Memo Review Memo: 415 - Phase Correction using Submillimeter Atmospheric Continuum Emission Matsushita, Matsuo, Wiedner, and Pardo Reviewer: Mark Holdaway Date Received: 2002Aug23 Review: I think it is likely that the basic points of 415 are correct, ie, that if you are doing WVR with continuum measurements, they will be more sensitive in the submillimeter, less sensitive to liquid water, and also that multi-frequency measurements will permit rejection of liquid water. However, because of a host of questions about this method's utility, I don't think we should change the design of the ALMA to meet the requirements of this calibration method. Some issues I have, small and large: * 11.5 um is the specification for the instrumental part of the phase error. Larger total phase errors (ie, in part due to the atmosphere) will occur. * It is stated that 183 GHz WVR is the most convincing phase correction method. It would be more accurate to say that 183 GHz is the most hoped for phase correction method. So far, no radiometric phase correction scheme has produced the required accuracy for ALMA, and worse, we still don't have a quantitative understanding of the errors involved. Fast switching should at least be mentioned. * It is never stated how much better (or worse) this technique might be than 183 GHz WVR in the presence or absence of liquid water. In part that is because we don't know how to quantify the WVR residual errors. * A critical piece of missing information that is required to judge how useful the submillimeter continuum monitoring might be is: "what fraction of the time will there be appreciable liquid water over chajnantor"? I am guessing 10-20% Now, of that time when liquid water is a problem over the site, we have other questions to ask: - can spectral line 183 GHz WVR succesfully reject the liquid water? (183 GHz WVR is in the project to stay) - what fraction of that time has poor opacity, precluding observations at any but the lowest frequencies? - what fraction of that time would result in system temperature fluctuations so large as to make amplitude calibration unreliable? - the phase recovered by fast switching is not affected by liquid water, so if the opacity and Tsys fluctuations were not too severe, fast switching could recover some time affected by liquid water. I contend that these additional factors would make the utility of this method a small incremental improvement over what we are currently planning for the ALMA. Now, another set of issues: how easy is it to make the submillimter measurements? * very stable recievers or well-calibrated gains are required * small systematic errors (variable ground pickup) will be a problem * ALMA is not designed to perform dual frequency observations, and only two bands will be ready to go, one at a time, at any given time. * we don't yet know how well we can remove the liquid water with the dual frequency technique Some submillimeter observations could use this technique "for free". Only software and some algorithmic work will be required (assuming the recievers are stable enough, which they may not be if we are not forcefully backing this proposal). If we were observing at the millimeter, we would need to perform "fast frequency switching" to utilize this method. In that case, the errors we experience will be similar to the residual errors in fast switching: we will perform the calibration measuring the sky at submillimeter frequencies, switch to the target frequency (but not moving the antenna), and integrate on source while the atmosphere changes above us. (Yes, we DON'T need to move the antenna, but the residual errors in fast switching are always dominated by the time terms, ie, the atmosphere changing above us with time. The time overhead required to change frequencies is about the same as the antenna move time and settle time.) If we desired the multi-frequency approach to explicitly remove the liquid water, we have the same sort of problems, but we will ALWAYS have to do the frequency switching thing. For the SMA, this is probably a good research topic. Multi-frequency observing is possible, and the SMA may provide more of a research and development environment. For the ALMA, I think we need more solid evidence of improvement for a major change in design to be implemented.