I re-investigate the semi-transparent vane calibration scheme proposed for ALMA, taking into account expected saturation behavior of the receivers. This memo improves on memo 371 by using more accurate derivations and better estimates of the receiver saturation temperatures. It is shown that to reach a given precision in calibration, the saturation on the vane must be less than this precision. Suitable values for the vane transparencies are given. The vane transmission can be calibrated in a few minutes (at mm wavelengths) to 1 hour (above 275 GHz) by measurement of an astronomical source (nearby quasar). A derivation of the saturation temperature from a measurement on the vane and on an ambient load is presented. The case of partial saturation on the vane is explored. Using the derived saturation temperature in this case requires a more accurate measurement of the vane transmission, but is the only way to reach the specified accuracy. Given the simplicity of the vane system (passive, slow device, in the receiver cabin), compared with the complexity, speed and location of the dual-load system, I recommend that ALMA develops and adopts such a scheme for the receiver calibration.
View a pdf version of ALMA Memo #423.
Last modified: 2002-11-18
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