NATIONAL RADIO ASTRONOMY OBSERVATORY

Green Bank, West Virginia

DATE:

TO: VLBI Network

FROM: R.J. Maddalena

SUBJECT: NRAO-GB 140-ft telescope calibration, 5 GHz, September 1992

Calibration of Antenna Efficiency

The temperatures used throughout the calibration of antenna efficiency, eta_A, were determined from pointing and source temperature measurements obtained during the VLBI session. A total of 239 measurements provided adequate sky coverage (Fig. 1) for a good measurement of efficiency.

The antenna temperatures used in the calibration were not corrected for the effects of the atmosphere. The sources and flux densities given in Table 1, in combination with the weighted average of the measured antenna temperatures for those sources when they were within 2 hours of the meridian, were used to establish the dependence of eta_A on declination at zero hour angle. (eta_A, when the physical aperture of the telescope is used _A equals 1.93 T_A/S where S is the 5 GHz flux in Jy.). At 6 cm, efficiency does not change much with declination along the meridian. The flux densities in Table 2 were determined from the meridian efficiency and the weighted average of T_A when the source was within 2 hours of the meridian.

The fluxes from table 1 and 2 and the measured T_A at the points indicated in Figure 1 were used to estimate eta_A across the sky; spherical harmonics were then fit to the values of eta_A. The coefficients found from the fit are given in Table 3 while the efficiency surface is shown in Figure 3. The derived efficiency is almost identical to what has been derived before.

Figures 4 - 30 display as functions of hour angle, the measured and calculated efficiencies located at the indicated declinations.

Table 1: Primary Calibrators

0237-23 3.30 ± 0.73 (2)

0521-36 8.11 ± 0.07 (1)

0851+20 2.60 ± 0.05 (1)

0917+624 1.51 ± 0.01 (1)

0954+556 2.19 ± 0.52 (2)

2021+61 2.67 ± 0.05 (1)

2203-18 4.03 ± 0.04 (1)

3C123 16.5 ± 1.12 (4)

3C147 7.99 ± 0.57 (3)

3C161 6.63 ± 0.55 (3)

3C218 13.5 ± 1.7 (3)

3C231.0 3.74 ± 0.67 (2)

3C274 60.5 ± 0.8 (4)

3C295 6.38 ± 0.56 (3)

3C309.1 3.49 ± 0.34 (2)

3C330 2.16 ± 0.38 (2)

3C348 12.0 ± 0.1 (1)

3C433 3.85 ± 0.34 (2)

3C438 1.52 ± 0.13 (2)

3C454.3 8.92 ± 0.08 (1)

3C48 5.25 ± 0.40 (3)

NGC 7027 5.45 ± 0.05 (1)

OQ208 2.56 ± 0.05 (1)

(1) Previous reports on the calibration of the 140-ft telescope.

(2) Kuhr et al. (1981, Astr. Ap. Suppl., 45, 367).

(3) Baars et al. (1977, Astr. Ap., 61, 99).

(4) U.S. VLBI Network Handbook

Table 2: Secondary Calibrators

1518+047 2.35 ± 0.04

3C345 7.50 ± 0.12

Table 3: Result of Fit

A10 = 0.0333 ± 0.0079

A11e = 0.0524 ± 0.0096

A20 = 0.0021 ± 0.0086

+ A20Y20

where: Y00 = 1

Y10 = cos(theta)

Y11e = sin(theta) cos(phi)

Y20 = 3/2 cos²(theta) - 1/2

and theta = 90-Dec, phi = hour angle in degrees.