A Broad-Band Microwave Source Comparison Radiometer for Advanced Research in Radio Astronomy

Drake and Ewen: Microwave Source Comparison Radiometer

:

(a) i4-J

I(4

I1-4

0

(b) 2!4j4Id-

AMPLITUDE IN UNITS OF CTo

Fig. 8-Probability densities of detected output voltage forlinear and square-law detectors.

(c)

The spectra of these powers and voltages are givenin Fig. 9. The essential characteristics of these spectraare approximately independent (for our purposes, whichdictate an invariant principal component of noise) of thetype of detector. Bandwidths B1, B2, and B3 representrespectively the predetection, audio-filter, and final-filter bandwidths.From the figure it is apparent that the choice of the

audio frequency, fa, for modulation is independent ofthese spectra out to the vicinity of fa=B1. There theavailable over-all power is reduced, but snr is not af-fected. It is seen that the successive filters improve snr

successively.

D-C

L-F

i

f'B to,i fsB f \I1. I

OUTPUT SIGNALVOLTAGEISPROPORTIONAL TOINPUT SIGNAL POWER

I

B

l l l~~~~~~~~~~~~~~~~~~~~~~~~

t~~~~5 I

o fa

II

4%I--mg

>_<i

I

.t,

f =B, fo

\11I

5S ~ ~~~ I

0 f-B

Fig. 9-Approximate power and voltage spectra.

ACKNOWLEDGMENT

The author wishes to thank the many members of thestaff of the Ewen Knight Corporation for stimulatingdiscussions that led to several of the results reportedhere. In particular H. I. Ewen and F. D. Drake broughttheir knowledge and experience in the field of radio as-

tronomy to bear on the details of the analysis.

A Broad-Band Microwave Source Comparison Radiom-eter for Advanced Research in Radio Astronomy*

F. D. DRAKEt, MEMBER, IRE AND H. I. EWENt, SENIOR MEMBER, IRE

Summary-A sensitive microwave radiometer system operatingat short centimeter wavelengths has been developed which will allowlarge extensions of the known spectra of a large number of radiosources; facilitate the measurement of source polarization; give in-

formation on galactic structure and the sources of galactic radioemission; provide new data on the physical structure of planetarynebulas; and provide a means of measuring more accurately plane-tary temperatures, and the precise position of the brighter radiosources.

A traveling-wave tube radiometer operating at 8000 mc with a

bandwidth of 1000 mc and sensitivities of the order of 0.010K is* Original manuscript received by the IRE, November 8, 1957.t Harvard College Observatory, Cambridge, Mass., and Ewen

Knight Corp., Needham Heights, Mass.

described. The radiometer is more than one order of magnitudemore sensitive than other existing radiometers operating at 8000 mc.The very serious effects of gain fluctuations, acting on small residualsignals, when trying to achieve very high sensitivities, are discussed.A means of eliminating such effects by introducing compensatingnoise has been found successful. Radio observations with this radi-ometer in conjunction with a 28-foot parabolic reflector have shownthat: 1) The predicted sensitivity is achieved. 2) Zero-level stabilityis extremely high. 3) It has been possible to detect in detail the dis-tribution of radio brightness at this wavelength in the vicinity of thegalactic plane. 4) Radiation from the planets Jupiter and Saturn hasbeen detected, this being the first detection of Saturn as a radiosource. 5) Radiation from two planetary nebulas has been detected,this being the first detection of these objects as radio sources.

z0

-jI

0cc0-

Z) O \r

Iv e

If t 1

1958 53

PROCEEDINGS OF THE IRE

INTRODUCTION

RADIO observations of celestial sources at centi-meter wavelengths have been impeded by two

causes.

1) The radio brightness of radio sources, in general,decreases rapidly with decreasing wavelength;

2) The sensitivities of available radiometers decreaseas one goes to shorter wavelengths.

These two causes in combination have largely dis-couraged attempts to observe the sky extensively at theshort centimeter wavelengths, even though shorterwavelengths allow one to achieve more narrow antennabeams with a given antenna dimension.

It has been known for several years that these wave-

lengths may provide information vital to the solution ofmany outstanding astronomical problems. Among theastronomical problems which now may be more effec-tively attacked at short centimeter wavelengths, one

may list:

1) The spectra of discrete radio sources.-Spectra in-formation now will be greatly extended with respect toavailable information.'-5 These spectra provide impor-tant clues with regard to the mechanism of radio emis-sion operating in a radio source.

2) The polarization of radio sources.-These meas-

urements are facilitated at short wavelengths becausethe earth's ionosphere affects the polarization of the ra-

diation to a much smaller degree at these frequenciesthan at lower frequencies.6

3) The origin of the radiation in the vicinity of thegalactic plane.-An accurate knowledge of this radia-tion, evaluated in conjunction with longer wavelengthdata, will determine whether the radiation is of thermalor nonthermal origin.

4) Galactic structure as determined from the distri-bution of regions of ionized hydrogen.-Clouds ofionized hydrogen are thermal emitters, which dominatethe radio sky at short centimeter wavelengths.

5) The physical structure of planetary nebulas.-These objects, whose position in the picture of cosmicevolution is still poorly understood, are difficult to study

I F. T. Haddock, C. H. Mayer, and R. M. Sloanaker, "Radio ob-servations of ionized hydrogen nebulae and other discrete sources ata wave-length of 9.4 cm,' Nature, vol. 174, pp. 176-177; July 24,1954.

2 F. T. Haddock and T. P. McCullough, Jr., "Extension of radiosource spectra to a wavelength of 3 centimeters," Astron. J., vol. 60,pp. 161-162; June, 1955.

3 V. M. Plechkov and V. A. Razin, "Results of measures of the in-tensity of radio emission of discrete sources at wavelengths of 3.2and 9.7 cm," Proc. Fifth Conf. on Questions of Cosmogony, Academy ofSciences of the USSR, Moscow, pp. 430-435; 1956.

4 N. L. Kaydanovsky and N. S. Kardashev, "Results of observa-tions of discrete sources of cosmic radio emission at a wavelengthof 3.2 cm," Proc. Fifth Conf. on Questions of Cosmogony, Academy of

Sciences of the USSR, Moscow, pp. 436-437; 1956.5 N. G. Roman and F. T. Haddock, "A model for nonthermal

radio source spectra," Astrophys. J., vol. 124, pp. 35-42; July, 1956.6 C. H. Meyer, T. P. McCullough, and R. M. Sloanaker, "Evidence

for polarized radio radiation from the Crab Nebula," Astrophys. J.,vol. 126, pp. 468-470; September, 1957.

photographically with high precision, but some aspectsof these objects may be studied with a microwave radi-ometer of sufficient sensitivity.

6) The temperatures of the planets.-The extensivework already done at the Naval Research Laboratory,Washington, D. C., at 3-cm wavelength, indicates thevalue of such observations, since temperatures obtainedby radio techniques differ markedly, in some cases, fromtemperatures determined by other means.

7) The precise determination of the positions ofradio sources.-The narrow antenna beams obtainableat short wavelengths facilitate position determinationsboth by decreasing the confusion produced by havingseveral sources in the beam antenna simultaneously, andby providing a radiometer response that is more sensi-tive to changes in anitenna pointing. Furthermore, atshort wavelengths one may observe sources of smalloptical size, such as Venus, thereby providing an ac-curate calibration of antenna pointing error.

CONSIDERATIONS IN RECEIVER DESIGN

The receiving system to be described was specificallydesigned for the measurement of very low-level thermalnoise powers associated with antenna temperatures pro-duced by faint celestial sources of small angular size.The "signal" produced by these sources has a noise-

like character similar to the thermal and shot noise ofthe receiving system. The design criteria for the systemthen involve distinguishing a small change in the noisepower output level of the receiver when the signal noiseis introduced at the terminals of the antenna.The minimum detectable signal of this system repre-

sents a change in the output-power level of the receiverof one part in 400,000 or a detectable antenna tempera-ture change of 0.01°K in the presence of an equivalent4000°K-system noise.The minimum detectable signal is determined by

fluctuations in the receiver output level in the absenceof a signal. These output-level fluctuations are producedby spurious gain fluctuations within the active circuitsof the receiver and by the statistical fluctuations in anoise-like waveform.The normal method that has been used to reduce gain

fluctuations and other spurious effects is to modulatethe signal at a frequency at which the amplitude of sucheffects is negligible. The amplitude of the signal maythen be determined by means of a coherent detectordriven at the modulation frequency.7

Dicke's original description7 of such a radiometer in-volved a 30-cps modulation of the signal by means ofmechanically switching the receiver input between theantenna terminals and a resistive load at "room" tem-perature. Comparison was obtained then between thethermal noise presented at the terminals of the antenna

7R. H. Dicke, "The measurement of thermal radiation at micro-wave frequencies," Rev. Sci. Instr., vol. 17, pp. 268-275; July,1946.

54 January


and the thermal noise of the resistor. Various othermethods of modulation have been employed; however,each involves the fundamental concept of sequencing"signal" and "comparison" information through activeamplifier circuits in a modulation pattern which in mostcases can be represented by a square wave. Each cycleof the square wave contains one-half period of signalinformation followed by a half-period of comparison in-formation.

In many systems this simple modulation scheme ofswitching to a resistive comparison load has been suffi-cient to reduce the effect of gain fluctuations below thelevel of statistical noise fluctuations. The receiving sys-tem described in this paper involves the successful ap-plication of further techniques to reduce the effect ofgain fluctuations. Such techniques become increasinglysignificant for systems with low statistical noise fluctua-tions.

Theoretically, the fluctuations in the output-noiselevel of the receiver produced by the statistical nature ofthe noise waveform can be reduced to any desired de-gree by increasing the integration time after detection(narrowing the post detection bandwidth).Expressed in equivalent temperature units the rms

value of these fluctuations AT is

AT = Equivalent system noise/x/Br , (1)

where the equivalent system noise can be obtained fromthe noise figure F of the system by the expression

(F - 1)To. (2)

To is the reference ambient temperature, 290°K. In(1) above:B =the predetection bandwidth, andr = the time constant of the integration network after

the detector.

Presumably we can increase r or B and thereby re-duce AT to any desired value. There is, of course, apractical limitation to the maximum value of r, which isintroduced by the amount of available observing time;this maximum value may be determined either by achange in source position as a function of time or a timedependent change in the amplitude of the power re-ceived by the antenna. An increase in B may be limitedby the bandwidth of the signal, the current state ofequipment development, man-made interference "noise,"and many other factors.To date, instrumentation at short wavelengths has

consisted of superheterodyne receivers. The sensitivityof such systems has been limited by the relatively nar-row bandwidths that pan be easily achieved. Existingsuperheterodyne receivers provide a maximum sensitiv-ity of the order of 0.5°K, with a five-second integrationtime. The introduction of traveling-wave tubes at shortcentimeter wavelengths has offered the possibility ofachieving, at the cost of a slight increase in noise figure,

extremely wide bandwidths, of the order of 3 kmc at afrequency of 8 kmc. In actual application, it was founddesirable to limit the bandwidth to about 1 kmc inorder to avoid interference. The sensitivity of such asystem, in theory, can be more than an order of magni-tude greater than that given by existing superhetero-dyne receivers.

In order to test the efficacy of traveling-wave tuberadiometers in radio astronomy application, such a radi-ometer was built and then was tested by actual observa-tion of celestial sources.

TRAVELING-WAVE TUBE RADIOMETERA simplified block diagram of the basic receiving sys-

tem is shown in Fig. 1. The receiver is basically a trftype consisting of three cascaded traveling-wave tubeamplifiers. The center frequency of the interstage filtersis 8000 mc and the bandwidth of each filter is 1000 mc.

1TWT-FLTR TWT FIT TWT

Fig. 1-Block diagram of basic receiving system employingtraveling-wave tubes.

The measured system noise figure is 111 db, pro-viding an over-all equivalent system noise of about4000°K. With an integration time of 100 seconds thevalue of AT from (1) is about 0.01°K. Hence, the fluctu-ations in the output power level of the receiver due tothe statistical nature of the noise waveform are verysmall indeed.Now let us direct our attention to the effect of gain

fluctuations. With an equivalent system noise of40000K a gain change of ±1 per cent would produce a+ 40°K change in the receiver output. To reduce thiseffect to 0.01°K would require stabilizing the gain of thereceiver to one part in 400,000 or 0.00025 per cent. Aneasier approach would be to modulate the signal bycomparison to a resistive load7 as described above.

Fig. 2 is a simplified block diagram of the receivermodified by inserting a ferrite switch at the input to thefirst twt. Modulation of the signal is obtained by switch-ing between the antenna terminals and a resistive loadat "room temperature."

Fig. 2-Block diagram of receiving system with addition ofcomparison switching circuits.

The general expression for the minimum detectablesignal is then:

AT = K[ i + ( )-G) (AT, + TA)] (3)

1958 55

FERRITESWITCH

--o


whereK =a constant introduced here to simplify the ex-

pression by eliminating other effects not dis-cussed in this paper,8

F=the system noise figure,To=the reference ambient temperature, 290'K,B =the predetection bandwidth (1000 mc),rr=the time constant of the integration network,

G(t) =the gain of the receiving system at time t,Go=the average value of G(t) during the period of

observation,aAT. = Tc2 -Tcl,T11=temperature "observed" by the receiver with

the switch connected to the resistive load,T,2=temperature "observed" by the receiver with

the switch connected to the antenna terminals,and with no radio source in the antenna beam,

TA = effective antenna temperature, "signal" pro-duced by a radio source.

This expression for AT may be divided into two parts.

1) The effect of statistical noise fluctuations:

- (F -1)ToAT= KV B- (4)

2) The effect of gain variations:

_ - G(t) -Go\AT= KE(T G + TA) (5)

Go

The first part obtains a value of 0.01°K as describedabove. For a 1 per cent variation in gain, the second partobtains a value of 2.9°K, if we assume to a first approxi-mation that with no signal:

Tc, = 2900K

Tc2==O°K.

It is evident from (5) that gain fluctuations limit theminimum detectable signal by an amount equal to thepercentage of the gain change multiplied by the noiseunbalance at the input terminals of the receiver, in theabsence of a signal. To achieve a sensitivity of 0.01°K, itis then necessary to balance the noise temperatures Te1and T,2 to better than 1°K, if gain fluctuations are nogreater than 1 per cent.

Various methods have been proposed to achieve con-dition Tjl= TC2. For this particular system the most con-venient one was to introduce additional noise at inputterminal 2 and increase T,2 until balance was obtained.

Fig. 3 is a block diagram of the receiver in the "noisecompensated" form. The addition of noise to terminal 2was achieved by inserting a fractional amount of thenoise power available from an argon gas discharge noisesource into the side arm of a directional coupler. Themain arm of the coupler was inserted in the antennatransmission line between terminal 2 of the receiver and

8 P. D. Strum, "Considerations in high-sensitivity microwaveradiometry," this issue, p. 43.

Fig. 3-Block diagram of receiving system with the addition of"noise compensating" circuits.

the antenna terminals. The addition of a variable at-tenuator between the noise generator and the couplerprovided an easy means for adjustment of the amounitof noise added to terminal 2.The addition of approximately 300°K of equivalent

noise power to the input of the receiver does, of course,increase the over-all system noise from 4000°K to4300°K. However, this produces a negligible effect oniAT as given by (4) when one considers the fact that thisaddition of 300°K of noise improves system sensitivityfrom 2.9°K to approximately 0.01°K. An effective sensi-tivity of 0.01°K was achieved in practice, thus providinga receiver more than one order of magnitude more sensi-tive than existing receivers at this frequency.To improve further the sensitivity of such a system

and, in fact, to determine the ultimate limitations on thesensitivity of the system described herein, we mustanalyze in greater detail the dependence of the terms in(3) on time and antenna position.

TP2 may be represented by the expression:T2= TB + TT + Ta2 + T1

where individual component contributions to TC2 ex-clusive of the noise compensating component are:

TB=the equivalent black body temperature of thegalactic background radiation field "observed"by the antenna,

TT= temperature due to tropospheric effects,T2= temperature due to losses in the ferrite switch

and side lobe contributions of the feed systemand reflector,

TP=effective temperature due to man-made inter-ference.

At a frequency of 8000 mc:

TB is less than 1°K. However, it is a function of an-tenna position. For fixed antennas it is a functionof time as a consequence of the earth's diurnalrotation.

TT will depend primarily on the water vapor andoxygen content of the atmosphere in the solidangle of the antenna main beam. It will, there-fore, be dependent on time as well as antennaposition. The absolute magnitude of this effectmight be as high as 30°K.

T<¢2 will depend on the geometrical configuration ofthe antenna system and topographic and reflect-ing properties of the antenna surroundings. Itwill depend primarily on position, and even for awell-designed antenna pointing toward thezenith this effect might be as large as 20°K.

56 January


Tr will, of course, depend on time, position of theantenna, and physical proximity of sources ofman-made noise.

The problem for the future is simply expressed as thedetermination of the function T12(t, 0,4) for each system.

If we include the noise compensation required for bal-ance, then:

TVNGT (2A+ + TB + TT + T.2 + Tr

whereTNG=effective noise generator temperature,

A,+A2=attenuation of the variable attenuator plusthe coupler.

The first term of T,2 can be made time dependentonly, with a long thermal time constant, by applicationof known techniques to temperature stabilize compo-nents.

T,1 can be represented by the expression:

L = To+ Tal,

with an equatorially-mounted 28-foot paraboloid witha solid aluminum surface for the purpose of makingactual tests on celestial sources of radiation. The beam-width to half-power points is 18' of arc. Because thereceiver has been in a developmental status and underconstant revision, precise calibration has not been at-tempted nor has the antenna gain been carefully meas-ured. The results to date are, therefore, of a preliminarynature and stated antenna temperatures may be in errorby a factor of two.

Fig. 4 is a reproduction of the radiometer response asthe radio source Cassiopeia A was allowed to driftthrough the antenna beam. Such a "drift curve' shouldresemble a rectilinear plot of the antenna beam pattern,if the radio source is much smaller than the size of theantenna beam, as is the case here. The time constant inthis case was 80 seconds, and the peak antenna tem-perature recorded for this source was about 2°K. Thetheoretical rms fluctuation amplitude at the radiometeroutput should be about 0.02°K under these conditions.The observed amplitude as recorded is approximatelyequivalent to this predicted value.

whereTo= ambient temperature of the load, and

Ta, =temperature component produced by losses inthe ferrite switch when connected to position 1.

If To varies appreciably with time, the resistive ele-ment can be placed in an oven and stabilized to 0.01°Kduring the time of observation. Tal will be dependentonly on time and its thermal time constant can be in-creased by various techniques.Many of the effects discussed above can be easily con-

trolled if the desired system sensitivity is no greaterthan 0.01°K, as was the case in the development of theradiometer described here. However, one of the pur-poses of this more detailed discussion is to point out theproblems that will be associated with radiometers em-ploying very low internal noise amplifiers. With a solidstate Maser, for instance, the term F -1/1/T in (3) willbe reduced by a factor of 1000 at microwave frequencies.Fluctuations in the output-power level of the receiverwill no longer be determined by the statistical fluctua-tions of the noise waveform. This fluctuation, presentlythe weak link in radiometer systems, will become thestrongest link and the heretofore negligible effects repre-sented by the second term

K[(G() TC + TA)]

will completely determine receiver performance. Thereceiver reported here is in the "twilight" zone betweenyesterday and tomorrow in radiometer development.

ASTRONOMICAL OBSERVATIONS

The radiometer has been operated in conjunction

Fig. 4-Radiometer response as the strong radio source CassiopeiaA drifted through the antenna beam. Maximum antenna tempera-ture = 2°K. Receiver time constant = 80 seconds.

Fig. 5 is a "drift curve" of the radio source associatedwith the galactic nebulosity M17. The time constantfor this record was 80 seconds, and the peak radiometerdeflection was about 3°K. The antenna was drivenaround the polar axis at a speed slightly greater thanthat of the earth's rotation, so that, although only about10 minutes of right ascension displacement are shownby the tracing, about two and one-half hours were re-quired for the observation. The most remarkable qualityof this tracing from an instrumentation standpoint, be-sides the high sensitivity obtained, is the high stabilityof the zero-intensity level, which drifted at a rate ofonly 0.1°K per hour. The small rate of zero-level drift isquite encouraging to radio astronomers, who have longfound a lack of zero-level stability to be a major obstaclein obtaining optimum observational data. The zero-leveldrift existing in this and other figures may be attributedto drift in the temperature of the resistive load againstwhich antenna temperatures are compared. Oven stabil-ization of the resistive load was not included during thisseries of tests.The source labelled A in Fig. 5 is associated with the

emission nebula M17. The source labelled B in Fig. 5 liesvery near the position of the galactic plane as found by

1958 57


t AFig. 5-Radiometer response as the region of the nebulosity M17 passed through the antenna beam. Maximum antenna temperature=

3°K. Time constant =80 seconds. Source A is the radio source associated with M17, Source B lies near the galactic plane.

D C B A

_- TIMEFig. 6-Radiometer measure of radio emission from Jupiter. Maximum antenna temperature 0.15°K. Time constant= 320 seconds. Be-

tween A and B, the antenna pointed J3 east of Jupiter; between B and C, the antenna pointed at the planet; between C and D, theantenna pointed 3° west of Jupiter.

radio investigations at longer wavelengths. Since, opti-cally, there is no emission nebulosity that might be re-sponsible for this source, it appears quite likely that thissource actually is the galactic plane. The narrow widthof the response to this source then indicates that thegalactic plane appears quite narrow at centimeter wave-lengths, possibly being only a few minutes of arc inwidth, as has been predicted by astronomical theorists.To the left and right of source B, one may observe anextended source of radio emission, which is probablyassociated with galactic nebulosities, such as IndexCatalog 4701, close to the galactic plane.'9 This tracingprobably marks the first observation at these wave-lengths of faint emission in the vicinity of and at the po-sition of the galactic plane.

Fig. 6 is an observation of thermal radio emissionfrom the planet Jupiter. The time constant was 320seconds, giving a theoretical sensitivity in this case of0.010°K. Referring to Fig. 6, the antenna was po-sitioned 2 east of Jupiter at point A on the scan and

' F. T. Haddock, C. H. Mayer, and R. M. Sloanaker, "Radio emis-sion from the Orion Nebula and other sources at X 9.4 cm," Astrophys.J., vol. 119, pp. 456-459; March, 1954.

then allowed to track this position to establish a radi-ometer zero level, with Jupiter just outside the beampattern. At B, the antenna was pointed directly atJupiter and allowed to track the planet. During thisportion of the scan, the radiometer output should de-scribe an exponential curve asymptotic to the antennatemperature produced by the planet. At C, the antennawas pointed 2° west of Jupiter, and again allowed totrack. During this portion of the scan, the radiometeroutput should describe another exponential until thezero level is again reached. The radiometer responsedoes follow the predicted path and indicates a peakantenna temperature due to the planet of 0.150K. If itis assumed that the gain of the antenna is one half thatof a perfect antenna, a black body temperature slightlygreater than 2000K is indicated.

Fig. 7 is a radiometer measurement of the emissionfrom the planet Saturn, taken in the same manner as theJupiter observation. The antenna tracked east of theplanet from A to B, pointed at the planet from B to C,and west of the planet following C. The antenna tem-perature due to the planet as measured by this scan is0.04°K, corresponding to a flux of about 4X(10-26)

58 January


' ....

....... . . . __ _. .__ __._ .

t'__.__._ .., -__..'_._._'. mm_. t.'*_....._.'''.... . ..

s ss i- 9--. . .....

* = >M!NiaSg

-ID l t

atA

- TIME

Fig. 7-Radiometer measure of radio emission from Saturn. Maximum antenna temperature 0.04°K. Time constant=320 seconds. Be-tween A and B, the antenna pointed j" east of Saturn; between B and C, the antenna pointed at the planet; between C and D, theantenna pointed j" west of Saturn.

watt m-2(cps)-' or 5.5 X (10-16) watts of power de-tected at the antenna terminals. This is probably thesmallest antenna temperature detected to date with amicrowave radiometer. In this case, one may not deter-mine a black body temperature for the planet withoutfirst knowing whether the well-known rings are con-tributing to the radio emission. If the particles makingup the rings are much smaller than the 3.75-cm receiverwavelength, or are very cold, the particles will con-tribute very little emission at this wavelength. Since wedo not have a reliable measure of the size of the ringparticles or their temperature, it is possible to invert theargument, accepting the planetary temperature givenby other means,10 and determine whether or not therings are present radiowise. To eliminate calibrationerrors as much as possible, the predicted ratio of theantenna temperature of Jupiter to that of Saturn wascomputed for both cases; i.e., with and without thepresence of the rings. Since the observations of Jupiterand Saturn were made in quick succession, taking theratio of the observed antenna temperatures from theseobservations should eliminate systematic calibrationerrors. From published data for the date of the observa-tions, July 24, 1957, the ratio of the antenna tempera-ture Jupiter to Saturn should be 4.7 if the rings aretransparent, and about 3.0 if the rings are opaque. Theobserved ratio is 4.3, indicating that the rings are notradio sources at 8000 mc, which may mean that theyprincipally consist of particles smaller than 3.75 cm indiameter, or are at a temperature less than about 20°K.A "drift curve" of the planetary nebula NGC 7293

(the 'Helix" Nebula) is shown in Fig. 8. This repre-sents the first radio observation of a planetary nebula.

10 C. W. Allen, "Astrophysical Quantities," University of London,London, Eng., 1956.

This tracing was made in the same manner as the trac-ings of Cassiopeia A and the region of M17, with an 80-second time constant, and indicates a maximum an-tenna temperature for the nebula of 0.250K. It is desir-able with weak signals to take a mean of several observa-

Fig. 8-Radiometer response as the planetary nebula NGC 7293passed through the antenna beam. Maximum antenna tempera-ture = 0.2 5°K. Time constant= 80 seconds.

tions, as this allows one to make a statistical analysis ofthe data to derive a measure of its reliability. Five ob-servations of NGC 7293 have been combined to obtain amean curve. The resultant curve and the probableerrors derived for the experimental points are shown inFig. 9. The mean probable error is 0.022°K. The actualscatter among the points themselves suggests that theactual probable error is somewhat smaller than this.This can occur because the observer must draw an ar-bitrary zero level through each tracing of the radiosource before deriving antenna temperatures from thetracing, and this zero will always be slightly in error.The effect is to introduce no error in the relative po-sitions of the derived experimental points, but to en-hance the probable errors derived in a straightforwardway from the observational data. It is fortunate thatthis is the case, as one can be sure that the derivedprobable errors are too large, and hence the data areactually more reliable than the probable errors indicate.In this case, the actual scatter among the points indi-cates that the true probable error is of the order of0.0100K.

1958 59


ANTENNATEMPERATURE

*K30

.25

.20

-.10

-.05

- Nt i3 29 g8 6I 2522kRIGHT ASCENSION

Fig. 9-Mean of 5 observations of NGC 7293, with probable errorsof the observed points indicated. The mean probable error is0.022°K; the arrow indicates the optical center of the nebula.

From the published optical data,"-"3 the expectedantenna temperature for the nebula was computed. Theuncertainties in the optical data make it possible for thistemperature to be in error by at least a factor of five.In the case of NGC 7293, assuming the antenna gain tobe half that of an ideal antenna, the predicted antennatemperature is 0.16°K, which is in excellent agreementwith the observed antenna temperature of 0.26°K fromFig. 9. The data of Fig. 9 show a response to the neb-ular signal which is broadened about 6' of arc over theresponse from a point source. This is to be expected,as this nebula is about 15' of arc in diameter, and hasfaint optical extensions extending at least as far as 10'of arc from the center of the nebula. The arrow in Fig.9 indicates the optical center of the nebula, and it isseen that the center of the radio emission as determinedfrom the present observations deviates appreciablyfrom the optical center, and actually lies very near theedge of the nebula. The difference between the twocenters seems too large to be attributable to instrumen-tal errors and is probably real. It is of interest to notethat the center of radio emission apparently deviatesfrom the optical center in the same direction as faintextensions of the nebula, recently observed on large-scale, high quality, photographs. Further radio and op-tical studies of this nebula are clearly indicated.

Fig. 10 is a reproduction of a drift curve of the plane-tary nebula NGC 6853 (the "Dumbbell" Nebula). Atime constant of 80 seconds was used, and a maximumantenna temperature of 0.10°K is indicated. Four suchobservations have been combined to give the resultsshown in Fig. 11. The mean probable error from thedata is 0.017°K, although, as before, the scatter of thepoints among themselves suggests that the true prob-able error is of the order of 0.008°K, which is representa-tive of the extremely high-receiver sensitivity. Theantenna temperature predicted from the optical data is

11 D. H. Menzel and L. H. Aller, "Physical processes in gaseousnebulae. XII. The electron densities of some bright planetary nebu-lae," Astrophys. J., vol. 93, pp. 195-201; January, 1941.

12 T. Page and J. L. Greenstein, 'Ionized hydrogen regions inplanetary nebulae," Astrophys. J., vol. 114, pp. 98-105; July, 1951.

13 I. S. Shklovsky, "A new scale of distances to planetary nebu-lae." Astron. J. Soviet Union, vol. 33, pp. 222-235; 1956.

Fig. 10-Radiometer response as the planetary nebula NGC 6853passed through the antenna beam. Maximum antenna tempera-ture = 0.10°K. Time constant = 80 seconds.

58 S5RIGHT ASCENSION

Igh

Fig. 11-Mean of 4 observations of NGC 6853, with probable errorsof the observed points indicated. The mean probable error is0.017°K; the arrow indicates the optical center of the nebula.

0.07°K, which is in remarkable agreement with the ob-served temperature of 0.09°K. In this case, the opticalposition of the nebula, as indicated by the arrow inFig. 11, is in close agreement with the radio position.This was expected since the dimensions of this nebulaare roughly 6'X8'.

CONCLUSIONThe traveling-wave tube radiometer described in this

report fulfills the theoretical sensitivity deduced fromits electronic characteristics. The inherent high stabilityachieved is of great value to radio astronomers. Thishigh sensitivity and stability was achieved by carefulconsideration and treatment of circuit componentswhich do not affect the simple theoretical considerationsof circuit sensitivity.

Astronomical results indicate that one may readilyobserve the temperature of the planets with a radiom-eter of the type described. For the first time, it is possi-ble with ease and reasonable speed to make a detailedstudy of galactic radiation at 3- to 4-cm wavelengths.Preliminary observations described here suggest thatmuch important data will accrue from such a study. Theradio observations of planetary nebulas, achieved forthe first time through the use of this radiometer, haveestablished a new field for the application of radio as-tronomy. The first results reported here indicate thatsuch radio information will greatly improve the accu-racy to which some parameters of the nebulas are known,and may bring to light important aspects of these ob-jects which have not been observable until now.

ACKNOWLEDGMENTThe authors are grateful to Peter D. Strum, Harry E.

Adams, and A. William Gruhn of the Ewen KnightCorp. for the extensive, unprecedented engineering de-velopment required to achieve the optimum equipmentperformance described here.

60 January

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A Broad-Band Microwave Source Comparison Radiometer for Advanced Research in Radio Astronomy