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.
Last modified: 2004-04-13