Basic technical approach

The obvious options are line measurements at 183 GHz, 22 GHz, and in the mid-infrared (10 or 20 microns), or measurement of the (sub)mm continuum as for example used at IRAM. The latter is unlikely to provide accurate enough path estimates and could not easily accommodate a wide range of conditions. 22 GHz is now essentially ruled out by the size of the optics. The feed would be  250 mm diameter to measure the interferometric phase and at least 500 mm for correcting the pointing. Sensitivity would in any case be problematical - a cooled system would certainly be required. The use of infra-red radiometers is a new suggestion from Dr David Naylor (Lethbridge, Canada). The principle is essentially the same as with the millimetre radiometers but uses water vapour emission bands in the mid-IR. The system uses detectors cooled to 77 K. We could not use the telescope’s optics so to measure the pointing corrections we would probably need either several detectors per dish or some optical relay system to give an appropriate spreading of the beam. The initial report on sensitivity and stability looks encouraging, but questions such as how much the results are affected by the temperature and pressure in the fluctuating layer and the effects of cirrus clouds have yet to be investigated. This needs to be done before we can judge whether this might be a viable option for ALMA. Meanwhile the baseline should remain 183 GHz.