Accurate amplitude calibration at millimeter and submillimeter wavelengths is a difficult goal to achieve due to the temporal variability of the emissive and absorptive properties of the Earth's atmosphere and the lack of an accurate astronomical flux standard. The difficulties with deriving a uniform amplitude calibration system has resulted in the three step calibration process used at millimeter and submillimeter single dishes and interferometers. The second step in this process involves the chopper wheel calibration technique. Chopper wheel calibration is used to derive the antenna temperature of an astronomical source corrected for atmospheric extinction. An analysis of the uncertainties in two variants of this technique, one which uses a single calibrated load and a second which uses two calibrated loads, has been derived. The conclusion of this analysis is that the one-load chopper calibration system is more uncertain than the two-load chopper calibration system for application at submillimeter wavelengths. The main reason for the larger uncertainty of the one-load chopper calibration system is the fact that it requires a knowledge of the mean atmospheric temperature, which is inherently difficult to obtain. Of the two calibration systems, the two-load chopper system has the potential for reaching a calibration accuracy of approximately 1% for all bands, as specified for the ALMA receiving systems. We will also address the problem of deriving the third and final step on the amplitude calibration ladder; the conversion of atmosphere-corrected antenna temperatures (T^*_A) to astronomical source radiation temperatures.
To place this discussion in the context of general temperature scales used in millimeter and submillimeter astronomy, a discussion of temperature scales and telescope efficiencies is given in an appendix of this paper.
View a PDF version of ALMA Memo
#318.
Download a PS version of ALMA Memo #318.
Last modified: August 9, 2000
alma-memos@nrao.edu