Annu. Rev. Astron. Astrophys. 1980. 18: 489-535 Copyright © 1980 by Annual Reviews. All rights reserved

(c) Calibration

The basis of absolute receiver calibration is the blackbody law; it is the only radiation source that is calculable in actual practice. The implicit assumption is that an isothermal enclosure that does not reflect or become transparent radiates a blackbody spectrum. There are a host of problems in making a practical calibrator approach the idealized limit and furthermore, given a good calibrator, in being assured that the act of placing the calibrator at the radiometer input does not affect the performance of the receiver or alter the geometry at the input in an incalculable way. The calibrator should appear to the radiometer as similar to the measured source as possible.

Most of the low frequency and all the high frequency experiments have used cryogenic reference bodies to reduce the demands on receiver linearity and to minimize the change in operating point of the square law detector when switching between the calibrator and the sky. The temperature-vapor pressure relation of liquid helium is known to better than 0.1% and with reasonable care cryogenic thermometry can be performed reliably to this precision. The calibrators are constructed by terminating a waveguide or light pipe on a prismatic or conical absorber several wavelengths thick that is in good thermal contact with the cryogen. The termination is designed to trap the radiation through multiple reflections by the poorly reflecting absorber material. The central difficulty in the calibrator designs has been the thermal gradients at the transition from the cryogenic environment to the warns world outside, the problems being emission by the warmer sections of the waveguide and from windows used to avoid condensation of air in the liquid helium. These contributions are not negligible; in some experiments, in fact, they are comparable with the emission by the absorber. As the frequency increases this problem becomes more acute.

The emissivity of the calibrator is determined by reflection measurements, using = 1 - R, and is generally larger than 99% for narrow-band calibrators. Broad-band calibrators are more difficult to design. The reflectivity of the calibrator has to be known because, in use, emission by the warm receiver components (i.e. waveguides, local oscillators, horns) is reflected by the calibrator back into the receiver and this effect is not balanced out when the receiver looks into the sky.

Most of the detailed considerations involved with the interaction of the calibrator, receiver, and the thermal gradients are eliminated if the entire apparatus is maintained at cryogenic temperatures. This has been the practice in high frequency measurements and is clearly indicated for any new precision measurements at lower frequencies.