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Load Calibration at Millimeter and Submillimeter Wavelengths

Jeff Mangum (NRAO Tucson)

Revised 2002/10/18
Original 2002/09/12

Keywords: Calibration

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 three variants of this technique, two which use a single calibrated load and a third which uses two calibrated loads, has been derived. The conclusion of this analysis is that the one-load chopper calibration systems are more uncertain than the two-load chopper calibration system. This is especially true at submillimeter wavelengths. The main reason for the larger uncertainty of the one-load chopper calibration systems is the fact that they require 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.

ALMA Memo 434 has been revised as of Oct 17, 2002. To view or downloaded the revised version, please use the following links:

View a pdf of revised ALMA Memo #434.

Download a postscript of revised ALMA Memo #434.

Download a zipped, postscript of revised ALMA Memo #434.

Original version of the document (Sept 12, 2002) is available through the following links:

View a pdf version of ALMA Memo #434.

Download a postscript version of ALMA Memo #434.

Last modified: 2002-10-18