Calibration Group Phone Telecon Meeting Minutes - 2003Feb07
Attending: Bock, Butler, Carter, Cernicharo, Guilloteau, Hasegawa, Hills,
Holdaway, Lucas, Mangum, Mundy, Pardo, Pety, Pintado, Richer, Saito,
Viallefond, Welch, Wilson, Woody, Wright
(apologies to anybody I missed - there were a few connects/disconnects
during the telecon).
Agenda:
- amplitude cal devices. see the following documents:
Update on dual load device testing
Update on semi-transparent vane testing
Jack Welch's new amplitude calibration proposal
- milestones (especially II.A)
- memo reviews (especially 423)
- WVR status
- next phone meeting
- possible face-to-face meeting (ALMA week - June 2-6?)
Minutes:
- Douglas Bock explained the dual-load tests they had done (and written
up) at BIMA. New tests in the last few days show large variations in
the standing wave pattern as a function of temperature (10% or more in
the amplitude/position) and focus position. The variation is smooth
with focus position change, so it appears that the standing wave is
between the receiver cabin and the subreflector. Richard Hills had
a question about the variation with focus - is it amplitude or
frequency that changes? Also, how is the switching done? Douglas
replied that he wasn't sure on the changes - data was so fresh.
Also, on the switching, there is a 320 msec mirror positioning cycle,
100 msec of which is spent integrating on each of 2 mirror positions,
the remaining 120 msec moving and settling. They can go twice as
fast, but doing so did not improve things (some electronics problem
was suspected). Douglas will write up this new experience (combined
with past measurements) into a memo, which should be done soon.
- Jesus-Martin Pintado presented a summary of the semi-transparent vane
test program. The first run of tests will be Feb 18-20, and will
concentrate on measuring losses and Tvane. There is no time yet
granted to do sky tests. It will be difficult to get time on the
telescope before the end of April, because of bad weather, which has
delayed many bolometer experiments. Mel Wright asked which
frequencies were being tested. Jesus replied that 3mm would be the
main frequency, but dual-frequency operation at 1 and 3mm is possible.
They will test variations as a function of angle and temperature.
Butler and Hills noted that they should try to do tests as the
physical temperature of the vane is varied. Jesus replied that they
would try to do these tests, by simply allowing the temperature in
the Rx cabin to vary (but this is difficult because the temperature
is regulated by passing air through it). Matt Carter pointed out that
some of these tests can be done also in Grenoble in the lab (and that
they would be done). Richard pointed out that scattering from the
material can be a problem, and must be tested in a different way
(a separate document was circulated to this effect just before the
telecon). Matt replied that they might try to measure this on the
antenna range in Grenoble. Dave Woody asked if the load could be made
into a true reflecting dielectric? Matt pointed out that there were
polarization problems with that. Richard thought that this question
needed more thought though.
- Jack Welch presented a new idea on absolute amplitude calibration
(document circulated previously to group). This requires a separate
ACA antenna to be outfitted with horns which have accurately
calibrated gain. This solves the true absolute calibration problem,
by determining the flux density of sources on an absolute scale.
Jack was motivated by the Sloan survey, which uses a smaller separate
telescope to look at calibration sources. Proceeds in three steps:
1 - determine gain of ACA antenna very accurately; 2 - use that to
measure flux density on a few sources (planets?), via SD measurements
and hot and cold loads; 3 - transfer this to nearby (to the sources
being observed over the next few hours or so) quasars. For step 2,
need to chop out the sky - at BIMA, they just did on-off's. Jack
says that a better way is to either use a chopping secondary, or to
chop the standard gain horn against the main dish. Problem with
chopping the standard gain horn is the beam overlap. Jack tried this
in early BIMA days, with relatively good results (at long [8.5mm]
wavelengths). Need to measure the gain of the ACA antenna accurately
and often, and know (as well as possible) how it varies as a function
of elevation, wind, temperature, and other parameters. It requires
a dedicated, fully outfitted, ACA antenna. Stephane Guilloteau
pointed out that this is $6-7 M, which is expensive, but is, after
all, only 1% of total project cost. A problem is determining the
extinction correction. BIMA tests were not affected much by this,
as it was done at 28 GHz. It is likely that sky tips will not be
good enough. Jack advocates tracking fringe phase on a strong source
as a function of elevation. Butler & Stephane pointed out problems
with this (time variation of opacity, decorrelation, etc...). To make
this work, we need waveguide switches (to switch between the standard
gain horn and the ACA, fairly rapidly). Some discussion on ferrite
vs. quasi-optical switches. Jack likes ferrite switches because even
though they can be lossy, they are quite fast. Matt Carter pointed
out that they do not work well in the sub-mm (Richard Hills agrees).
Stephane pointed out that it would be nice to know how much manpower
would be required to design and build such a system. Jack will try
to provide the answer to this question. Jack will also produce a
much more detailed write-up on the past BIMA observations like this
(it will be a proper journal paper).
- Butler summarized situation with milestones. Several that are of
concern:
. All ATM-related milestones. Pepe Cernicharo says that Juan Pardo
will start work on the library around March 1, and that the library
should be ready by the middle of March. This is a bit delayed, but
is an improvement over only having the executable available (as
the situation was left after the Leiden meeting).
. 'extra devices.' Richard Hills agreed that he and John Richer
would take the lead on this. He wants folks to send him ideas on
what they think is needed. He mentioned that a 2-D scanning
183 GHz radiometer (essentially a scanning WVR) might be nice,
for example. Simon Radford pointed out a cloud-cam poster at the
AAS, and will send Richard a pointer to the poster/investigators.
Simon also points out that the site IPT needs to have an indication
of where we want things to go, and some infrastructure requirements
(like, we need power, communications, a shack, etc..., at the place
where these auxiliary instruments will go).
. hot load necessity. Stephane Guilloteau and Matt Carter had a
discussion on whether there is even room for this beast in the
widget space. There is, but it is tight. Not much else has been
done on this though.
. WVR document. Richard Hills had forgotten about this, and has
vowed delivery, modulo European funding gymnastics. Deadline
should be moved to March '03.
- Butler proposed to have a telecon later this month, since our major
milestone (Level 2) is due at the end of the month. Perhaps the
morning of Feb. 28 (email queries will be sent out).
- Butler proposed that a face-to-face meeting might be appropriate
at ALMA week, which occurs June 2-6, in Victoria, B.C. Tetsuo
Hasegawa noted that it might be very nice to discuss ACA issues then.
We need more information on what the structure of ALMA week will be,
but it seemed that there was agreement that this would be a good
thing (having the face-to-face meeting during ALMA week).
dutifully scribed by bjb. 2003Feb10.