[asac][almanews] ALMA Memos 434 Released

[Stacy Oliver]

ALMA MEMO #434

Load Calibration at Millimeter and Submillimeter Wavelengths

Jeff Mangum (NRAO Tucson)

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.


View a pdf version of ALMA Memo #434.
http://alma.aoc.nrao.edu/memos/html-memos/alma434/memo434.pdf

Download a postscript version of ALMA Memo #434.
http://alma.aoc.nrao.edu/memos/html-memos/alma434/memo434.ps

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