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)
Agenda
Minutes:
- 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.