MMA Imaging and Calibration Group
Minutes for meeting Tuesday, 21 May 2002 at 4:00pm EDT.
Date: 21 May 2002
Time: 4:00 pm EDT
pm EDT (2:00 pm Socorro, 1:00 pm Tucson)
Phone: (434)296-7082 (CV SoundStation Premier Conference phone 3rd floor).
Past minutes, etc on MMA
Imaging and Calibration Division Page
Agenda
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News. -Wootten
News from my point of view. Report for February-March for ALMA/US Science IPT.
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Y+ Configuration - Butler, Holdaway, Otarola
a link to
images of some masks that Angel and I made from Bryan's latest mask.
Loose Mask
Tight Mask
or, to get the fits files, go to
http://www.tuc.nrao.edu/~mholdawa/
and hunt.
The idea behind using two different masks is that the loose one will have
better (u,v) coverage but more expensive roads; the tight one will have
poorer (u,v) coverage but cheaper roads. How much better, how much
cheaper? Well, we'll find out. I suspect the tight one will actually do
fine (it isn't that tight)... maybe we can define an even tighter one
based on how the Y+ config set comes out for this mask?
These masks were generated from:
- Bryan's shadowing criteria, 100m pipeline constraint, science
preserve
- Cleaning up Bryan's initial mask (taking away points on top of
Chascon, and the rough science preserve edge.
- Adding a ~ 5 km extension to the west
- Adding the blob on the underside of the western "wing" which
corresponds to a "finger" between aroyos which was not
otherwise available to us due to the lack of DEM info in
that region
- Taking away points with TOPO gradients greater than 15%
- Subjective carving away of splotchy points and places we don't
want to bother putting antennas, the process which gives
the Y+ array its shape.
* This process was facilitated by a totally wonderful AIPS++
software module I wrote yesterday which makes it simple
to carve out and give back various regions of the mask
based on mouse clicks on the display image.
Dick Kurz reports from Chile: "It was a
beautiful day at Chajnantor when the Finance Committee visited on last
Thursday except we had had the first snow of the winter (we ran into a
rain squall on the road to Paranal on Tuesday). This complicated the
logistics, but with Simon's help all those who really wanted to got to
the site. "
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Meetings Past - Wootten
Report from ACC telecon??
Sitescape is ready for your trial. Check it out!
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Other Items-Wootten
The AAS is meeting in Albuquerque in June. Some plans for the new ALMA
backdrop are at stuff.
Issues: Calibration Group Set Up
Gasp! A new calibration group email exploder, almacal@nrao.edu, has
been set up for Calibration Group discussions.
Let's discuss some items from ALMA Week:
Firm next steps, in my view:
1) produce a real ALMA version of the Project Book rather than the current
grandfathered MMA version. SG working on this in concert with others but
I must be out of this loop.
2) evaluate the dual load system on the Vertex antenna and the semi-transparent
vane on the A/C/E antenna (or both on both). In my view neither of these
systems is ready for production. Both have been tried in the past and
abandoned for reasons we think a little engineering could solve. But we
need some commitment from the FE group, already overstressed by the need for
a receiver(s)... Because of the interaction between front ends, antenna
and the calibration system I think we need at least one receiver at the ATF
ready to test when evaluation finishes on 1 Jan 2004. (I think it should be
a prototype band in an RAL dewar with Carter's system but...) Some major
questions remain--for instance do we need two loads for the semi-transparent
system? Can we measure coupling of the dual load system well
enough? Enumerating these is an action item in itself.
See Memo 372, also JGM is working on revision of his memo.
3)
The project maintains a direct photonic testing program with the
hope that it eventually may offer a more stable though probably not (?)
cheaper system. A major problem is the laser
RIN noise. The Tucson group will see if an optical filter can cut that
down this month. In the meantime, I think that we should continue with
some photonic calibration tests. Since we will have a system photonic to
100 GHz anyway, and since the evaluation receiver only has two polarizations
at 3mm, I think we should aim to see if a 3mm photonic signal injected into
the system is viable for sideband separation calibration in total power.
We should aim for that when evaluation finishes; we could compare these
tests with the interferometric separation of sidebands. A next step would
be to sweep the signal for a test on amplitude calibration of the passband.
A phase-locked signal could test phase calibration also but I'd put that
as lower priority to these first two tests, which only require the amplitude
stable signal for a few minutes. (I think the phase calibration would
need a phase-locked signal, right?) Lastly, it would be useful to see if the
system could be used to measure the relative phase between the two
polarizations. As noted at the meeting, various reflections could complicate
this so I'd put this test at lower priority. It seems to me that whatever the
fate of direct photonic to the highest frequencies, we should at least
evaluate the system on the ATF/Prototype Interferometer at 3mm to
discover in which details the devils hide. If the tests only uncover angels,
they can help transport us to higher frequencies...
3)Comments? Discussion?
3)3-1) How stable?
3)3-2) Over how long?
3) Subtexts:
CDL has 'decided' to go with single=ended unbalanced mixers.
This takes lots of power (LO); Is this the correct philosophy?
How was this decision reached? Based on R. Simon's numbers, some AEC
members feel photonic option may be too 'expensive'. Photonic group
feels these numbers are inaccurate but accurate figures must await a final
evaluation of the photonic system. A system which would allow later change
to photonic system has been proposed; these have not been approved but in
my view should be very seriously considered. So the question: what role
should the photonic system play in ALMA calibration. I tried to define
a first role, involving as few people and extra pieces of equipment as
possible. Basically, we use a 3mm signal (e.g. the holography signal)
to test measurement of SB rejection ratio, bandpass shape, and angle between
orthogonally polarized feeds. I understand that that transmitter is
*time, amplitude stable for long 'enough' (Larry wants this all defined,
of course, as he should)
*polarization--what is the role of reflections off of structures? The
polarization state at the emitter is well-defined, I understand.
SG Comments: Agree with most if not all concerns. However, I believe
- do not count on the system as far as phase correction is concerned.
Accordingly, phase calibration strategy should not count on that.
- Development should be oriented toward a tunable and/or broadband,
amplitude stable device. Timescale required for amplitude of order of a few
seconds. Phase noise to be stable over the same timescale.
- Action item: Ask Cal Grp/Bill Cotton how such a device could be used for
polarisation measurement:
- either absolute
- or just relative (stability of phase/amplitude difference between the 2 receivers over hours timescale)
3)ASAC comments (Sep 2000):
For the coherent signal scheme, it will be important to demonstrate that
the accuracy obtained will exceed that of more conventional methods.
This system is particularly useful in 3 areas:
3)1. bandpass calibration
3)2. sideband gain ratio determination
(ALMA spec for SSB tuning is 10 dB sideband rejection.)
3)3. polarization
3)The first 2 items require proper control of the power vs. frequency, which may
be difficult. Baseline ripples apart, bandpass calibration can be achieved
on astronomical sources within 1-2% accuracy in a reasonable time at
millimeter wavelengths. It will be more difficult to do astronomically at
sub-mm wavelengths where the potentially high signal/noise of the coherent
scheme gives it an advantage. Measuring the sideband gain ratio is easy at
mm wavelengths, but time consuming in the sub-mm region unless the
specification is relaxed to about 5% accuracy. Here again, if the coherent
scheme can be made accurate to the 1-2% level, its inherent high signal/noise
will be an advantage.
3)In a phone conversation 13 May, John notes that Ellison will measure
the noise soon. John will measure phase and amplitude noise at 3mm in the
coming weeks. He notes that the extra noise in the photonic scheme is the
same at all ALMA bands, so although it may show the photonic system to
poor effect when someone cites it adding 15 K to a 40 K rx temperature
at e.g. 100 GHz, this wouldn't look so bad at 950 GHz where we don't know
how to get LO power from either source at the moment. Also, Shillue is
heading to Japan with the RAL photomixer to do tests there.
4) How serious is compression? At the PDR there were a number of
items for which the front end group was supposed to respond.
Now we need to ask for an estimate of Tsat for each band. How much
compression can be tolerated? We may need some negotiation with the
receiver people on how many junctions are needed. Four are planned for
3mm; we used 6 on the 12m. Tony Kerr thinks that junction number is not
a major problem. Kerr wrote to me about this already:
" Multiple junction SIS
mixers are not more complicated than single junction mixers. It might
be thought that it would be too difficult to get N good junctions in a
single mixer, but our experience with mixers made at UVA, JPL, and
Hypres, and IBM is that this is not a significant factor. In fact, the
junctions in an N-junction mixer are N times the size (area) of those in
an equivalent single junction mixer, and therefore usually have better
characteristics. Probably the major obstacle to using multiple
junctions is that people would have to re-design their mixers! Other
factors are the possibility that it may be harder to suppress the
Josephson currents in all junctions simultaneously (by magnetic field)
and the somewhat higher LO power required, but I do not think these are
serious problems in most bands."
SG comments:
"IRAM people have a different experience with the technology they use.
Going from 1 junction to any other number implies an isolated area in the
design, which cause problem in the lift-off process used to fabricate the
circuit. LO power may also be an issue at 300 GHz if we don't use balanced
mixers (which we don't want because of the serious additional complexity)."
5) WVRs. Kate talked with Matt Carter regarding
positioning etc. of the WVR, and with Juan Pardo regarding modelling
effort. I think it was good to see closer contact between these groups
as it was clear the widget space wasn't being accounted well.
We do have a WVR section, terribly out of date, on the ALMA web with URL:
http://www.alma.nrao.edu/development/cal_imaging/phasecal.html
This needs updating so I asked Kate for contributions...or links.
SG comments:
" Good. Other on-going action items:
- refurbish Juan Pardo code (planned for May-June-July)
- make it available as a stand-alone code
- make it available in AIPS++
6) The atmosphere. I think we need to put something justifying an
FTS into the Project Book chapter, and giving a short discussion of
how ATM fits into the project. We need to consider how this and the
network of sensing devices (I don't think any observatory with a telescope
is so well-instrumented as Chajnantor is without one) provides feedback
to the dynamic scheduler (which needs to be put back into the project).
SG comments:
" YES I'll write down a section on that with Juan Pardo for
the project book. Jeff and Juan have discussed the implications of the
better ATM model on the single-load calibration device, and as far as I can
tell, agreed that a single load is now sufficient even at sub-mm wavelengths
provided the new ATM model is used."
7) Total power. Mark is working on finishing his memo in the ALMA
context. A plan for evaluating e.g. nutator use vs. OTF should result
from that. We can confront that plan with the ATF at 1 and 3mm but
a real test will be in the submillimeter and can probably only be
done at Chajnantor. In any event, I hope Mark can push the simulations
to the point of suggesting whether we have the right nutator specifications,
or need a Mark II design. e.g. you mentioned that the JCMT chops
equatorially for the best cancellation; should that be a spec on the
ALMA nutator? So I conveyed to Mark comments from the discussion--to
consider using sources in the image library (10' being more relevant that
30'), to consider different forms for the nutating function (e.g. a sine
wave), and to consider the directionality of the water vapor screen.
Other
Lucas' list of simulation requirements to be discussed in
Granada
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Star Wars -- Others?
Any other reviews of the movie?
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Upcoming Meetings - Wootten
May 2 ASAC agenda.
MAY 6-9 Astrophysics
of Life Space Telescope Science Institute Wootten attending
May 24 ESO Committee of Council
June 2 - 6 AAS Meeting Albuquerque
JUN 18-21 Scientific
Frontiers in Research on Extrasolar Planets at Carnegie Institution
See ALMA abstract. Program now online with two ALMA posters:
Session: Ground-based Planet Search Instruments: ALMA
Jean-Francois Lestrade, Observatoire de Paris/LERMA
"Astrometry at millimeter wavelengths with ALMA to search for extrasolar planets or to determine their orbits"
Henry Alwyn Wootten, National Radio Astronomy Observatory
"Extrasolar Planet Research with the Atacama Large Millimeter Array"
JUN 24 AMAC Meeting Munich
AUG 17/24 URSI General Assembly,
Maastricht, the Netherlands Butler attending
AUG 22/28 SPIE Symposium on Astronomical
Telescopes and Instrumentation, Waikoloa, Hawaii. Brown, Wootten attending
SEP 9-13 Winds, Bubbles
and Explosions
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Travel
Oh, always
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