[mmaimcal] WVR decision

[Al Wootten]

Folks

If you have any comments on the item on today's agenda for the WVR decision,
please let me know asap.

The DH meeting is at 1pm EDT.

A review of the state of water vapor radiometry was held in Tucson on 
7 June 1999. The following facts arose from this meeting: 

1) Continuum water vapor radiometry has been employed at IRAM for some years 
to correct data for the excess path length caused by differing
amounts of water vapor along the line of sight to astronomical objects. 
Simultaneous 1.3mm and 3mm observations are used for this, resulting in
improbed images about 70% of the time during which no liquid water vapor is 
present. A similar system is under development at Nobeyama and the
technique has been explored at BIMA. Although the system works well, it fails
when liquid water is present.

2) Water vapor radiometry in the 22 GHz line has been employed with good 
results using a system developed at OVRO, primarily by Dave Woody.
Path length variations are corrected down to about 200 microns in virtually 
all weather, except for rain or snow. Similar systems are being
constructed for use at the VLA, the AT, IRAM, and BIMA. They have proven 
stable, the radiometry shows good correlation with phase,
coherence correction is straighforward and previously unusable data has 
been recovered. Residual delay errors are at the 100-200 micron level
at OVRO. The system under development is expected to improve this to below 
50 microns.  This system works quite well and could be employed on ALMA.

3) Water vapor radiometry in the 183 GHz line has been demonstrated to 
provide good correction at Mauna Kea by Wiedner, Hills, Yun and others
(see ALMA Memo No. 252 and Wiedner's thesis). Path length corrections 
better than 60 microns were demonstrated under conditions which apply
at Chajnantor 80% of the time. This is within a factor of a few of the 
ALMA goal of achieving 0.15 radian phase calibration at 230 GHz. Substantial
improvement should be achievable on the performance of the receiver which
 would allow ALMA realization of the goal.  A system at 183 GHz has great
promise for the ALMA site, where this line should show a line core optical
depth around one much of the time.

We conclude that water vapor radiometry in the 183 GHz water line will 
provide a most promising technique for correcting interferometric data for
phase noise due to water vapor path fluctuations in the troposphere. We 
recommend that construction of a water vapor radiometry system
operating at 183 GHz for this purpose become a part of the ALMA project. 
The systems currently stationed at Chajnantor should be further
improved to develop into the final more sensitive system. 

Clear skies,
Al