Calibration Group Phone Telecon Meeting Minutes  -  2003Mar12

Attending: Bacmann, Butler, Carter, Guilloteau, Hills, Holdaway, Kohno, 
   Mangum, Pintado, Radford, Saito, Wootten
   (apologies to anybody I missed - there were a few connects/disconnects
   during the telecon).


 - amplitude cal devices, and near-term decisions to be made.
 - milestones
 - other?


 - Stephane went over the note that he sent just before the meeting
   ( click here for it ), and 
   discussion followed.  Butler pointed out that the thing to do,
   probably, is to pursue (from the "Possible decision paths" part of
   Stephane's writeup) point (1) at the ASAC meeting in Grenoble on April
   2-3, ask the FE IPT for (2a) and/or (2b), and follow (3a) - i.e., 
   abandon the dual-load in the subreflector.  Wootten agreed, pointing
   out that "delta development" on the dual-load was small, and didn't
   warrant continued support of that system.  Mangum agreed, but pointed 
   out that the cost savings from (3a) would not necessarily allow for 
   (2a) and/or (2b), since if we are left with only the S/T vane, then 
   the contingency goes up for that system, because of the risk involved.
   Hills pointed out that we should still pursue a partially reflecting 
   (polythene or teflon?) vane, at 45 deg angle to the feed, directing 
   the beam off into a load.  Polarization may be a problem with this
   system though.  Carter pointed out that with the current system, the
   LO is reflected back through the feed, and the 45 deg. reflecting 
   vane would not suffer from this problem.  Hills reminded everybody 
   of the vane temperature and scattering problems with the S/T vane.  
   Carter noted that the scattering had been measured, and was "not a 
   problem", but would not hold to a specifically small number (1%) when 
   pressed by Hills.  Butler pointed out that with the reflecting vane 
   you need to fill the beam, and this might be difficult given space 
   constraints in the Rx cabin.  Carter agreed.

 - Jesus-Martin Pintado presented a summary of the semi-transparent vane 
   tests to date.   Click here  for a 
   short writeup of the tests.  They have had a few hours of test time 
   on the 30-m telescope.  Because of time constraints, the test that 
   they did was conducted with the S/T vane pretty close to the Rx.
   Absorption by the vane was measured by looking at cold and ambient 
   loads.  Several measurements were made for each frequency, and 
   averaged.  The scatter between the measurements on different days is 
   up to 10%.  Tilting of the vane up to 5 deg was possible.  They did 
   not measure Tvane or true vane losses.  Kohno asked if the measurements
   were stable when looking simply at the ambient or cold load - Pintado
   answered that they were.  The future test plan is to try to move the 
   vane further from the feed, to make real astronomical observations 
   and compare with the current calibration devices on the telescope, 
   and to test some new materials sent to them by Carter.  These tests 
   will not occur until late April/early May, however, and so will not 
   happen fast enough to help in the decisions discussed above.

 - Discussion turned to milestones.  Butler has to get moving on trying
   to meet II.A.  He will attempt to get this done while in Tucson next
   week.  Other milestones are related to the document due end of June.
   Hills was asked about II.B.2 (the "auxiliary" calibration devices, or
   what is sometimes called the "weather station").  He has worked on it,
   but not much, given other time constraints.  He will work on it next
   week.  He solicited input from other members of the cal group on what
   kinds of devices might be desired or needed.  Butler pointed out that
   the intent was to have the ATM software available to attempt to see
   really what we needed, but that will not happen.  Pintado noted that
   Pardo is now working full-time on trying to get a working ATM library
   available for delivery, and this should occur in early April.  Thus
   the document produced for II.B.2 will not contain this analysis, which
   will have to come at a later date.
 - Final discussion concerned the issue of polarization calibration.  It
   is now clear (and imminent) that Myers will be leaving the ALMA 
   project to go full-time on AIPS++.  This, as has been known for some
   time, will leave a void in the polarization calibration area.  We 
   need to fill that void, but it is unclear how.  People in the cal
   group were encouraged to think about possible people to work on this
   problem (either already in the science IPT or not).

dutifully scribed by bjb, with input from sg.  2003Mar12.