Calibration Group Phone Telecon Meeting Minutes  -  2002Oct11

Attending: Becmann, Butler, D'Addario, Guilloteau, Hasegawa, Hills,
  Holdaway, Isaak, Koda, Lucas, Mangum, Mundy, Pety, Radford, Richer,
  Saito, Wilson, Woody, Wootten, Wright
  (apologies to anybody I missed!)


  - results from ASAC meeting
  - action items left over from last telecon:
    . gain compression - is it a problem?
    . photonic device
  - discussion of memo review process and reviews already received
  - new memos/proposals
    . memo 434 (mangum)
    . larry's new proposal
  - discussion of level 2 milestones, specs documents, timeline...
  - next phone meeting?
  - next face-to-face meeting?


 - Richer reported on the results of calibration discussions at the
   ASAC meeting in Socorro a month ago.  He summarized Guilloteau's
   presentation thereat.  The report has not yet been finalized, but
   when it is (and has been presented to the ACC), it will be 
   distributed to the calibration group.  The most important points
   were that the ASAC was keen to see a more detailed timeline (this
   should be answered to some degree by the Level 2 milestones), and 
   to get another update in 6 months or so (since the calibration group
   is so new).  Butler brought up the restructuring of the ASAC, and
   Richer & Wilson (with affirmation from Guilloteau & Wootten) answered
   that it was still uncertain what the new structure will be, or how
   that will affect communication with our group (i.e., whether there
   will be a specific 'calibration liaison' still).  We will just have
   to wait and see.

 - Old action items were then discussed:

   . Guilloteau pointed out that gain compression was dealt with in
     memo 423 - unfortunately no reviews of that memo have been
     received yet (Myers & Blake are on the spot here).  Further
     discussion was deferred until those (or other) reviews are

   . Butler pointed out existence of article relevant to photonic
     calibration device in Electronics Letters
     (Hirata et al., Elect. Lett., v.38, p.798-800 - online at:
     for those that have access) - which he had found out about from
     Wootten, who had found out about it from Payne.  Wootten pointed
     out that NRAO (through Payne) has a contract with NTO which has
     now gotten through the business office, and will involve testing
     of some photonic devices (testing at SRON - including work with
     Moseley's group and with the Herschel group).  We will need to
     keep up with this work.

 - We then turned to the memo review process.  Butler summarized the
   current situation - of 23 original requests for reviews, we had
   4 declines; 4 who have never responded; 6 who agreed to review but
   have not done so yet; 9 completed reviews; and 2 completed 
   unsolicited reviews.  Unfortunately, we have 2 memos (including
   memo 423 as pointed out above - 402 is the other one) which have had 
   no review to this point.  Butler thought the exercise was still
   useful, despite mixed response, since it has encouraged discussion 
   and will assist greatly in coming up with the 2 documents necessary
   to meet the upcoming Level 2 milestone deadlines.

   Guilloteau pointed out that he intends to formally retract memo 422,
   since it has errors, and he would like it not to be mistakenly taken
   as valid work (Butler notes posthumously that it's not clear how
   this can be done).

   Hills commented that the information in reviews received on memo 
   352 is being incorporated into a new document describing the WVR
   systems.  It is unclear what form this report will take and how it
   can be distributed, however, since it is really a proposal to the
   European side of ALMA to continue the work there.  Hills then 
   reported briefly on some work done at JPL on MMIC-izing a 183 GHz 
   WVR (this was work that had been reported to Hills & Butler by P. 
   Napier at the URSI GA).  There are some problems with the system, 
   notably 1/f noise, and it is early on in the development, but the 
   Cambridge group will keep abreast of further developments (if 
   JPL/NASA pumps money into it, it could get interesting).

 - We next discussed 2 new memos/proposals:

   . Mangum summarized memo 434 (see it at: ).
     At low frequencies, either a single-load or dual-load system seem
     to be capable of reaching the required 1% accuracy.  At frequencies
     > 350 GHz or so, however, they each have problems.  For the 
     single-load (semi-transparent vane) system, the difficulty is in
     measuring the mean atmospheric temperature.  For the dual-load
     system, the difficulty is in determining the atmospheric opacity.
     Also, for both, the sideband gain ratio must be known to ~1%.
     This prompted a lively discussion on opacity variation across the 
     ALMA site.  How much does it vary?  How densely will it have to
     be measured (in space & time) - and how accurate (expensive) will 
     the devices need to be?  Will the WVRs help in this respect?
     All of the amplitude calibration strategies assume accurate 
     knowledge of various atmospheric quantities, so this is a serious
     issue.  Holdaway volunteered to have a look at current site data 
     to try to determine spatial variations from temporal sampling.

   . D'Addario summarized his new proposed scheme for calibration
     ( ).
     The main point here is that the calibration sources should have 
     relatively well-known (and simple) structure, and can be monitored
     for absolute flux density, so use the power of the interferometer 
     when observing them to derive the interferometric gain.  This gain
     then must be transferred over to the single dish measurements.
     Also, the point that the cross correlation quantity is not simply
     a correlation coefficient, but can rather be turned into a real
     'cross-power' measurement, is important (Butler - this is just 
     what we normally call the Van-Vleck correction - or the 
     quantization correction, along with a known 'gain' - isn't it?).
     This prompted lively discussion.  Guilloteau pointed out that this
     scheme was not really so different than what was proposed in memo
     372.  He also pointed out that decorrelation is an important issue.
     D'Addario stated that he assumed that it was measured and 
     corrected for.  Woody brought up self-cal and power non-linearity 
     issues.  Guilloteau reminded that self-cal leaves the absolute 
     power (or flux density, if you will) level unknown.  This brought 
     us to the monitoring of the secondary sources.  How often?  How 
     does it interact with scheduling?  This is a tricky issue, and is 
     not only applicable to the D'Addario scheme.  Several folks brought
     up the problem with not measuring the sideband gain ratio.  This 
     is only a problem with the single dish observations, of course.
     D'Addario admitted that this required more thought (Butler - and, 
     in his defense, the ink is so fresh on his proposed scheme that 
     issues like this were bound to arise).  Another issue is the
     measurement of the bandpass, which D'Addario assumes does not vary
     except electronically - this ignores the atmospheric contribution.
     Butler asked if D'Addario would be providing a more formal
     description of the proposed scheme (e.g., in an ALMA memo) - 
     D'Addario was noncommittal, stating he needed to think more about
     some of these issues.  Wootten pointed out that we will be having
     a calibration sub-group meeting in Tucson on October 21 (at least
     Wootten, Guilloteau, and Butler will be there, and hopefully
     Mangum, D'Addario, and Radford can sit in on at least parts), and
     that it might be nice to have an update then (or we can at least
     discuss any progress or changes to it then).  Radford pointed out
     that we should really finalize a schedule for this meeting, so that
     he can plan accordingly.

 - Time was pressing, so detailed discussion of the Level 2 milestones
   was deferred.  Butler promised to send them out to the group after
   Wootten, Guilloteau, and van Dishoek had finalized the list along
   with descriptions.  Discussion can follow by email, and it will be
   an agenda item for the next telecon.

 - Brief discussion of date for next telecon produced November 15 as
   a possibility.  Butler will send out an email asking folks if this
   date is OK for the next one.

dutifully scribed by bjb.  2002Oct11