, C. D. W., and WINSLOW, D. K., 1965, Proc. I.E.E.E.,53, 1604 .

LAMBERT, L ., 1962, I.R.E. natn . Conv. Rec ., 10, 6, 69 .CROFUT, W ., 1967, Microwave ,., January 1965 .COLE, T. W., 1973, Proc . Inst. elect. electron . Engrs, 61, 1321 .COLE, T. W., 1973, Astrophys. Lett ., 15, 59 .See, for example, MELEN, R., 1973, Electronics, May 24, 106 .COLE, T. W., and ABLES, J. G., 1974, Astron. Astrophys., 34,149 .BRIGGS, B. H., and HOLMES, N., 1973, Nature, Phys. Sci., 243, 111 .

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