I consider the problem of calibrating the amplitude scale for the ALMA receivers, focussing particularly on the question of whether cold loads are necessary to achieve the best possible accuracy. Cold loads are of possible value below about 300~GHz. In these lower frequency bands the atmospheric opacities are low, so the sky appears cold; and SIS mixers are more likely to exhibit gain compression, so it is best to calibrate them with loads which are similar to the sky temperature. I estimate, however, that reflections and losses in dewar windows will likely lead to calibration uncertainties of at least 3% with cold loads, and argue that they are not worth the complexity.
Calibration techniques where the loads intercept only a fraction of the receiver beam can achieve better accuracy. One possibility is to mount the loads behind a hole in the center of the subreflector, as described by Bock et al. in ALMA memo 225. Another possibility, a variant of the traditional chopper wheel method, is to block the beam with a partially transparent vane. An advantage of the latter method is that the load filling factor, or absorption of the vane, may be calibrated on the telescope with an astronomical observation; furthermore, this technique may be useful for calibrating solar observations.
Measuring the atmospheric opacity with a specialized instrument at the center of the array may be preferable to calibrating it separately at each antenna. I estimate that the opacity varies by sim 3% over scales of 1.5 km.
View a PDF version of ALMA Memo #321.
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Last modified: August 30, 2000
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