[asac][almanews] ALMA Memo 478 Released

[Stacy Oliver]

ALMA MEMO #478 

Distance to Possible Calibration Sources as a Function of Frequency for
ALMA 

Bryan Butler 
National Radio Astronomy Observatory

2003-10-07

The assumption that so-called "fast switching" calibration will always
be done by 
observing a calibrator at 90 GHz is examined. It is found that it may be
better to
observe the calibrator at the same frequency as the target, since
suitable calibration
sources may be just as close at higher frequencies as they are at 90 GHz
(all roughly
1° for continuum bandwidths). In the end it will depend on the
characteristics of the
sources (size, mostly). Observing the calibrator at 90 GHz will require
an additional
calibration to be performed on fairly short timescales (of order
minutes) to calibrate
the relative complex gain between the target and calibrator frequency,
since the electronics
will change between scans. This extra calibration is not required if
calibrator and target
are observed at the same frequency. In addition, observing the
calibrator at the same frequency
as the target removes the requirement of having very accurate
atmospheric modelling in order
to transfer the phase and amplitude from calibrator frequency to target
frequency (especially
problematic in regions of dispersion near strong telluric lines).
Because of the uncertainty
in the sizes of the sources, it is not recommended to change the current
paradigm of calibrating
at 90 GHz, but the option of calibrating at the target frequency should
be kept in mind, and
exercised if it turns out that the sources considered herein are
appropriate to use as calibrators
(small enough). 

View a pdf version of ALMA Memo #478. 
http://www.alma.nrao.edu/memos/html-memos/alma478/memo478.pdf

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