Calibration Group Phone Telecon Meeting Minutes  -  2003-Sep-15

Attending: Bacmann, Butler, Hills, Lucas, Mangum, Mundy, Pety, Radford,
   Richer, Viallefond, Welch, Woody, Wootten
   (apologies to anybody I missed)



 - Project news.  John summarized results from the ASAC meeting.  The 
   main charge was on amplitude calibration specification, and gain 
   accuracy, which was probably erroneously combined into a single
   charge by the board.  

   The ASAC was asked to comment on receiver stability specifications 
   (currently the science IPT is asking for 1E-4, while the FE IPT is 
   using 1.5E-3 as the current [as of about one week before the ASAC 
   meeting] spec).  The FE IPT presentation on gain stability was given 
   by Charles Cunningham to the ASAC.  He introduced the new 
   specification - calling for stability of 1.5E-3 in 1 second.  The 
   recent Holdaway memo (not yet a formal memo, and presented by Al 
   at the ASAC meeting) showed that this results in noise 15x worse 
   than the specification favored by the ASAC, and is 15x thermal - i.e., 
   it will limit the imaging in thermal continuum when total power is 
   needed and polarization even when just doing normal (no total power 
   needed) imaging.  Pragmatically, it appears that this is what will 
   occur for early receivers at least.  It was thought by the group 
   that this stability spec, if we can't do better than it, is 
   catastrophic.  Richard pointed out that it may not be so bad for 
   polarization in interferometry mode if it integrates down (you just 
   have to spend longer calibrating).  Lee pointed out that if the 
   effective noise temperature is set by the receiver stability then 
   polarization is still a problem.  Richard thought that if one could 
   achieve .1% after some integration then the interferometric 
   polarization shouldn't be too badly affected.  Lee pointed out that 
   the interesting polarization science is at the few % level 
   polarization, so if you want to get 10-sigma or so on this, then .1% 
   is barely good enough.  There was still some uncertainty in the 
   group on whether this was bad for the correlated data (Richard 
   pointed out that one of the linear Stokes is a direct correlation, 
   while the other is a difference).

   The ASAC was asked to state how science degrades with a loosening of 
   the amplitude calibration spec from the current 1/3% mm/submm.  
   The presentation on amplitude calibration was given by Bryan at the 
   ASAC meeting.  It was clear to the ASAC that a reformulation of the 
   current spec was needed, breaking it into two parts - the absolute 
   flux density scale, and the fluctuating part.  Tetsuo Hasegawa gave 
   a presentation at the ASAC (not currently available) showing the 
   results of some simulations he and collaborators had done showing 
   how imaging fidelity degrades with fluctuating (non-calibrated) 
   amplitude errors.  This is supposed to be written up into a memo in 
   the next few weeks.  Bryan relayed that it showed that the fidelity 
   really went to crap if the amplitude fluctuations were > 5% or so, 
   in fact becoming so bad that adding the ACA was meaningless.  The 
   ASAC was confused about the current definition of the hardware for 
   amplitude calibration, whether there was a well-defined path to go 
   forward with it, and whether it was possible to "upgrade" it in the 
   future.  Bryan and Al pointed out that Stephane and Aurore's memo
   should help to make the science IPT case for what we really want or 
   need in terms of the amplitude calibration widget.  The ASAC is 
   concerned that if the absolute flux density scale specification is 
   relaxed then it will not be possible to do accurate line ratio or 
   spectral index studies.  We will be able to do this if we at least 
   know (very accurately) the spectral index of a few sources, but the 
   ASAC is worried that we don't even know that.  The ASAC has not yet 
   completed the study of what science will be compromised if the spec 
   is relaxed.

   The ASAC is now writing its formal report, which will be delivered 
   to the ALMA board before its November 2003 meeting (formally it is 
   supposed to be delivered by October 1).  John pointed out that while 
   the report is formally only accessible to the board, relevant parts 
   will be made available to the science IPT as necessary to, e.g., 
   make the case to the FE IPT to continue investigation of amplitude 
   calibration widget designs.

   Al gave a short report on what would be discussed at mini ALMA week.
   There may be some discussions relating to calibration, especially 
   with respect to milestones, and how ours in the science IPT interact 
   with other IPTs.

 - Bryan reported that the updated project book chapter 3 ("Calibration 
   of ALMA") is just ready to submit to the DAR process.

 - Bryan then started the discussion on the new calibration plan 
   document.  All those who were on the phone were happy with their 
   writing assignments.  Bryan pointed out that Al had put in a change 
   request to delay the L2 milestone by one month.  Al confirmed he had
   done this, but only at the level of telling Richard Simon - is this 
   enough?  Probably, since Richard seems to control such things, 
   somehow.  Bryan wants first drafts by October 1, in some modifiable 
   format (text, LaTeX, Word, etc... - not PostScript or PDF, e.g.).  
   We will discuss status at the next telecon.  There are still 
   uncertainties on sections - notably those on bandpass and single dish.

 - At this point, we segued into a discussion on a document from 
   Larry describing the overall system requirements.  This is supposed 
   to flow from a document being worked on by Al & Ewine
   detailing the scientific requirements of ALMA.  Al pointed out that 
   Larry's document has a significantly different spec on phase stability 
   than either the science requirements document or the updated project 
   book chapter 3.  Richard pointed out that he was surprised by this, 
   and disagreed strongly with Larry's approach regarding taking the 
   past results on WVRs to indicate future performance.  He thinks (as 
   most of us in the science IPT do as well) that the ALMA WVR will do 
   much better, since not only is it a more modern device, but is 
   integrated into the system from the beginning - very different from 
   previous WVR efforts.  Richard asked if this document was out for 
   public comment, and Al replied that as of this morning it was, as 
   Dick Sramek had sent it out for comment.  Richard asked whether Bryan
   would like to coordinate a response to this, to which Bryan responded
   that he would prefer that comments go directly to Larry.

 - Richard then relayed that he was quite worried that things keep 
   slipping on the amplitude calibration device.  He asked what it was 
   exactly that we needed to do, and how it was going to get done.  
   Bryan relayed that he thought it would be good to have Stephane and 
   Aurore's memo and then to go to the FE IPT with a formal request 
   that such a system be investigated.  Formal vs. informal discussions 
   and requests were then discussed briefly.  Al pointed out that 
   Charles is in CV right now, and he would go and talk to him about
   this.  If Stephane and Aurore do not submit their memo soon, we will 
   use the draft.  In any case, this is not in the baseline plan, and 
   as such, a change request will have to be put in, and perhaps even a 
   new work package issued from the FE IPT to get anything done on this.
   Al will investigate (with Charles).

 - We discussed briefly the date of the next telecon.  Bryan suggested
   October 16, but Al pointed out that this is during the AMAC meeting.
   Bryan then suggested October 9, which was accepted as a first cut on 
   the next telecon date.

dutifully scribed by bjb with input from haw on 2003-Sep-15.