MMA Imaging and Calibration Group

Minutes for meeting Mon, 19 April 1999 at 11am EDT.

Date: 19 April 1999

Time: 11:00 am EDT (9:00 am Socorro, 8:00 am Tucson)

Phone: (804)296-7082 (CV SoundStation Premier Conference phone).

Past minutes, etc on MMA Imaging and Calibration Division Page

Agenda

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MAC Meeting - Wootten

We discussed items arising at the MAC meeting on 14 April 99. The MAC would like to see the action list on its agenda each time.

Ed submitted minutes: Minutes of the Apr. 14, 1999 MAC Telecon

1) Acting upon a suggestion by Larry Rudnick, it was agreed to begin keeping a standing list of action items which will remain on the MAC agenda until they have been dealt with in the form of a report or other mechanism. These action items should begin with the list of issues raised in the annual report from last November's meeting.

2) Al reported more details for the Oct. Science Meeting to be held in D.C. The meeting will be initiated with a demonstration and reception for members of Congress in the Rayburn building, room 2325 beginning at 5:30 PM on 6 Oct. (Wed). It is hoped that each of us who plan to attend will contact our Senators and Representatives and invite them to attend the reception. This will likely be more successful if the invitation is accompanied by a personal offer to explain the project, demonstrations, and equipment on display. A computer link to the 12m on Kitt Peak will be there to demonstrate remote observing. Other observatories are also welcome to set up links to their telescopes if they wish. There will be a display of the Chajnantor site, high technology parts of receivers, and documentation of radio astronomical contributions to science and technology. There may also be some room for a few posters.

The science meeting, entitled "Science with a Large Millimeter Telescope Array" will occur on 7-8 Oct. This will be an international meeting with contributors from Europe, Japan, and the US. It will be limited to 200 participants. Information on the meeting can be found at:

http://www.mma.nrao.edu/science/science99.htmp

The meeting will be held at the Carnegie Institution of Washington, 1600 P St. NW. Accommodations are being arranged at the Omni Shorham Hotel. A limited number of rooms will be reserved, so early reservations are advised (but not until the negotiations with the hotel are finalized).

Following the science meeting, a joint MAC/SAC meeting will take place on 9th Oct. to discuss issues relevant to both advisory committees. This will begin with a press conference at 8AM to explain the international nature of this project, its magnitude, and its scientific goals to the media and the public.

Plans for the Oct. meeting seem to be progressing very well. Al and the LOC have clearly been putting a lot of effort into this for which the MAC is very appreciative.

AS THE TIME FOR THIS MEETING APPROACHES, MAC MEMBERS SHOULD BE THINKING OF ISSUES THAT WE WILL WANT TO HAVE DISCUSSED AT THIS MEETING.

3) We can now finally rest easy at night. It is extremely unsettling to have a project without a name. The combined project now has an official name and acronymn. The NSF and European reps agreed on "ATACAMA LARGE MILLIMETER ARRAY" or ALMA . In Spanish, ALMA means "soul". The official naming, was achieved via a contest initiated by Bob Brown. Lots of interesting names were submitted.

4) On Mar. 30th, a meeting of the NSF and European Negotiating Teams was held in Munich. A Memorandum of Understanding (MoU) for a joint design and development program for ALMA was drawn up and initialed. This MoU, which is expected to be officially signed within a few weeks, will govern the D&D phase of the project for the next 2 years.

Management Structure The MoU defines a management structure headed by a 12 member Project Coordination Committee composed of six members choosen by the NSF (in consultation with NRAO management) and six by the European Coordinating Committee. The project will be managed by a four member Executive Committee composed of the MMA Project Director (Bob Brown) and Project Manager (Peter Napier) and the LSA Project Manager (Dick Kurtz) and Project Scientist (Stephan Guilloteau). The Executive Committee will meet every 2nd and 4th Thursday of each month. The chair will rotate once per quarter.

Advisory Structure A joint Science and Technical Committee and a joint Oversight Committee will be established to provide advice to both the Executive and Coordinating Committees.

Continuing Work In the US, work will be managed by the MMA Project Team as is the case now. In Europe, it will be managed by the LSA Project Team.

5) Action Items Brief progress reports were given by Al on the following items.

Amplitude Calibration: A rapidly rotating mirrow, which alternately sees a temperature controlled load and the antenna, was installed behind the sub-reflector of one of the BIMA antennae in December. It is integrated into existing observing software and hardware, and has proven reliable since installation. It is still too early to evaluate how accurately this technique can provide amplitude calibration in routine observations, but the prospects look promising.

Water Vapor Monitor: Andy Harris has completed construction of a 16 channel correlator module and is working on the A to D converters for reading the data out. In collaboration with David Woody, the correlator will be installed (hopefully in June) at OVRO to learn about line shapes, altitude info on the line emission, and hardware issues associated with integration into an operating system. Software is being developed by Lee Mundy and Johannes Staguhn at UMd.

Longest Baseline: The question was raised about scientific arguments for baselines longer than the initially considered 3 km. A 10 km baseline was certainly discussed at the "Atacama Array" meeting in Tokyo. The MAC recommends that options for baselines of 10 km and longer not be precluded by hardware limitations except for reasons of technical infeasibility or prohibitive cost. Ideally, this should be limited by atmospheric effects and site limitations.

Lowest Frequency: John Carlstrom has agreed to write a summary of the science that ALMA can contribute in the 30-45.5 GHz bands. The range of frequencies that will be covered is still under discussion, although Bill Brundage has suggested that it span the protected astronomy bands at 31.3-31.8 GHz and 42.5-43.5 GHz. The present aim is to cover the range from 30 to 45.5 GHz with sensitivity maximized at 33 GHz. This will also provide overlap with the upper frequency range of the VLA/VLBA.

The Project Book: The final version of the MMA Project Book was scheduled to be issued on 15 April.

6) Scheduled Meetings Holography preliminary design review (PDR) will take place on 19 Apr (today) in Tucson. The NSF MMA Oversight Committee will meet in Tucson on May 12-13. The IF System PDR will meet on 17 May in Socorro The Antenna Vendors prebid meeting will be on 18 May in Socorro THE NEXT MAC MEETING WILL BE 19 MAY AT THE USUAL TIME.

For those of you who might be interested, I have put an Excel Spread Sheet which calculates the line and continuum sensitivities for ALMA for any choice of the number and size of antennae, integration time, bandwidth, etc. This can be accessed via anonymous ftp at:

ftp.astro.wisc.edu

cd /d/ftp/outgoing/ebc/ALMA

It should be opened in Microsoft Excel. All the equations etc. should appear at the top of each column if you want to change anything. I think most parameters are self explanatory by the column headings. I only caution that this was done for my own amusement and no claims for user utility or buglessness are made. If you have trouble, send me an email or give me a call and I will try to guide you through it.

---------------------------------end of MAC minutes

Examine the WWW pages at www.mma.nrao.edu/science/science99.html for errors. Try out the registration, please. The MAC would like to see the action list on its agenda each time. --------

Total Power Mapping --'Holdaway Holdover' Memo - Butler

  • We discussed Holdaway's Protomemo (postscript) on total power mapping.

    The plan is that the memo will be the basis for establishing specifications on lots of things. The first goal is to finish the memo, hopefully with as many references to measurements as we can put our hands on. The material in secton 2.2, for instance, can be checked by comparing the Chajnantor interferometer results with the total power measurements from the tipper and the 183 GHz and 22 GHz radiometers (the latter not yet operational). In section 3, Mark's model of the nutating subreflector doesn't contemplate using different waveforms, which might optimize the noise somewhat. Some of the difference in Table 3 surely arises from having equally apportioned on and off times for beam switching, while having many ons per off for OTF. One unwritten goal of the memo is to recommend whether or not all 64 antennas must have chopping secondaries, or whether OTF mapping can replace the functionality of the chopping secondary. The present analysis suggests that this is so. Another goal is evaluation of the specifications for 1/f noise. For this, we require that correlated noise not dominate the other terms in Equation 11. This resulted in specifications for 1/f noise which are quoted in the project book, but without justification. So the real goal of the memo is to quantify three noise sources, approach them theoretically, and make recommendations for the design of the array based on the findings. Bryan will work on this.

    What practical experiences can we confront with the memo's theoretical treatment? John Richer has a copy of the memo, and may work on obtaining JCMT experience to confront the expectations in the memo. As an observer, I would be uncomfortable with the memo as it is being issued, with no grounds in practical experience. I asked Goeran Sandell for his reactions:

    -for (sub)mm one needs to chop at a frequency typically faster than 3 Hz (Bill Duncan should have direct powerspectra of the atmosphere). JCMT uses 4 - 7.8 Hz. The standard is 7.8125 Hz even though it is close to a telescope resonance. The memo does not really address the correlation time of sky noise, i.e. how fast do you really have to switch.

    - a larger telescope cancels out the atmosphere better for the same chop, i.e. assuming that most of the noise variations happen relatively close to the telescope in a cylinder described by the near field.

    - complicated chopping schemes are supposed to help, e.g. two or three position chopping, but in reality we have seldomly seen much improvement. These also affect the duty cycle and the only thing which matters in the end is S/N for a given total time.

    - my experience is that often really dry weather conditions, i.e. superb transmission often is accompanied by lots of sky noise. That is, even though you now should seemingly integrate down much faster, you are now killed by sky noise (even with the JCMT 7.8125 Hz). Having an array really helps (correlated sky noise) but even then it is sometimes a killer. (I could not find something that would have readily explained this. Maybe if I plug some numbers into (10) or (11)).

    - Again, quoting from my SCUBA experience. Short chop throws (and fast) cancel out the sky. I see hardly any difference between a 30 and a 60" chop throw (for 850 micron, beam 14.5"), but going to 80 or 100" increases the noise by close to a factor of 2. Lesson, Chop close, 3 - 4 times HPBW.

    - JCMT probably has one of the best controllable subreflectors that exists. I never wrote up a final report on my tests, but it cost me years of my life. I can chop in any direction, you can give it a figure eight, if that is what you want. We've done wonders with maximizing duty cycle and the stability of the chop. The guy who did the controller is still available and one of Richard Prestage's best friends.

    Antenna:

    - Antenna slewing rates. Could not fully understand this part or why an aperture synthesis telescope would have to do on the fly mapping with scan reversal. But for continuum one will really have to scan off source. No EKH-algoritms will do the trick. You need the true sky background.

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    M51 Far, far away -- Yun

    Min has made a simulation of M51 Long, long ago. I thought it was a bit sobering. We considered the fact that some CO cloudlets will have moderately high brightness temperatures, in addition to appreciable size, out to some fairly interesting z. Many of the effects we would like to study in galaxy evolution commence at a lookback time corresponding to z=0.5. We should see if the array resolution can be matched to the sizes of relevant cloudlets at this z, and higher ones, to see what progress we will make in studying these distant galaxies. If we can match the beamsize to the object, we should not be subject to the distance squared loss of flux.

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    22/183 GHz meeting plans - Ensemble

    We could have this meeting over the phone, or face to face, perhaps on the occasion of the MOC meeting or? We have decided that both most of us, and a sizable contingent of interested outsiders, will be present at the imaging 99 conference in Tucson. On 9 Jun, people will be leaving; on 8 Jun we have the banquet. I propose we try to meet for two hours or so on the afternoon of 7 June to have our meeting.

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    Correlator Dumptime - Rupen

    Michael was going to lead a discussion of how the upscoping of the array affects the discussion in his memo No. 194 . However, we ran out of time.

    Other items: Al should try to locate a search engine for this site, as it is becoming cluttered.

    Harvey is back from IRAM, where he reports that Guilloteau and Gueth are studying multibeam interferometric mapping.

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    Action Items 26Apr99

    UPCOMING REVIEW: On 7 June 1999 at the Imaging99 conference in Tucson we review results from the OVRO and BIMA phase correction systems.

    DECISION: 183 GHz or 22 GHz phase correction?

    DECISION: Is a nutating secondary necessary?

    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: Holography PDR 19 April MEETINGS: MOC meeting 12-13 May 1999, Tucson. MEETINGS: MAC meeting 19 May 1999 at noon. ------

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    Travel

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    T. Helfer:

    A. Wootten: 12-13 April 43m observing 28 May - 3 Jun AAS 5-9 Jun I99 9 - 13 Jun CSO

    J. Mangum:

    M. Yun: 20 Aoril - ? at CSO

    B. Butler:

    S. Radford: