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!)
Agenda:
- 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?
Minutes:
- 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
received.
. 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:
http://ioj.iee.org.uk/journals/el/2002/15/20020553.html
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:
http://alma.aoc.nrao.edu/memos/html-memos/abstracts/abs434.html ).
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
( http://listmgr.cv.nrao.edu/mailman/private/almacal/2002-October/000067.html ).
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