[asac][almanews]Correction of links to revised ALMA Memo 434

[Ted or Stacy Oliver]

***Please see corrected links below for access to revised ALMA memo 434.***

ALMA MEMO #434

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.
http://alma.aoc.nrao.edu/memos/html-memos/alma434/memo434vb.pdf

Download a postscript of revised ALMA Memo #434.
http://alma.aoc.nrao.edu/memos/html-memos/alma434/memo434vb.pdf

Download a zipped, postscript of revised ALMA Memo #434.
http://alma.aoc.nrao.edu/memos/html-memos/alma434/memo434vb.pdf






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