Absolute brightness temperature measurements at 2.1-mm wavelength

ICARUS21, 254--261 (1974)

Absolute Brightness Temperature Measurements at 2.1-ram Wavelength' B. L. U L I C H ~ Millimeter Wave Observatory, Department of Electrical Engineering, The University of Texas at Austin, Texas 78712

Received September 13, 1973; revised October 23, 1973 Absolute measurements of the brightness temperatures of the Sun, new Moon, Venus, Mars, Jupiter, Saturn, and Uranus, and of the flux density of DR21 at 2.1-mm wavelength are reported. Relative measurements at 3.5-mm wavelength are also presented which resolve the absolute calibration discrepancy between The University of Texas 16-ft radio telescope and the Aerospace Corporation 15-ft antenna. The use of the bright planets and DR21 as absolute calibration sources at millimeter wavelengths is discussed in the light of recent observations. I. INTRODUCTION O b s e r v a t i o n s of t h e t h e r m a l radio emission of the Sun, Moon, a n d planets a t millimeter w a v e l e n g t h s are i m p o r t a n t in t h e e v a l u a t i o n of theoretical a t m o s p h e r i c a n d surface models. I n m a n y cases a knowledge of t h e absolute i n t e n s i t y of the o b s e r v e d r a d i a t i o n a n d its v a r i a t i o n w i t h w a v e l e n g t h provides powerful cons t r a i n t s on model p a r a m e t e r s . T h u s it is a d v a n t a g e o u s , w h e n e v e r possible, to m a k e a b s o l u t e l y calibrated m e a s u r e m e n t s of b r i g h t n e s s t e m p e r a t u r e s . H o w e v e r , large radio telescopes are difficult to calibrate, a n d m a n y o b s e r v a t i o n s are m a d e relative to a s t a n d a r d source. T h e m o s t obvious calibration sources a t millimeter w a v e lengths are t h e b r i g h t e r planets, b u t their s p e c t r a in this w a v e l e n g t h r a n g e are n o t well defined. I n an effort to d e t e r m i n e the absolute s p e c t r a of the planets to a high degree of precision, a n d t h u s to p r o v i d e good s t a n d a r d sources for u n c a l i b r a t e d radio telescopes, Ulich et al. (1973) (herea f t e r called P a p e r I) h a v e m a d e extensive o b s e r v a t i o n s a t 8.6-mm a n d 3.1-mm w a v e lengths w i t h a 16-ft d i a m e t e r a n t e n n a . I n

this p a p e r I r e p o r t the extension o f this p r o g r a m to 2 . 1 - m m wavelength. I n Section I I I describe the a n t e n n a a n d receiver used for the 2 . 1 - m m observations. Section I I I contains a discussion of t h e observing procedure. Section I V presents the results, a n d in Section V I discuss their relation to previous d a t a a n d existing theoretical models. Section V I is a s u m m a r y of the results a n d presents an a s s e s s m e n t of various sources as reference calibrators.

II. INSTRUMENTATION T h e a n t e n n a used for the 2 . 1 - m m observ a t i o n s was The U n i v e r s i t y of T e x a s Millimeter W a v e O b s e r v a t o r y 4.88-m (16ft) d i a m e t e r radio telescope n e a r F o r t Davis, Texas. T h e a p e r t u r e efficiency a t the o p e r a t i n g f r e q u e n c y of 141 G H z was d e t e r m i n e d b y c o m p a r i n g the p o w e r received b y the 16-ft a n t e n n a f r o m a t r a n s m i t t e r in the far field to the p o w e r received b y a s t a n d a r d gain horn. This conical h o r n was c o n s t r u c t e d so t h a t its effective a r e a could be a c c u r a t e l y calculated. Following the m e t h o d of Cogdell (1969) I calculated the h o r n gain to be 35.0 =E 0 . 1 d B 3. T h e relative received p o w e r was m e a s u r e d

1 This work was sponsored by the National Aeronautics and Space Administration under Grant NGL 44-012-006. 2 Presently at the National Radio Astronomy 3 All errors quoted in this paper are l a or Observatory, Tucson, Arizona 85717. 68% confidence level errors. Copyright © 1974 by Academic Press, Inc. 254 All rights of reproduction in any form reserved. Printed in Great Britain

2.1

MM BRIGHTNESS TEMPERATURES

with a precision cutoff waveguide attenuator at a 60 MHz I F frequency. The gain on the electrical axis of the 16-ft antenna was determined to be 74.2 ± 0.3dB, corresponding to an aperture efficiency of 50.3 ± 3.6%. Actual observations were made with dual beams separated by 5.5 min of arc and positioned symmetrically about the axis of the antenna in a plane of constant declination. The off-axis aperture efficiency was 48.9 ± 3.5%. Pattern measurements using the transmitter in the far field indicated symmetrical beams with a half power beam width (HPBW) of 110". No evidence of astigmatism was visible, and the sidelobes were about 20dB below the peak of the main beam. Since the antenna gain measurement was made looking near the horizon, several tests were performed to identify possible changes in antenna properties at high elevation angles due to gravitational distortion. First, power patterns were taken about 120 ° apart in azimuth. Since the telescope has an equatorial mount, the direction of gravitational force underwent a rotation of about 90 ° with respect to the primary reflector. The two sets of patterns were identical, indicating no significant gravitational distortion is present. Secondly, principal plane half power beam widths determined by fitting a two-dimensional gaussian to five point observations of planets agreed with the pattern range measurements and were constant at different source altitudes. In addition, a comparison of simultaneous solar, lunar, and planetary observations over a wide range of elevation angles showed t h a t if the atmospheric extinction corrections determined by the extended source data were applied to "point" sources, no additional variation of peak antenna gain was evident. Thus all tests showed no noticeable effects of telescope orientation on the gain or beamwidth of the antenna. The coupling efficiency to a uniformly bright disk of the same angular diameter as the Sun and Moon was determined by calculating the fraction of the power which misses the extended source. Losses due to spillover (7.6%), central blockage (4.0%),

255

and spar blockage (3.5%) have been calculated by Cogdell (1973). The central blockage is larger than the value given in Paper I since a larger receiver mounting flange was installed at the prime focus in 1972. The width and amplitude of the error pattern due to random reflector errors was determined from scans of the Sun, and 8.4% of the scattered energy was estimated to miss the extended source. Combining these losses produces a coupling efficiency of 78.4~:4.2% for the 16-ft antenna at 2.1 mm. The uncertainty in the coupling efficiency is associated mainly with the scattering by random errors in the reflector surface. A dual beam Dicke radiometer utilizing a switchable ferrite circulator, a 141 GHz klystron local oscillator, a Schottky diode mixer, and a 1-2 GHz transistor I F amplifier was designed and constructed especially for precise absolute measurements. The radiometer output was synchronously detected, integrated, digitized, and punched on paper tape for later analysis. For observing extended sources the receiver was converted to a single beam load switched radiometer through the use of a remotely controlled waveguide switch. The RMS receiver noise was typically 1K for an integration time of 1 sec. Relative calibration was maintained with a weakly coupled neon noise tube. The thermal equivalent of this calibration signal was determined by comparison with the radiation of a well-matched termination which was thermally cycled using liquid baths at four temperatures between 77K and 365K. Small corrections were made for VSWR and waveguide losses. III. OBSERVATIONS Solar observations were made by taking drift scans through the apparent center of the disk. Lunar deflections were measured by a simple OFF-ON-OFF technique with the OFF position about 2 ° away from the center of the Moon. Planets were observed using the dual beam arrangement. The source was tracked first in the primary beam and then in the reference beam. The length of integration was set so t h a t each

256

B.L. ULICH

b e a m t r a c k e d across the same p a t h in the sky. This assured good cancellation o f sky emission and s t r a y pickup. E i g h t hr of b l a n k sky integrations over a wide range in hour angle showed t h a t , within the sensitivity of the receiver, this observing technique completely cancelled s t r a y signals. As described in detail in P a p e r I, five point observations were m a d e to correct for a n y small residual pointing errors. Because of its relatively weak signal, DR21 was observed only at the predicted position so t h a t sufficient integration time for a reasonable m e a s u r e m e n t was accumulated during a single observing period. The receiver was periodically calibrated b y injecting a signal from the neon noise tube. D a t a were t a k e n within 4 hr of transit to minimize atmospheric extinction losses. The zenith a t t e n u a t i o n was calculated from atmospheric tipping d a t a t a k e n e v e r y few hours. Typical values ranged from a m i n i m u m of 0.1 dB on a v e r y d r y winter night to a m a x i m u m of 2.3dB on an e x t r e m e l y wet b u t clear s u m m e r day. A complete description of the d a t a reduction procedure has been given in P a p e r I. The t o t a l systematic error in a n t e n n a gain, t h e r m a l calibration of the radiometer, and telescope pointing is a b o u t 6 % for e x t e n d e d sources and a b o u t 7 % for point sources. I n all cases the quoted s t a n d a r d deviations are the q u a d r a t u r e sum of r a n d o m receiver noise errors and systematic calibration errors.

brightness t e m p e r a t u r e measured with the conical h o r n was 6200 ± 280K, which differs from the previous result b y less t h a n 2%. This close agreement confirms the calculated b e a m efficiency at 2.l-ram wavelength. The weighted average of the two semi-independent results (using the same t h e r m a l scale) is 6 1 6 0 ± 2 2 0 K as given in Table I. Observations of the m e a n center of the new Moon on December 5, 1972 yielded a Sun/new Moon ratio of 4 2 . 6 0 ± 1.00 and a new Moon central brightness t e m p e r a t u r e of 145 ± 9K. Additional solar observations on the 16-ft a n t e n n a in July, 1973 at 8.6-ram wavelength have revealed t h a t the solar brightness t e m p e r a t u r e originally reported in P a p e r I is in error because of nonlinear receiver response. The revised solar temp e r a t u r e at 8 . 6 m m is 9480 ± 570K. Fig. 1 is a plot of the solar c o n t i n u u m from 1-mm to 2-cm wavelength. I n c l u d e d are the d a t a r e p o r t e d here and in P a p e r I, the 1-cm and 1.9-cm results of W r i x o n and Hogg (1971), and the average of the 1.2-mm d a t a b y Ade et al. (1974). Venus, Mars, J u p i t e r , and S a t u r n were observed for a b o u t 6 hr each on an average of 6 days between N o v e m b e r 21 and D e c e m b e r 2, 1972. The measured 2.l-ram brightness temperatures, which are calculated assuming a u n i f o r m l y bright disk, TABLE I ABSOLUTE BRIGHTNESS TEMPERATURES OR FLUX DENSITY AT 2.1-~]:M WAVELENGTK

IV. RESULTS E q u a t o r i a l drift scans of the Sun at 2 . 1 m m were t a k e n on 12 days between J u l y and December, 1972. The average measured solar brightness t e m p e r a t u r e was 6080 ± 370K. Since d a t a which showed t e m p o r a l e n h a n c e m e n t s o f the solar flux were excluded, this t e m p e r a t u r e represents an average across the equatorial region of the quiet Sun. An i n d e p e n d e n t check on the calculated b e a m efficiency was made b y observing the Sun with the same receiver and the s t a n d a r d gain horn previously used to measure the a p e r t u r e efficiency of the 16-ft antenna. The solar

TB (K) or

Source

S (f.u.)

_+

1a a

Sun New Moon Venus Mars Jupiter Saturn Uranus b DR21

6160 145 300 211 168 164 115 18.3

+ _+ _+ + _+ __ _ ___

220 9 21 16 12 12 16

2.0

a The standard deviation quoted here is the quadrature sum of random receiver noise errors and systematic calibration errors. b Cogdell et al. (1974).

2.1 MM BRIGHTNESS TEMPERATURES 14

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Fro. l. Recent solar brightness temperature measurements between l-ram and 2-cm wavelength. are listed in Table I. During the Mars observations the pole was inclined 16 ° toward the E a r t h and the longitude of the central meridian varied from 40 ° to 190 ° . F u r t h e r solar and planetary observations with the same equipment were made in June, 1973 by Cogdell et al. (1974). Their data are in excellent agreement with these results, indicating t h a t random errors are small and the total uncertainty is dominated by systematic calibration effects. The measured 2.1-mm brightness temperature of Uranus is 1]5 ± 16K (Cogdell et al., 1974). Observations of DR21 for about 6 hr on each of 8 days in November and December, 1972 yielded an average observed flux density of 17.5 ± 1.5 f.u. Correcting for partial resolution of the source by the factor

257

accurately extrapolated to short millimeter wavelengths. However, recent observations between 2 m m and 350~m indicate a tremendous increase in the flux density of DR21 with increasing frequency. The 2.1-mm value reported here is 18.3 ± 2.0f.u., which is above the extrapolated spectrum by a significant amount. Observations at 1.4mm by Rather (1973) indicate a flux density of 41 ± 10f.u., Rieke et al. (1973) have measured 1000 ± 400f.u. at 350/~m, and Lemke and Low (1972) have reported a flux density of 750 i 150f.u. at 21-~m wavelength. Figure 2 is a plot of the flux density of DR21 between 21-/~m and 3.8-em wavelength. I t clearly shows the rapid rise in flux density (increasing approximately as the cube of the frequency) in the short millimeter wavelength region. According to Gezari et al. (1973) the thermal opacity of dust grains can result in a variation as large as the fourth power of the frequency. Thus the observed rapid increase m ay be entirely accounted for by the presence of large amounts of dust grains. In Paper I we presented the results of absolute measurements near 3-ram wavelength on the 16-ft antenna of The University of Texas, which indicated a 17% absolute calibration discrepancy for small angular diameter sources between this instrument and the Aerospace Corporation 15-ft antenna. In an a t t e m p t to resolve this difference, the temperatures of the bright planets were measured relative to 3000

C s = 1.0 + 0.15 (10.03)/B 2

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where B is the antenna H P B W in minutes of arc (Dent, 1972), and including systematic calibration errors results in a true flux density of 18.3 ± 2.0f.u.

DR 21 300 IO0 3O

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In Paper I we derived an absolute spectrum of the H I I region DR21 based on the relative measurements by Dent (1972) at centimeter wavelengths and an analysis of the millimeter spectra of Jupiter and Saturn. The suggestion was made t h a t the spectrum of DR21 could be

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FIG. 2. Spectrum of DR21 showing the rapid increase with frequency near 1 mm due to the wavelength dependent opacity of dust grains.

258

B . L . ULICH

the flux density of the calibration source DR21. These data were taken with the 36-ft radio telescope of the National Radio Astronomy Observatory 4 on K i t t Peak, Arizona and a dual beam Dicke radiometer operating at a wavelength of 3.5 mm. Independent observations by myself and E. K. Conklin were in excellent agreement and produced the average ratios listed in Table II. Data taken previously with the same equipment by Epstein (1973) resulted in Mars/DR21 and Jupiter/DR21 ratios identical to those in Table II. However, observations by Epstein with the Aerospace Corporation 15-ft antenna at 3.3 mm are not in agreement with these results. Two sets of data taken in 1967 (Fogarty et al., 1971) resulted in Jupiter/DR21 ratios of 11.7 ± 2.3 and 5.1 ± 0.6. Observations in 1973 have yielded a ratio of 14.2±1.8 (Epstein, 1973). Thus, while three independent observers have measured compatible ratios at 3.5 mm with the NRAO 36-ft radio telescope, the 3.3-ram Aerospace results differ significantly. Possible systematic effects in the calibration of the Aerospace antenna are currently being investigated (Epstein, 1973). By assuming a flux density for DR21 one can use the ratios in Table II to calculate the planetary disk brightness temperatures. The expected flux density at 3.5mm, based on extrapolation of longer wavelength observations and the least squares fit spectrum derived in TABLE II MEASURED 3.5-5IM RATIOS OF P L A N E T A R Y BRIGHTNESS TE~IPERATURES TO T H E T R U E F L U X D E N S I T Y OF D R 2 1

Paper I, is about 16.2f.u. However, the true value is somewhat higher because the second component peaking at about ]00/~m contributes to the observed intensity even at 3-mm wavelength. Taking into account the 2.1-mm, 1.4-mm, and 350-Fm results, the excess contribution of the second component is about 0.7f.u. at 3.5-mm wavelength. Thus, the total flux density of DR21 is about 16.9f.u., and from the measured planetary ratios the calculated brightness temperatures are 350 ± 35K for Venus, 212 ± 4K for Mars, 174 i 4K for Jupiter, and 150 ± 7K for Saturn. Assuming the extrapolated value of 16.2f.u. for DR21 leads to lower limits for the planetary temperatures being smaller by about 4%. A comparison of the temperatures in Table II with the 3-mm data in Table I of Paper I leads to the conclusion that, if the flux density of DR21 at 3.5-mm wavelength is 16.9f.u., The University of Texas results are probably high by about 3%, which is smaller than the quoted standard error of 7%, and t h a t the Aerospace Corporation temperatures are probably too low by about 13%, which is only slightly larger than the quoted calibration uncertainty of 10%. The microwave spectra of Venus, Mars, Jupiter, and Saturn are shown in Figs. 3-6. The basic data have been tabulated by Ulich (1973). The filled circles represent the 2.1-mm and 3.5-mm data reported here. In addition, the 1.4-mm observations of Rather et al. (1974) and the 150-Fm and 350-Fm data of Armstrong et al. (1972) have been included. As can be seen in the figures, recent observations at short wavelengths are quite consistent

TB(K) ± la

Source

Ratio _ lo

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(If DR21 ----16.9 f.u.)

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Venus Mars Jupiter Saturn

20.7 __ 2.1 12.5 ± 0.2 10.3 ± 0.2 8.9 ± 0.4

350 ± 35 212 ± 4 174± 4 150_+ 7

4 The National Radio Astronomy Observatory is operated by Associated Universities, Inc. under contract with the National Science Foundation.

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FIG. 3. Microwave spectrum of Venus.

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2.1MMBRIGHTNESS TEMPERATURES 260

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FIG. 4. Microwave spectrum of Mars. and determine the planetary spectra with considerable precision. The brightness temperature of Venus is seen in Fig. 3 to decline smoothly in the millimeter region with no apparent spectral features. The new Mars data definitely show a slight increase in brightness temperature at millimeter wavelengths and are consistent with simple homogeneous model calculations assuming thermophysical properties similar to those of the lunar soil (Janssen and Welch, 1973). Thus Mars is a good calibration source since its brightness temperature varies only slightly with wavelength and in a predictable fashion. In the past Jupiter has been widely used as a calibration source since its millimeter spectrum was believed to be relatively fiat. However, Fig. 5 indicates a comparatively complex shape due to a deep and very broad ammonia absorption feature near 1.35-cm wavelength. In view of its complex spectrum, Jupiter is not an ideal calibration source. More absolute measure-

ments are needed in the millimeter region to accurately determine its brightness temperature spectrum. The microwave spectrum of Saturn is shown in Fig. 6. Its shape is qualitatively similar to t h a t of Jupiter except near l-ram wavelength. Low and Davidson (1965) measured a brightness temperature at 1.2mm referred to the disk of 140 ± 22K when the rings were nearly edge-on. Recent data by Rather et al. (1974) at 1.4 mm indicate a higher disk temperature of 194 ± 2 1 K . These observations were made when the rings were inclined by about 27 ° to the line of sight. Both measurements were made with antenna beams larger than the apparent diameter of the ring system. Since the observed Saturn/ Jupiter brightness temperature ratio increased from 0.90 ± 0.14 (Low and Davidson, 1965) to 1.29 ± 0.05 (Rather et al., 1974) as the rings opened up, it seems -- 220

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FIG. 5. Microwave spectrum of Jupiter.

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FIG. 6. Microwave spectrum of Saturn showing the increase in apparent disk brightness temperature at l-ram wavelength due to emission from the rings.

260

B.L. ULm~

certain t h a t the rings contribute significantly to the l-mm radiation. The assumption has been made that the emission from the disks of Jupiter and Saturn are constant with time. The 2.1-mm data were taken with the rings at nearly maximum inclination and imply a marginally significant increase in the apparent disk temperature due to ring emission. Thus Saturn is not a good calibration source at wavelengths shorter than 3mm, since its total emission will vary with the changing aspect of the rings. Fig. 7 is a plot of some recent microwave observations of Uranus. Included are the data reported in this paper, the centimeter wavelength results of Mayer and McCullough (1971), Pauliny-Toth and Kellermann (1970), and Webster et al. (1972), and the 1.4-mm point by Rather et al. (1973). In addition, several measurements relative to Jupiter have been shown. The ratio measured by PaulinyToth and Kellermann (1970) at 9.5mm yields a disk brightness temperature of 138 ± 17K if Jupiter is assumed to be 155K as discussed in Paper I. At 8.2mm Uranus is 146 ± 16K (Kuzmin and Losovsky, 1971) if Jupiter is 160K. At 3.5mm Pauliny-Toth and Kellermann (1970) measured 138± 9K if Jupiter is 174K as previously discussed in this paper. I f the data of Epstein et al. (1970) at 3.3mm are adjusted upward to match this more accurate Jovian temperature, their measurements indicate a brightness temperature of 119 ± 15K for Uranus. Sufficient data has now been accumulated to give a precise picture of how the observed 250

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VI. SUMMARY In this paper I have presented new absolutely calibrated 2.1-mm wavelength measurements of the brightness temperatures of the Sun, new Moon, Venus, Mars, Jupiter, Saturn, and Uranus, and of the flux density of DR21. In addition I have reported measurements of the bright planets relative to DR21 at 3.5-mm wavelength. The planetary brightness temperatures derived by assuming a flux density of 16.9f.u. for DR21 indicate t h a t near 3-mm wavelength the absolute calibration of The University of Texas 16-ft radio telescope is probably about 3% high, which is within the quoted standard error of 7%, and t h a t the absolute calibration of the Aerospace Corporation 15-ft antenna is probably about 13% low, which is only slightly larger than the quoted calibration uncertainty of 10%. Saturn, Jupiter, and DR21 are not good reference calibration sources at wavelengths less than 3 mm because their spectra are not predictable with precision in this region. Jupiter is shown to have a relatively complex millimeter spectrum, and more absolute measurements and theoretical calculations are needed. The spectrum of Mars is in accordanee with simple theory, and thus Mars is the most accurately known strong calibration source at wavelengths less than 2 mm.

URANUS

75 5o

brightness temperature varies with wavelength. The general shape is qualitatively similar to the microwave spectrum of Venus. The 2.1-mm value reported here was calculated assuming the diameters given by Newburn and Gulkis (1971). The other data were calculated assuming apparent sizes quoted in the "American Ephemeris and Nautical Almanac," which differ by a small amount compared to experimental errors.

ACKNOWLEDGMENTS i IMM

3MM

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I a , n p l e a s e d t o t h a n k Dr. E . K . C o n k l i n for t h e use of u n p u b l i s h e d 3.5-ram o b s e r v a t i o n s of V e n u s , J u p i t e r , S a t u r n , a n d D R 2 1 . I also w i s h

2 . 1 MM BRIGHTNESS TEMPERATURES

to thank Dr. E. E. Epstein for informative discussions concerning the calibration of the Aerospace Corporation 15-ft antenna and for permission to quote unpublished data.

REFERENCES ABE, P. A. R., RATHER, J. D. G., AND CLEGG, P. E. (1974). Observations at 1.4 m m wavelength with the N R A O 11 meter telescope; I : limits to solar limb darkening derived from antenna beam parameters. To appear in Astrophys. J. AR:i~ISTRONG, K. R., HARPER, D. A., AND LOW, F. J. (1972). Far-infrared brightness temperatures of the planets. Astrophys. d'. 178, L89-L92. COGDELL, J. R. (1969). Calibration program for the 16-ft antenna. The University of Texas at Austin, Millimeter W a v e Sciences Tech. Report No. NGL-006-69-1. COGDELL, J. R. {1973). Private communication. COGDELL, J. R., DAVIS, J. S . , WILLS, B., AND ULRICH, B. (1974). To be published. DENT, W. A. (1972). A flux-density scale for microwave frequencies. Astrophys. J. 177, 93 99. EPSTEIN, n . E., DWORETSKY, M. M., MONTGOMERY, J. ¥~r., AND FOCvAI~TY,W. G. (1970). Mars, Jupiter, Saturn, and Uranus: 3.3 m m brightness temperatures and a search for variations with time or phase angle. Icarus 13, 276-281. EPSTEIN, E. E. (1973). Private communication. FOGARTY, W. G., EPSTEIN, E. E., MONTGOI~ERY, J. W., AND DWORETSKY, M. M. (1971). Radio sources: 3.3-mm flux and variability measurements. Astron. J. 76, 537-543. GEZARI, D. Y., JOYCE, R. R., AND SIMON, M. (1973). Observations of the galactic nucleus at 350 microns. Astrophys. J. 179, 167-170. JANSSEN, M. A., AND WELCII, V~r. J. {1973). Mars and J u p i t e r : Radio emission at 1.35 cm. Icarus 18, 502-504. KUZMIN, A. D., AND LOSOVSKY, B. YA. (1971). Measurements of Uranus radio emission at 8.22 ram. Icarus 14, 196-197.

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LEMKE, D., AND LOW, F. J. (1972). 21-Micron observations of H I I regions. Astrophys. J. 177, L53-L58. Low, F. J., AND DAVIDSON, A. W. (1965). Lu n ar observations at a wavelength of 1 millimeter. Astrophys. J. 142, 1278-1282. MAYER, C. H., AND McCuLLOUGH, W. P. {1971). Microwave radiation of Uranus and Neptune. Icarus 14, 187-191. NEVVBURN, R. L., JR., AlqD GULKIS, S. (1971). A brief survey of the outer planets Jupiter, Saturn, Uranus, Neptune, Pluto, and their satellites. J e t Propulsion Laboratory Tech. Report 32-1529. PAULINY-TOTH, I. K., AND KELLERMAN1N-,K. I. (1970). Millimeter-wavelength measurements of Uranus and Neptune. Astrophys. Lett. 6, 185-187. RATHER, J. D. G. (1973). Private communication. RATHER, J. D. G., ULICH, B. L., AND ADE, P. A. R. (1974). Planetary brightness temperature measurements at 1.4 m m wavelength. To be published. RIEKE, G. H., HARPER, D. A., Low, F. J., AND Am~STRONG, K. R. {1973). 350-Micron observations of sources in H I I regions, the galactic center, and NGC253. Astrophys. J. 183, L67-L71. ULIC~[, B. L. (1973). Absolute brightness temperature measurements at millimeter wavelengths. The University of Texas at Austin, Elect. Engr. Res. Lab. Tech. Report No. NGL.006-73-1. ULICH, B. L., COGD]~LL,J. R., AND DAVIS, J. H. {1973). Planetary brightness temperature measurements at 8.6 m m and 3.1 m m wavelengths. Icarus 19, 59-82. WEBSTER, W. J., WEBSTER, A. C., ANDWEBSTER, G. T. (1972). Interferometer observations of Uranus, Neptune, and Pluto at wavelengths of 11.1 and 3.7 centimeters. Astrophys. J. 174, 679-684. ~VRIxON, G. T., AI~D HOGG, D. C. (1971). Absolute measurements of solar flux at 16 and 30 GHz. Astron. and Astrophys. 10, 193-197.