Agenda for meeting Tuesday, 8 February 2000 at 4pm EST.
Date: 8 February 2000
Time: 4:00 pm EST (2:00 pm Socorro, 2:00 pm Tucson)
Phone: (804)296-7082 (CV SoundStation Premier Conference phone).
Past minutes, etc on MMA Imaging and Calibration Division Page
Minutes
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From the ALMA/US DH meeting.
From the NSF budget WWW site: In FY 2001, funding for projects within the MRE account will include: $17.44 million to initiate construction of EarthScope; $16.4 million to continue construction of detectors for the Large Hadron Collider; $6.0 million for additional research and development of the Millimeter Array; $12.0 million to initiate construction of the National Ecological Observatory Network (NEON); $28.2 million to continue construction of the Network for Earthquake Engineering Simulation (NEES); $13.5 million to continue the modernization of South Pole Station; and $45.0 million for Terascale Computing Systems.
Correlator PDR report is ready.
Project book updates on track--Test Interferometer by 7 Feb, ALMA by 31 Mar.
New ALMA costing due by 11 Feb.
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The issue of single vs. double sideband receivers and phase switching is very complex and the implications for the system design needs to be very carefully worked out and discussed.
The current system and correlator design assumes that the receivers will be single sideband or sideband separating. Sideband separating receivers were initially proposed when the MMA was predominantly a millimeter wave (as opposed to a submillimeter wave) instrument to be located at a US site. The baseline design for ALMA now extends to 950 GHz and possibly beyond and is slated for a site significantly better than any US site. Achieving sideband separation for all of these bands will be more difficult and the potential improvement in system performance at the lower frequency bands will be less.
In light of this, I believe it is important that the system at this time be able to fully utilize DSB receivers, i.e. not simply suppress the image sideband. Many observations, such as line searches, continuum measurements and complex regions like Orion, will be correlator limited. There is a factor of two improvement in observing time for receiver noise dominated DSB frontends if the sidebands are separated in the correlator for these observations.
This means that the correlator should be able to properly process the IF from double sideband receivers. Most if not all of the present millimeter interferometers utilize double sideband receivers with sideband separation accomplished via phase switching of the 1st LO and binning of the correlator output. This system works extremely well and essentially processes twice the on-the-sky bandwidth for the same amount of correlator hardware at the cost of adding four-state binning of the correlator output.
There are issues associated with sideband and spurious response suppression that may need to be looked at in more detail. The result of CW frequency offsets, 180 deg 1st LO modulation with bit inversion of the digitizer output along with the effects of .5 msec time slices for the correlator processing can be complex. There could be subtle artifacts that show up in the correlator output. Some of the questions that come to mind for the present baseline design or any other phase switching scheme are:
1) If the 180 deg Walsh sequence is to complete in .5 msec, what phase lock relock times are required for the 1st LO?
2) If the 180 deg Walsh period is .5 msec then the meaningful minimum integration time becomes ~50 msec). Is this acceptable?
3) In the highest resolution mode, the 90 deg sideband suppression offset frequencies and possibly the 180 deg phase switch periods will be within the frequency resolution of the correlator, i.e. lag length longer than the clock period of the Walsh steps. What artifacts does this produce?
4) Are the 180 deg Walsh states synchronous with the 90 deg frequency offsets, and if so what effects does this have on the output?
5) What is the effect of incomplete cross correlation at the ends of the .5 msec time slices for the 90 deg frequency offsets and the 180 deg Walsh sequence averaging?
6) Similarly, if there is any dead time between the .5 msec slice or 16 msec "frames", then the averaging over an integral number of sequence cycles is compromised.
7) Even the type of phase switch sequence to use is not obvious and is an active area of research. Eric Keto (SMA), Steve Scott (OVRO) and Lynn Urry (BIMA) are even now discussing how to do this for much smaller sparse arrays. All of the desirable properties you want from phase switching are hard to achieve in a limited sequence time.
My guess is that ALMA will have to support two phase switch modes:
1) fast complete integration times of ~16 msec with imperfect image and artifact suppression to be used for solar and on-the-fly mosaicing.
2) slower complete dump times >50 msec with full phase switch state binning at the correlator output for the best possible artifact suppression and with image recovery (i.e. DSB receivers). This mode will be used for most observations and will be required for difficult detections requiring long integration times.
The phase switch sequence for 64 antenna will require Walsh sequences that are ~100 cycles long. If binning can be accomplished at the .5 msec time scale with blanking for phase lock recovery, then a full sequence and hence the minimum integration time would be ~50 msec. The LTA could accomplish this for the widest band mode where each plane contains .5 msec of data, but it may be impractical for the highest resolution modes where lags are distributed between planes and possibly between quadrants. These modes may require longer cycle times.
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This is coming up. Larry noted some items in an email exchange we had.
In SWSSR mel wright 456 wrote:
Can we observe several narrow lines simultaneously ? E.g. different molecules in a cold dust core, or time dependent parent and daughter molecules in a comet ?
I naively responded: > Yes, there will be four dual polarization windows avalable to be placed > anywhere within the two sidebands, as I understand the system. > Hence one may wish to observe one narrow line at e.g. 267 GHz (HCO+ 3-2) > while observing another at 243 GHz. Or CO 1-0 at 115 GHz and N2H+ at > 93.2 GHz.
But I was not taking into account the actual LO plan, as Larry pointed out:
a. If we're lucky, *some* bands will have sideband separating mixers that cover the full +-(4..12 GHz) from the first LO. Some might be DSB, some might cover less than the specified bandwidth, and some might have poor performance away from the center of the IF band. Remember that so far no one has built a receiver with this bandwidth, and no one has built a practical sideband-separating receiver, for any mm-wavelength band.
b. Even with an "ideal" front end, your first example is impossible because it has the two lines 24 GHz apart, so each would be at the extreme upper end of the IF, and we would have to tune the 1st LO just right. Your second example has them 18 GHz apart, which is getting feasible (+-9 GHz). It is unreasonable to count on simultaneously observing lines further than 22 GHz apart (+-11 GHz), nor between 6 (in the same sideband) and 9 GHz (+-4.5) GHz apart.
c. The tuning flexibility is pretty good but not complete. You can always get two lines nicely centered if they fit in the IFs (as described above), but for four lines it gets trickier. If all of them are in the lower (upper) half of the IF band, there are two (not four) separately tunable 2nd LOs available. You can separate them with the digital filters, but if you are unlucky you may not find a combination of LO and filter frequencies that centers all 4 lines in their respective final bandwidths.
d. Of course the hardware can do the trivial task of Doppler tracking, since it can do the much more demanding task of fringe tracking! Whether you *want* to do so in real time is perhaps an issue, but it only affects high level software. When observing different lines in different baseband channels, you can track them separately if you want. (Though not if they are in the *same* 2 GHz BB channel, even if in different correlator channels.)
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Joint ALMA Science Advisory Committee Meeting 14 Feb 2000.
Minutes are being approved and discussed. Next meeting 1630 UT on 14 Feb 2000.
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The next meeting will occur on 23 Feb 1999. Perhaps we can have some summary of the discussion this week.
Min updated the "array design and development" part of the ALMA web site. The working version is now in the main area area. A face-to-face review is being planned for 20-21 March 2000 in Tucson. This is the same day as the JRDG meeting and reciever CDR in CV but perhaps the conflict is not severe for many except Al and Stephane.
Min is rewriting the configuration portion of the Project Book.
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Bryan and I? need to submit an abstract to IAU Symposium 202.
I am somewhat unhappy about all the blather about FIRST, etc in IAU Symposium 204: THE EXTRAGALACTIC INFRARED BACKGROUND AND ITS COSMOLOGICAL IMPLICATIONS but no mention of ALMA. What do others think?
Menten is giving a talk at IAU Symposium 205: "Galaxies and their constituents at the highest angular resolutions"
A number of joint discussions but without posters for the most part.
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During January, the Calibration and Imaging Division concentrated on configuration issues, including continuing work on the problem of where to place the compact configuration at the site, given topographic and other constraints, and the creation of a mask for antenna placement. An e-mail reflector was arranged for the configuration working group. A work plan was created, identifying issues and designating individuals to lead the efforts. Planning continued for the CoDR 20-21 March in Tucson. A Site Monitoring Review was scheduled for 22 March in Tucson. WWW sites with Interest Groups and email addresses were set up for configurations, water vapor radiometry and site testing. Representatives attended the Science and Software Requirements meeting and the AIPS++ sponsored "Pipelines in Radio Astronomy" meeting in Socorro. Wootten attended the Correlator PDR; the proposed correlator design can fulfull nearly all the science requirements which now exist or can be reasonably forseen, with good upgrade paths for future needs. The ASAC meeting was held on 10 Jan, at which the ASAC charter was developed and plans for the 10-11 March meeting developed. Work began on Project Book updates and on the ALMA construction costing exercise. ----------------------------------------------------------------------
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Action Items 8Feb2000
DECISION: Configurations--where are we? March meeting plans...
DECISION: Implementation of 183 GHz WVR?
DECISION: Is a nutating secondary necessary?
DECISION: What is the total power specification on the ALMA?
DECISION: What is the effect of 1/f noise in the HEMT amplifiers of SIS receivers upon our ability to combine total power and interferometric images into a faithful representation of the sky?
MEETINGS: SAC phone meeting 14 February 2000 for members only.
AGL meeting 16 Feb 2000, followed by general meeting 17-19 Feb 2000, Tokyo.
System/Test Interferometer PDR Garching 28-29 Feb 99.
MOC meeting 2-3 March 2000. D. C.
ASAC face-to-face meeting 10-11 March 2000. Leiden.
JRDG face-to-face meeting 20-21 March 2000. CV
Configuration face-to-face meeting 20-21 March 2000. Tucson.
SSR face-to-face meeting end of April, Paris.
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Travel
AW -> AAS 12-16 Jan 2000. AW -> Tokyo 16-21 Feb 2000. AW -> Leiden 8 - 13 March 2000. ------