Planetary brightness temperature measurements at 1.4-mm wavelength

ZCA~US23, 448--453 (1974)

Planetary Brightness Temperature Measurements at 1.4-mm Wavelength J. D. G. R A T H E R Lulejian and Associates, Inc., Torrance, Calif. 90503 B. L. U L I C H National Radio Astronomy Observatory, Tucson, Arizona 85726 AND

P. A. R. A D E Queen Mary College, University of London, England Received February 25, 1974; revised April 15, 1974 Precise relative measurements of the disk brightness temperatures of Venus, Mars, Jupiter, and Saturn have been made at a mean wavelength of 1.4 mm. The rings of Saturn contribute significantly to the observed total emission. Other results include a better understanding of the properties of the NRAO 11-m antenna near its high frequency limit and of atmospheric degradation of observations in this wavelength range.

I. INTRODUCTION The o b s e r v i n g capabilities of the l l - m radio telescope of t h e N a t i o n a l R a d i o Astronomy Observatory I at Kitt Peak, Arizona h a v e b e e n t h e subject of detailed s c r u t i n y a n d i m p r o v e m e n t for some years. This i n s t r u m e n t is in routine use in the w a v e l e n g t h r a n g e f r o m 2 to 10 m m a n d r a n k s a m o n g the m o s t precise reflector a n t e n n a s in existence, b o t h in t e r m s of m a x i m u m usable frequemcy a n d of t h e r a t i o of d i a m e t e r to m i n i m u m o p e r a t i o n a l w a v e l e n g t h . I t has b e e n a m a t t e r o f considerable interest, therefore, to i n v e s t i g a t e t h e possibility of e x t e n d i n g the routine o b s e r v i n g capabilities of t h e 11-m telescope to the s h o r t e s t possible w a v e l e n g t h s . Studies of t h e c o m p o s i t e p r o b l e m of o p t i m i z i n g new 1 - m m receivers to m a t c h t h e p r o p e r t i e s of the a n t e n n a a n d a t m o s p h e r e h a v e led to a n i m p r o v e d unders t a n d i n g of all these factors. I t is n o w 1 The National Radio Astronomy Observatory is operated by Associated Universities, Inc. under a contract with the National Science Foundation. Copyright © 1974 by Academic Press, Inc. All rights o f reproduction in any form reserved. Printed in Great Britain

possible to m a k e precise m e a s u r e m e n t s of r e l a t i v e l y w e a k sources in r e a s o n a b l e i n t e g r a t i o n times, p r o v i d e d t h a t p r o p e r a c c o u n t is t a k e n o f t h e changing characteristics of the telescope a n d a t m o s p h e r e . U n d e r f a v o r a b l e conditions of t e m p e r a t u r e and total precipitable water vapor, our p r e s e n t receiving e q u i p m e n t has a n r m s noise level of a b o u t 3 J y (1 J y = 10 -26 W m -2 H z -1) a f t e r one h o u r of integration. E x p e r i m e n t s u n d e r identical conditions e x c e p t t h a t t h e d e t e c t o r viewed a c o n s t a n t ambient temperature background have resulted in t y p i c a l r m s noise levels of 1.5 J y a f t e r one h o u r o f integration. H e n c e short t e r m fluctuations of t h e a t m o s p h e r e or c h o p p e r noise c o n t r i b u t e significantly to t h e s y s t e m noise w i t h t h e p r e s e n t b a n d p a s s , which has its centroid a t 1.4 m m w a v e l e n g t h a n d long a n d short w a v e l e n g t h cutoffs a t a b o u t 3.0 a n d 0.8 m m , respectively. S h a r p e r filter cutoff characteristics, t o g e t h e r w i t h g r e a t e r d e t e c t o r sensitivity, should reduce the effect of a t m o s p h e r i c fluctuations. A t these w a v e l e n g t h s t h e m e a n a t m o s pheric o p a c i t y is a strong function of t h e 448

PLANETARY BRIGHTNESS TEMPERATURE MEASUREMENTS

total precipitable water vapor. At K i t t Peak, the mean observed optical depth typically varies from 0.25 at 2 mm total precipitable water vapor to 0.7 at 8 mm total precipitable water vapor. The opacity varies markedly from day to night and m a y change abruptly with wind and temperature variations. It is necessary, therefore, to make periodic observations of sources outside the atmosphere in order to precisely determine the atmospheric extinction. The planets nicely fulfill this need and we have attempted to calibrate the brighter planets with high precision. Both gain calibration and atmospheric extinction measurements can be quickly and conveniently accomplished by observations of two planets at different elevations. The same information m a y be obtained somewhat more laboriously by several observations of one planet or by observations of the Sun or Moon and a planet.

~I.

INSTRUMENTATION

The I 1 meter antenna has an altitudeazimuth mounting which is controlled by an on-line computer. Directly coupled torque motors position the antenna from encoder dat~ having a resolution of 1.2 arc sec. Pointing corrections for telescope flexure, refraction, etc are contained in the computer control program which generates celestial tracking for the mounting. The overall rms pointing accuracy is 5 are sec (Conklin, 1972). The observer m a y stipulat~ an acceptable wind-load pointing error (typically l0 are sec) beyond which the computer does not accept data. The aperture efficiency VA decreases in accordance with the theory of Gaussian deformations elaborated by Ruze (1966) for an rms surface error of 0.14 mm. The 1.6-0.8 mm wavelength atmospheric windows are thus in the regime of rapidly declining aperture efficiency. I t is important t~ not~ t h a t this rapid decrease of power in the main beam causes the dish itself to act as a low-pass filter. Tests with capacitive mesh bandpass etalons of the type developed by Ulrich (1967) have shown t h a t essentially no focused power

449

enters the receiver shortward of 800 /xm wavelength. Thus, we do not experience the difficulties t h a t plague observers who use large optical telescopes for far-infrared observations wherein the highly variable atmospheric transmission in the five "windows" between 1 mm and 300/~m and the unknown spectral index of sources outside the solar system make accurate calibration very difficult. Details of the antenna beam shape determination have been reported by Ade, Rather, and Clegg (1974). The main beam is gaussian with a full width at half maximum of 65 arc sec, while the error pattern has a full width at half maximum of 6.4 arc rain. The aperture efficiency is 9 ± 2%. Although the efficiency is low, the effective area is still comparable to t h a t of the largest t~lescopes in existence for focusing 1 mm wavelength radiation. The measurements reported here were made with a receiving system designed and built at NRAO by John Rather. The detector element was a liquid Helium cooled, Gallium-doped Germanium bolometer built by Infrared Laboratories, Inc. The system had more than an order of magnitude greater sensitivity than previous l-ram receivers used on the l l-m t~lescope because of a newly designed cooled-lens system within the cryostat which transfers the fast (f/D = 0.8) focused radiation from the dish to the detector with minimum loss. The short wavelength cutoff is determined by specially prepared filters which were built and calibrated by Peter Ado at Queen Mary College. The filters, all cooled to liquid helium temperature, consist of: (1) a capacitative mesh which is 80% transparent at 1.4 mm-2 and cuts off rapidly for wavelengths less than 800t~; (2) a Thallium-impregnated polyethylene filter which is completely transparent lcngward of 500t~ and completely opaque from 30/x to 200/x; and (3) a thin black paper which eliminates the short wavelengths which would be transmitted by the Thallium filter. The long wavelength cutoff is determined by the entrance iris holes (of. Andrejewski, 1953). The latter effect is dominated, however, by the v2 character of the thermal spectra

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of the planets. The net bandpass was measured on a polarizing Michelson interferometer at Queen Mary College and is incorporated into Fig. I. The receiver m a y be operated in either single beam (load switched) or dual beam mode. In the dual beam mode, the balance of the two beams defined by two holes in an iris plate may be precisely adjusted by remotely varying the size of one of the iris holes. Transition to single beam operation is accomplished by closing a knife-edge completely over one of the iris holes. The beams are separated by 2' 10", and experiments show t h a t there is no significant self-cancellation due to beam overlap. Geometric beam separation occurs at a zenith altitude of 17.5 km where all liquid water vapor should be frozen out. Beam chopping is accomplished with a modified Bulova type L-50 taut-band chopper with a reflective metal vane at ambient temperature. Figure 2 shows a scan of the two balanced beams through Jupiter.

The detector is cooled to 1.8 K by pumpdown of the liquid Helium. Valving and remote pressure monitoring are located near the receiver at the prime focus of the l l-m dish, permitting accurate measurement of the pressure. Preamplifieation is accomplished at the prime focus by an Advanced Kinetics Type F-60 low-noise preamplifier. A lockin voltmeter is employed for synchronous detection. Data collection is entirely digitized. The NRAO on-line data system can provide completely reduced data at the telescope as well as a magnetic tape record of the raw data (Moore and Rather, 1973). III.

OBSERVATIONS

In the foregoing discussion we have indicated that the vagaries of the atmosphere and telescope must be correctly monitored if calibrations are to be made with precision. This means that calibra-

PLANETARY BRIGHTNESS TEMPERATURE MEASUREMENTS

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tions done by the usual methods of radio astronomy are of only secondary importance, since differential hot-cold load measurements at the receiver provide no information about external effects. We therefore adopted the following procedure which provides all the necessary information : 1. The Moon or Sun were observed at several elevations to determine the atmospheric extinction. Since these are strong, extended sources, they fill both the main beam and much of the error pattern. The error pattern does not change significantly with time and provides very accurate information on atmospheric extinction exclusive of possible changes in the main beam aperture efficiency. The phase of the Moon changes slowly enough t h a t no significant change of the flux density in the error pattern occurs during the time required to make the necessary measurements. The sun is observed through the dome fabric, the attenuation of which has been determined with precision by observing the moon through the slot and through the fabric. Solar heating affects the figure of the telescope drastically if the dome is opened. 2. Bright planets were observed to

calibrate the system gain and verify the main beam aperture efficiency VA. Clearly, if VA changes with elevation angle of the telescope, the observed brightness of a "point" source, seen only by the main beam, will appear to vary with elevation in a different way t h a n the extinctiondominated brightness changes of the extended sources discussed above. In practice, it was found t h a t the observed changes in planet brightness matched the extinction curves of the Sun and Moon to high precision when the ambient temperature remained constant. Hence VA appears to vary only with dish temperature and not with elevation. This greatly simplifies data reduction, because changes in ~A due to ambient temperature appear only as changes in the overall system gain. Using the method outlined above, we measured the zenith extinction from several observations of the center of the moon. Interspersed with these observations, we measured the planets in "roundrobin" fashion. The results were highly repeatable, and the signal-to-noise ratio was generally large, as shown in Fig. 3. In reduction of the data, three further corrections were applied other t h a n atmospheric extinction. The first was a small correction,

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not exceeding 9%, for partial geometrical resolution of the planets by the main beam. (The broad error pattern is cancelled in the two beam mode to a negligible fraction of its single beam value.) Routine monitoring of the main bandwidth is accomplished by an automatic five-point observing program which positions the telescope alternately 30 arc see North, South, Center, East, and West of the commanded position. An online two-dimensional Gaussian fit is performed and the beam parameters are printed out summarily together with the antenna temperature of the Gaussian peak and its ensemble variation. This method, of course, also eliminates small errors in pointing. No significant change in measured beam shape has been found at any elevation angle of the telescope. The second correction to the data involved the use of the Planck function rather t h a n the Rayleigh-Jeans approximation in calculating the disk brightness temperatures. At 1.4 mm A this correction amounts to ~5% for Jupiter. The third correction was

for the oblateness of the disks of Jupiter and Saturn. In the case of Saturn the significance of the latter correction is completely overshadowed by the large excess of radiation from the rings as discussed below. The observation of Uranus was a "quick-look" on only one night. The signal-to-noise ratio is poor and the data are included here only as a matter of interest. In order to apply a brightness temperature T s to each planet, we have assumed 150 K for Jupiter. The results are summarized in Table I. IV. Discussion

The disk brightness temperatures listed in Table I were calculated by assuming Jupiter to be 150 K. The actual absolute scale was chosen by comparing our results with previous absolute measurements of Venus, Mars, and Jupiter at nearby wavelengths (Ulich et al., 1973, and Ulich, 1973). The best joint agreement with the extrapolated radio spectra of all three

453

PLANETARY BRIGHTNESS TEMPERATURE MEASUREMENTS TABLE I 1.4 MM PLANETARY BRIGHTNESS TEMPERATURES

Planet Venus Mars Jupiter Saturn Uranus

Measured ratio of brightness temperature relative to Jupiter 1.94 ± 0.12 1.50 ± 0.19 --

1.29 ± 0.05 0.77 ± 0.35

TB(°K)

Random error 1 a (°K)

Total error 1 a (°K)

291 225

18 28

150

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194 115

8 52

34 36 15 21 53

planets occurs when Jupiter is 150 K, and ters. Neither can we make a correction for Jupiter was adopted as the primary partial resolution of the ring by the main reference standard as a matter of con- beam because the geometric cross section venience. An additional 10% uncertainty of the emitting surface remains unknown. in the absolute calibration has been included in the quoted errors. The temperatures of Venus, Mars, Jupiter, and REFERENCES Uranus are all consistent with previous measurements and existing theoretical AI)E, P. A. R., RATHER, J . D. G., AND CLEGG, P. E . (1974). L i m i t s t o solar l i m b d a r k e n i n g a t models. The high disk brightness tema w a v e l e n g t h o f 1.4 m m d e r i v e d f r o m perature of Saturn of 194 i 21 K, however, a n t e n n a - b e a m p a r a m e t e r s . Ap. J. 187, 3 8 9 is significantly greater than the 140 ± 22 K 394. reported by Low and Davidson (1965), at ANDREWJEWSKI, W. (1953). Die B e u g u n g elek1.2 mm, particularly when one considers t r o m a g n e t i s c h e r W e l l e n . . . a n d e r kreisfSronly the errors in the Jupiter-Saturn migen 0ffnung im leitenden ebenen Schirm. ratios and not the total absolute errors. Zeitschrift fi~r angewandte Physik 5, 178. This increased disk temperature is un- CO~rKLIN, E. K . (1972). N a t i o n a l r a d i o a s t r o n o m y doubtedly due to emission from the rings o b s e r v a t o r y i n t e r n a l r e p o r t o n t h e p o i n t i n g of t h e 36-ft r a d i o telescope. of Saturn (Pollack et al., 1974). In both cases the quoted temperatures were calcu- L o w , F. J . AI~D DAVIDSON, A. W . (1965). L u n a r o b s e r v a t i o n s a t a w a v e l e n g t h of 1 r a m . Ap. J. lated for the area of the disk only and 142, 1278-1282. neglected the solid angle subtended by the MOORE, C. H . AND RATHER, E . D. (1973). T h e rings. However, the first observations by F O R T H p r o g r a m for s p e c t r a l line o b s e r v i n g . Low and Davidson were made when the Proc. I E E E 61, 1346-1349. rings were nearly edge-on as seen from the POLLACK, J . B., SUMMERS, A. AND BALDWIN, B. Earth, and our observations were made at (1973). E s t i m a t e s of t h e size of t h e p a r t i c l e s in nearly maximum ring inclination (March t h e r i n g s of S a t u r n a n d t h e i r cosmogonic 16, 1973). Thus the measured difference in i m p l i c a t i o n s . Icarus 20, 263-278. the two results nearly corresponds to the RUZE, J . (1966). A n t e n n a t o l e r a n c e t h e o r y A review. Proc. I E E E 54, 633-640. maximum possible expected signal from the rings. This is apparently the first ULICH, B. L. {1973). A b s o l u t e b r i g h t n e s s t e m p e r a t u r e m e a s u r e m e n t s a t 2.1 m m w a v e detection of ring emission at wavelengths l e n g t h . Icarus 21, 254-261. greater than 1 mm. Although the ring's ULICH, B. L., COGDE~, J . R . A~D DAWS, J . H . spectral index is probably steeper than (1973). P l a n e t a r y b r i g h t n e s s t e m p e r a t u r e t h a t of a black body due to increasing m e a s u r e m e n t s a t 8.6 m m a n d 3.1 m m w a v e opacity at shorter wavelengths, we are l e n g t h s . Icarus 19, 59-82. unable to correct the mean wavelength of ULRICH, R . (1967). F a r i n f r a r e d p r o p e r t i e s o f our observation for this effect without m e t a l l i c m e s h a n d its c o m p l e m e n t a r y s t r u c t u r e . Infrared Physics 7, 37-55. further observations with differential ill-