Measure the beam efficiency using standard hydrogen line calibration
Accurate measurements of the solar flux require accurate calibration of the SRT. The vane calibration, while absolute, is slightly influenced by reflections from the absorber and other methods of calibration are desirable as a check. Extended sources of radio flux can be used to measure the antenna beam efficiency or assuming a fixed value of beam efficiency as a check of the stability of the vane calibration. While this will not improve absolute flux measurements, which require accurate knowledge of the aperture efficiency, however it will improve relative measurements. In the early days of radio astronomy with small dishes several methods were used. The following methods are described in Methods of Exp. Physics L. Marton vol. 12, part c (ref. 1):
1. Pointing at some distant absorber – like the trees on a nearby mountain.
2. Observe a “calibration” region in the sky. S8 and region 7 are examples, see spectra below:
This region is sufficiently extended for the SRT – but the 50 kHz resolution of the SRT is not sufficient to resolve the line peak so the integrated intensity should be used. Integrated intensity observed.
(30 + 30 + 10 + 12) x 8 = 650 ± 50 K km/s
This region has a relatively weak H-line but is much more extended and is therefore less likely to be influenced by pointing errors. The nominal peak temperature (after baseline subtraction) should be about 2 K. The observed intensity integrated over velocity is
(3.3 + 1.6 + 1.8) x 8 = 53 ± 10
1. L. Marton , C. Marton “Methods of Experimental Physics,” vol. 12 Astrophysics, part C: Radio Observations, M.L. Meeks editor, Academic Press, 1976.
2. D.R.W. Williams, “Studies of four regions for use as standards in 21-cm observations,” Astron. Astrophys. Suppl. 8, 505-516, 1973.
3. G.T. Wrixon, C. Heiles, “Baseline Determination for 21-cm Hydrogen Line Astronomy,” Astron.&Astrophysics, 18, 444-449, 1972.