Date: 3 Dec 2002
Time: 4:00 pm EST (2:00 pm Socorro, 2:00 pm Tucson)
Phone: (434)296-7082 (CV SoundStation Premier Conference phone 3rd floor).
Past minutes, etc on
MMA Imaging and Calibration Division Page
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iv. Stability
(1) 1e-4 rms over 0.1 sec interval
(2) 1e-4 rms over 1 sec interval
i) Singledish 1 10-4 1s
ii) Interferometer 3 10-3 1min
iii) Diff 5 10-4 1 min meaning between two mixers same cartridge.
In practice this spec for total power will be hard (impossible?) to achieve, but all studies have shown that it is important that it be met for ALMA to work as advertised, that is, to include the total power flux in its images.
I talked about this with John Webber at the CDL yesterday. What matters to the astronomy is that the receiver be stable over the timescale on which the atmosphere changes considerably, about one second, so that an image might be constructed in that time which is not limited by the system. The receiver will be a key component of this.
The receiver stability will, I imagine, consist of several contributions each with a characteristic amplitude and timescale.
One of these will be the '1/f' noise associated with the IF amplifier. For a 4-12 GHz amplifier, John Webber suggested to me that 1 part in 10^4 was probably the limit.
Another will be other variations associated with the IF section. One component of this will result from thermal variations which could be controlled in the fashion described in ALMA Memo No. 131.
One component will result from thermal variations at the mixer owing to cycling of the cryostat. I presume that this will be similar in timescale to the plots which Jack Welch provided. Again, in Memo No. 131 Frye et al. described one approach, and noted that the fluctuation in gain with cryostat variations was lower the lower the physical temperature.
Another will result from changes in the power of the LO signal which the mixer sees. I'd guess this is perhaps more rapid than the thermal variations but since we don't know much about the LO source, I'd guess this is the hardest to characterize.
We discussed the low frequency 1/f noise problem and how to combat it.
From the MAP experience, the CDL estimates that the IF contribution to
this, from 1/f noise in the 4-12 GHz amplifier, is 10-4 1 sec alone. As noted,
there will also be contributions from the effect of the cooling cycle in the
dewar, affecting the gain of the mixer and amp, and from the effect of the
variation of LO power to the mixer, also affecting its gain. One alternative
might be to temperature stabilize the mixer and amp, applying heat to thermally
stabilize it. This approach has been used for years, by centimeter wave
receiver engineers in the 60s, and recently by BIMA (ALMA Memo No. 131).
However, the cartridge space is limited, as is the cooling capacity of the dewar
and this alternative only addresses one of the sources of gain variation in
the front end. Frye et al., in Memo No. 131, noted that the variation of
gain of the mixer with cryo cycling was lessened by operation at lower
temperatures. One alternative, used for the MAP receivers,
might be to construct special purpose receivers for the total power antennas.
In these receivers, which could be correlation receivers such as will be
employed on the GBT, the signal is electrically switched at a rate of
1 kHz or so between two feeds. This would mean special receivers, not
compatible with the rest of the array, for the total power antennas.
One drawback is that this makes demands
on dewar real estate; this might be acceptable for dedicated total power
antennas if some bands were not deployed on those special purpose antennas.
The feed separation is of course fixed, presumably at the same elevation; in
this configuration the receiver samples a different set of spatial frequencies
than in a nutating mode, or in a non-switched total power mode. What is the
optimum separation for these two feeds? What are the effects of this
receiver design on the images ALMA will make? If all bands will be
deployed in this manner, there may need to be more than one set of total
power dedicated antennas--is this acceptable, or are there some bands which
are more critical than others? For spectral line on-the-fly mapping, we
may wish to use four antennas from the interferometer array--is this
acceptable?
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See
a summary
of our MMAImCal discussion of 5 November on this topic.
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Telecon
minutes
from last telecon. Next telecon 13 December 2002.
List of the status of the reviews/reviewers. The key is: "C" - already
submitted a review; "U" = submitted an unsolicited review; "Y" = agreed
to review the memo; "N" - declined to review the memo; "NR" - no response.
Links are provided to reviews; please consult the archive for subsequent
discussions.
1 - Memo 423 - Steve Myers (Y), Matt Carter (N), Geoff Blake (Y)
2 - Memo 422 - Douglas Bock (Y), Lee Mundy (NR),
Jeff Mangum (C)
,
Dave Woody (U)
(Memo withdrawn)
3 - Memo 427 -
Robert Lucas (C)
, Ed Fomalont (NR),
Craig Walker (C)
4 - Memo 372 -
Jeff Mangum (C)
,
Larry D'Addario (C)
, Mark Gurwell (N),
Bryan Butler (U)
5 - Memo 415 - Jose Cernicharo (NR),
Mark Holdaway (C)
, John Richer (Y)
6 - Memo 404 & 403 - Mel Wright (N),
Dave Woody (Y&C)
, John Richer (Y)
8 - Memo 402 - Richard Hills (Y), Peter Napier (NR)
9 - Memo 352 -
Dave Woody (C)
,
Chris Wilson (C)
, Dick Plambeck (N)
10 - Memo 434 -
Stephane Guilloteau (C)
so, as yet no response from:
Lee Mundy - 422
Ed Fomalont - 427
Jose Cernicharo - 415
Peter Napier - 402
and awaiting agreed upon reviews from:
Steve Myers - 423
Geoff Blake - 423
Douglas Bock - 422
John Richer - 415, 404, 403
Richard Hills - 402
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JAN 5/9, 2003 201st Meeting of the American Astronomical Society
JUL 13-26, 2003 Twenty-Fifth General Assembly of the IAU
JUL 22/25, 2003 IAU Symposium (221): Star Formation at High Angular Resolution, Sydney, Australia. Tyler Bourke is a co-chair of the scientific organizing committee. Good opportunity to show ALMA capabilities.
SEP 22-26, 2003 4th Cologne-Bonn-Zermatt-Symposium on The Dense Interstellar Medium in Galaxies
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Travel
Oh, always
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