[mmaimcal] Re; Tau device

[Al Wootten]

I thought this morning's exchange might be of some interest to the group.


Wolfgang Wild writes:
 > Dear Stephane and Al,
 > 
 > during one of the JRDG's telecons, the possibility of an independent
 > devide for measuring the atmospheric tau was discussed. Presumably, this
 > would supply tau for all antennas as input into the calibration
 > procedure. What do you think of such a device ? Should it be considered
 > for the calibration ?
Yes, there are more there now than at any site on the planet, to my knowledge.
There are at present a number of such devices at Chajnantor, operating over
frequencies including 22 GHz, 183 GHz, 225 GHz, 350 microns, 260 microns and
200 microns, and for some of these from different sites, over relatively
rapid timescales.  Both the Japanese and the Smithsonian have operated
FTS devices, for various lengths of time; this gives the best measure of the
atmospheric transmission but I'm not sure of the integration times.
I anticipate that the fixed frequency tipping devices will be maintained
at least through late stages of construction, with some of them continuing
after that.  Their output would, for example, be of use as input
to the dynamic scheduler, in addition to calibration.

In fact, the JCMT for some years used the NRAO/CSO tipping
receiver at 225 GHz for providing calibration to the UKT14 photometer, perhaps
analogous to the system you envision.  I don't think that this procedure 
was ever entirely satisfactory.  In my experience, I have used receivers for
which the gain was a function of elevation, owing presumably to the relationship
between coolant and device in the cryostat; in such a case one might gain from
an independent tau meter.  However, the ALMA receivers shouldn't suffer from
this problem.  Also, if the time structure of the atmosphere
has a high frequency content, tipping wouldn't provide information on the 
shortest timescales (one reason for the use of the CSO radiometer by UKT14).
However, the 183 GHz WVR shoult be running at all times, providing a time
sequence of water vapor content of the atmosphere, though not at the frequency
of most interest.  Lastly, of course, the site is extensive and we know that
it can be snowing at Chajnantor while not at Pampa la Bola, suggesting that
a single monitoring station might not supply data relevant to the whole
array.  

The short answer is yes, I would imagine operation of at least some portion
of the current atmospheric monitoring network alongside of ALMA for various
reasons including calibration.

Al

Hi Wolfgang,

	Yes, we have been thinking of using a independent sounding
device to get the atmospheric opacity. Something like a tipping
radiometer with an FTS would be appropriate. The calibration scheme
I am currently working on with Rafael Moreno assumes such a device
exists. However, there are quite a number of additional problems
which need evaluation (homogeneity over the array, etc...).

	At present, we can't tell whether such a device will remove
the need for "sophisticated" calibration system in the receiver
or not. However, the total cost of the equipment is so low as compared
to ALMA that we believe it (radiometer + FTS) should be included
in the project.

		Stephane

Simon points out that the 22 GHz radiometer has in fact not yet run at
Chajnantor.  My mistake.