NATIONAL RADIO ASTRONOMY OBSERVATORYGreen Bank, West Virginia

ELECTRONICS DIVISION INTERNAL REPORT NO. 310

BEAM SHAPE AND SYSTEM TEMPERATURE OF85-3 TELESCOPE AT 610 MHZ

D. Anish Roshi, Frank Ghigo and Dana Balser

April 16, 2000

Beam Shape and System Temperature of 85-3 telescopeat 610 MHzD. Anish Roshi, Rank Ghigo and Dana BalserDate: 04-06-2000

This report discusses beam shape and system temperature measurements of the 85-3telescope at 610 MHz. A spectrum analyzer (Anritsu MS2602A) is used as a total powermeter for all the measurements. It is configured in the zero span mode. The center frequencyis selected as 609.25 MHz and the resolution bandwidth as 3 MHz. This frequency rangewas relatively free of interference. The calibration noise is switched at 1 sec rate using anHP(Hewlett — Packard) 3310A Function generator. The video bandwidth of the spectrumanalyzer is set to 10 Hz. An Epson printer is used to print the spectrum analyzer display.

1 Beam ShapeRadio source Virgo (RA(1950): 12:28:17.6, DEC(1950): 12:40:02; 49610MHz 390 Jy) wasused for the beam shape measurement. The scans taken in hour angle (slew rate = 30°/mm)and declination (slew rate = 20°/min) for the two orthogonal polarizations (channel A andB) are shown in Figs. la, b, c d.

2 System TemperatureFor system temperature measurement, the telescope was pointed to a few positions in the skyand the calibration noise was switched at 1 Hz. The system temperature is calculated using

vi /(v2 — v 1 ) x "'cal where v 1 and v2 are the voltages measured by the spectrum analyzerwhen the calibration noise is turned off and on respectively. These voltages are proportionalto the total powers. Teat is the measured calibration noise temperature. Table. I. gives themeasured values and the system temperature in terms of the calibration noise temperature.Similar measurements were made towards Virgo which are also included in Table. 1. Figs.2a to k show the spectrum analyzer outputs corresponding to all these measurements.

The calibration noise temperatures for the 85-3 receiver system were measured earlier byCoe (1989). At 610 MHz these temperatures were 4.6 K and 5.3 K for channel A and Brespectively. The mean off-source system temperature obtained using the noise temperaturevalues are 188.6 K for channel A and 295.2 K for channel B. The antenna temperature dueto Virgo can be calculated by taking the difference between the off-source system tempera-ture and the system temperature measured towards Virgo. The antenna temperature thusobtained are 112.2 K for channel A and 126.7 K for channel B. These values for antennatemperature cannot be obtained even if the aperture efficiency of the telescope is 100 %.Therefore we conclude that the present values of the calibration noise temperature are notthe quoted values in Coe (1989).

The system temperature can indirectly be calculated using the measurements towards

Off-source Tsyson T

sysof fTsysof 1

Table 1: Results of the measurements made on 85-3 telescopeRA DEC HA VI v v2-vi Fig. ref

'"..hh:mm:ss 0 . • (hrs) Ch A Ch B Ch A Ch B Ch A Ch B 12:08:17.6 12:40:02 ,,, -3.5 0.849 0.835 0.869 0.85 42.5 55.7 2a,b12:28:17.6 17:40:02 '- 0.951 0.975 39.6 2c12:28:17.6 17:40:02 ,-, -3.5 0.914 0.936 41.5 2d-, 10.00:00 ,-, 50:00:00 ,--, -1 0.93 0.953 40.4 2e12:28:17.6 12:40:02 r•-, -3.5 0.969 0.955 0.984 0.967 64.6 79.6 2f,g (Virgo)12:28:17.6 12:40:02 ,-, -3 0.95 0.965 63.3 2h (Virgo)12:28:17.6 12:40:02 r,, -3 0.956 0.97 68.3 2j (Virgo),,, 13:00:00 ,-, -40:00:00 -4.6 0.962 0.968 160.3 2k$

* All voltages are relative values.$ Note that the antenna is pointed to ground at this position

Table 2: Estimated System temperature for 85-3 telescope

RA(1950) DEC(1950) i = 0.4 0.5hh:mm:ss ° : I :" Ch A Ch B Ch A (K) Ch B (K) Ch A (K) Ch B (K) 12:08:17.6 12:40:02 1.54 1.43 55.2 69.3 69.1 86.712:28:17.6 17:40:02 1.65 45.9 57.412:28:17.6 17:40:02 1.58 51.4 64.3

10:00:00 50:00:00 1.62 48.1 60.2

Virgo. The ratio of the system temperature towards Virgo to that at an off-source positionis independent of the calibration noise temperature. Using this ratio the system temperaturecan be estimated if we assume an aperture efficiency (ij). Table 2 gives the estimated systemtemperature for different aperture efficiencies. For these calculations, an average of all themeasurements towards Virgo is used.

3 ReferenceCoe, J. R. 1989, NRAO memorandum.

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