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ALMA MEMO #434
Load Calibration at Millimeter and Submillimeter Wavelengths
Jeff Mangum (NRAO Tucson)
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
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