Prism beamswitch for radio telescopesJ. M. Payne and B. L. Ulich Citation: Review of Scientific Instruments 49, 1682 (1978); doi: 10.1063/1.1135345 View online: http://dx.doi.org/10.1063/1.1135345 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/49/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The Sardinia Radio Telescope AIP Conf. Proc. 1357, 317 (2011); 10.1063/1.3615148 Low Cost Radio Telescope Am. J. Phys. 32, 546 (1964); 10.1119/1.1970768 Radio Telescope Phys. Today 16, 94 (1963); 10.1063/1.3050952 Australian Radio Telescope Phys. Today 15, 44 (1962); 10.1063/1.3058062 Radio Telescope Phys. Today 14, 86 (1961); 10.1063/1.3057534

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Prism beamswitch for radio telescopes J. M. Payne and B. L. Ulich

National Radio Astronomy Observatory. Tucson. Arizona 85712

(Received 19 June 1978; in final form. 22 August 1978)

A dielectric prism and switching mechanism have been constructed for beamswitching a Cassegrain radio telescope. Spatially extended radio sources may be mapped without significant confusion utilizing the sensitivity and stability inherent in the conventional Dicke radiometer.

INTRODUCTION

The apertures of existing millimeter-wavelength radio telescopes are very large compared to the wavelength, and thus the widths of their diffraction beams are small (typically 1-5 arcmin). Many celestial radio sources are extended compared to the typical telescope beamwidth, and thus spatial maps of the source brightness distribu­tion may be desired. Of course, the antenna may be scanned over the region of interest with the receiver con­figured in a total power mode of operation. However, maps obtained in this manner will be degraded, due to instabilities and gain fluctuations in the receiver. The conventional Dicke radiometer provides increased sta­bility and reduces the effects of gain fluctuations, par­ticularly when it is operated in a balanced mode. An ideal radio frequency switch would be lossless and in­stantaneous, and the signal and reference power levels would be equal. A nutating subreflector1 is used on the II-m Kitt Peak radio telescope operated by the National Radio Astronomy Observatory. The axial direction of the telescope diffraction beam is shifted by a combina­tion of translation and of rotation of the secondary mirror of thef!13.8 Cassegrain optics. However, aber­rations and mechanical limitations restrict the maximum beam throw to about ±5 arcmin. Some astronomical experiments require unconfused maps of larger regions, and we have developed a prism beamswitch which al­lows such observations to be made. Basically, a prism is placed in the aperture of the feed horn at the Cassegrain focus. This prism deflects the diffraction beam of the feed horn so that it misses the secondary mirror and is instead directed toward a "cold sky" reference direc­tion. The reference direction should generally be away from the galactic plane to avoid confusion when observ­ing thermal sources in the plane of the galaxy. The overall telescope beam is simultaneously rotated and changed in shape from that of the diffraction pattern of the primary mirror to the diffraction pattern of the feed horn. This simple arrangement is analogous to the "cold sky" reference horn used with prime focus parab­oloids, but the quasioptical switch is to be preferred over existing waveguide switches because of the larger bandwidth and lower loss. A simple obstructing chopper could be used as a load switch, but the difficulties of maintaining a reference temperature close to that of the

antenna (typically -30 K) make a balanced system impractical.

I. DESCRIPTION

A schematic diagram of the secondary optical system of the telescope is shown in Fig. 1. The small diameter (46 cm) of the secondary mirror of the I1-m NRAO tele­scope on Kitt Peak requires a Cassegrain feed horn with a large aperture and a small half-power beamwidth (-2.1 0). To maintain the best balance of signal and ref­erence power levels, the feed beam should be deflected by an angle just equal to the subtended diameter of the subreflector. Thus the feed beam should be deflected by about 4°, and from Snell's law this corresponds to a wedge angle of 6.8° for a Rexolite prism (cross­linked polystyrene with an index of refraction = 1.59). Figure 2 is a plot of the lens-corrected feed horn power pattern with and without the prism over the aperture. The beam direction is shifted by the proper angle and a small loss (-0.4 dB) is noted due to reflections at the prism surfaces and to absorption within the dielectric material. The surfaces of the prism contain circular

Hyperbolo/dal Subreflector

Normal Beam

Pattern

r40j / .....

/ \ I I / I

I /

/ / I I

I / I I

I , I ,

Beam Pattern Deflected

By Prism

/1 Rotat/ng /1' _ Prism Lens--~

Horn T To Rece/ver

FIG. I. Schematic diagram of secondary optical system of radio tele­scope showing prism positions and beam deflection.

1682 Rev. Scl.lnstrum., 49(12), Dec. 1978 0034-6748/78/4912-1682$00.60 © 1978 American Institute of Physics 1682

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o ;;; - 1 ~ -2 = -3 o

'" a.> ~ o a.. a.> > .., o a.>

0::

-2 -1 o 2

With Prism~,

/ \ I \

I \ I \ I \ I \

I \ I \ I \

I \ I \

I \

/ \ I \

/ \ / \

./ '-

Beam Deflection (0)

FIG. 2. Power patterns of the lens-corrected feed horn at 3.3-mm wavelength with and without the prism over the aperture.

machined matching grooves2 to minimize the specular reflections and consequent signal loss. A good, polariza­tion-independent match may be obtained over a 30% bandwidth with this technique. A servoloop employing position and velocity feedback is used to rotate the prism about an axis through its base at a 0.7-Hz rate in a square-wave fashion. The transition time is 150 ms, and the input to the synchronous detector is blanked during this interval. A synchronous detector is used to demodulate the difference in the signal and reference power levels. Maps of radio sources more than 4° in diameter will be confused due to detection in the broad reference beam, and thus the prism beamswitch is used only for sources smaller than 4°.

II. EXPERIMENTAL RESULTS

Figure 3 is a plot of the measured antenna tempera­ture due to sky radiation as a function of telescope ele­vation angle. These data were taken at 3.3-mm wave­length with the NRAO I1-m telescope and represent typical observations under clear sky conditions. The

Air

60r-____ ~2~,6~2~,0~1.7~~----~~----~r.

50

~ 40 ::> .., o ~ 30 0.

E Q)

I- 20 o c: c:

~ 10 c: <[

o

--0_ ------0

T(Subreflector)- T(Prism)

FIG. 3. Measured antenna temperature at 3.3-mm wavelength due to sky radiation without the prism (open circles) and imbalance be­tween the normal primary beam and the deflected feed horn beam (solid circles).

1683 Rev. Sci. Instrum., Vol. 49, No. 12, December 1978

Mop of Sun Using Switching Prism

8

~1O' 10'

FIG. 4. Contour map of the Sun during partial eclipse at 21 h UTC on October 12, 1977. The prism beams witch was used on the NRAO 11-m telescope at a wavelength of 2.8 mm, and the data were taken by scanning the telescope. The contour interval is 600 K in brightness temperature. Right ascension and declination are denoted by a and 8, respectively.

open circles in Fig. 3 are the measured sky tempera­tures when the feed beam is directed at the subreflector. The solid circles in Fig. 3 are the measured differences in the antenna temperature when the prism was switched in and out of the feed hom aperture. This residual imbalance is small (± 3 K) and is about a factor of 5 lower than can be achieved with a constant temperature reference load.

The Sun is a bright radio source ~ 30 arc min in diame­ter and was used to test the prism beamswitching system. Figure 4 is a contour map ofthe Sun obtained on October 12, 1977 at 2.8-mm wavelength with the NRAO II-m telescope. On this date the Moon partially eclipsed the Sun as can be clearly seen in the figure.

The slower rate of the prism beamswitch (0.7 Hz) compared to the nutating subreflector (6.7 Hz) results in increased receiver output noise due to IIf com­ponents. In one second of integration time the rms receiver noise is typically 50 mK for a switching rate of 6.7 Hz and 150 mK for a switching rate of 0.7 Hz. Thus the receiver noise is significantly higher with the prism beamswitch and existing cooled mixer receivers, and correspondingly greater integration times are required to achieve the same signal-to-noise ratio. The nutating subreflector is clearly preferable for sources smaller than ~ 10 arc min diameter, but for sources between 10 arc min and 4° the prism beamswitch provides the best available means of obtaining unconfused maps.

ACKNOWLEDGMENT

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

1 J. M. Payne, Rev. Sci. Instrum. 47, 222 (1976). 2 T. Morita and S. B. Cohn, IRE Trans. Antennas Propag. AP-3,

33 (\956).

Prism beamswltch 1683

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