[asac][almanews] ALMA Memo 491 Released

[Stacy Oliver]

ALMA MEMO #491 

Does the ACA Need Phase Compensation?


M.A. Holdaway 

2004-04-06


We analyze the phase monitor data from Chajnantor over the period 1996 -
2001 and infer the level of phase fluctuations that would be seen on 10-
30 m, typical baseline lengths for the Atacama Compact Array (ACA). On
baselines of 10-30 m, fast switching phase compensation will be useless,
but water vapor radiometry (WVR) could improve the phase stability
marginally. We consider the effects of phase errors on imaging and on
the flux scale, and determine that (a) the ACA will require dynamic
scheduling in order to meet the ALMA scientific objectives, (b) given
dynamic scheduling, the ACA will likely not require any sort of active
phase correction such as WVR, and (c) by permitting high frequency ACA
observations during conditions with the lowest opacities and less than
optimal phase stability, WVRs could increase the ACA's efficiency,
especially for high frequency observations, and can probably be
justified on a cost-benefit basis. There is another reason why we would
want phase calibration on the ACA - the 12 7 m dishes cannot calibrate
themselves to the required accuracy in a reasonable amount of time. In
order to calibrate accurately, we need to correlate the 7 m dishes with
the four 12 m dishes dedicated to the ACA, or even better, with the full
ALMA array. If we are correlating with the four nearby 12 m dishes, we
do not need phase compensation on the ACA antennas, but if we plan to
correlate with the full ALMA, we absolutely need WVR and fast switching
capability.

View a pdf version of ALMA Memo #491. 

http://www.alma.nrao.edu/memos/html-memos/alma491/memo491.pdf

 

 

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