Water-Vapor Radiometry

Accurate phase calibration is a critical requirement for ALMA, and the baseline design of ALMA uses a 183 GHz receiver (mounted slightly off-axis from the astronomical beam) to measure a strong atmospheric water line. Under various assumptions about the atmospheric pressure and temperature, and the location of the turbulence, the electrical path above each antenna can be derived. Richard Hills and John Richer contributed a report outlining the status of the 183 GHz systems currently in place (Appendix E), and a series of suggestions for the requirements of a second generation system. Christine Wilson presented a report by David Naylor (Appendix F) on an alternative strategy that uses a 20$\mu$m photometer to measure water vapor fluctuations in the infrared. These reports were discussed in detail. The specific recommendations of the ASAC are:

1.

The water vapor radiometers are central to the scientific success of ALMA, and the project should ensure that their development is adequately resourced and integrated with all aspects of the ALMA system.

2.

The project should design and test preferably two (identical) prototype/pre-production 183 GHz radiometers as part of the Phase 1 project. These should be tested on reasonable astronomical sites when completed. The possibility of putting them on the 12-m prototype antennas at the VLA site during the test interferometer work is highly attractive, and the feasibility of this option should be investigated.

3.

The project should adopt a specification for the WVR system as follows: it should correct the atmospheric path above each antenna to an accuracy of 10(1+w_v)$\,\mu$m on a timescale of 1 second, over a period of 5 minutes and allowing for a change in zenith angle of 1 degree; w_v is the precipitable water vapor in mm.

4.

Although it is not possible to put very firm design constraints on the optics, the project should adopt as the specification that the maximum permissible offset between radiometer and astronomical beams be 10, and (if possible) smaller for the higher frequency channels.

5.

The project should check that the above specifications are sensible and adequate. In particular, the short timescale behavior of the atmosphere should be quantified to ensure that correction of phase on 1 second timescales is rapid enough.

6.

There are scientific and productivity gains to be made by correcting the wavefront tilt across each antenna (the so-called ``anomalous'' refraction). This effect most strongly compromises mosaic observations, and those at high frequencies. However, given that there are large periods of time when this effect will not be a major problem, the ASAC does not recommend adopting such a system as the baseline design at present. Further study of the loss of observing time this effect produces should be made, and this recommendation should be reassessed at future meetings.

7.

The baseline design for the water vapor radiometer remains a 183GHz system. The alternative Canadian solution using 20$\,\mu$m radiometers should be examined further, probably by the Canadians themselves, and further reports on progress should be brought to the ASAC. In particular, the correlation of the 20$\,\mu$m and 183GHz systems should be examined on the JCMT. The main theoretical problems of the 20$\,\mu$m technique that need to be investigated are its ability to sample the correct patch of atmosphere; its performance in differing cloud conditions; and the accuracy of the path estimation as a function of pressure, temperature, and water vapor distribution.

8.

The baseline design should use a cooled 183 GHz radiometer. Whether to cool or not is, strictly speaking, an engineering problem; there was some feeling that although not absolutely required to achieve the required sensitivity, the benefits of cooling in terms of stability and noise probably outweigh the costs.

9.

The project should examine the role of the system water vapor radiometers in the following: a) the amplitude calibration system, through their estimates of the atmospheric opacity above each antenna; and b) in single-dish mode observing, where they could be used to estimate the atmospheric emission. The scientific benefits of these techniques, and the extra requirements they place on the system, should be investigated.

10.

The project should accelerate its work on understanding the different atmospheric models used by the WVR systems to predict path errors from water line measurements.

11.

The location of the WVR is an engineering problem, and the solution likely depends on the degree of cooling required, and the final optical design adopted. There appear to be no show-stopping problems with locating it either in the same Dewar as the astronomical receivers, or in its own cryostat. The optimum engineering solution should be investigated. The ASAC does note that the simultaneous operation at 183 GHz and Band 1 receivers is not a scientific requirement, so it is straightforward to locate these systems in the same Dewar if that makes sense.