Minutes for meeting Mon, 11 January 1999 at 4pm EST.
Date: 11 January 1999
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
--------
Bryan Butler brought us up to date on the antenna RFP with an email:
the antenna pointing spec is currently:
0.7 arcseconds offset pointing accuracy, with pointing calibration every 15 minutes. blind pointing accuracy is to be 2 arcsec (this is a new spec which peter told me yesterday).
the 4 antenna motion specs are:
1 - Fast switching
1.5 deg (to within 3 arcsec) in 2 seconds
2 - OTFM (On-The-Fly-Mapping)
0.5 deg/sec - positions read off accurate to 0.7 arcsec, but antennas do not need to be pointed at the same position
3 - OTFIM (On-The-Fly-Interferometric-Mosaicing)
0.05 deg/sec - antennas point to same position to within 0.7 arcsec
4 - slew to new source
1.5 deg/sec in El, 3 deg/sec in Az
issues we discussed:
- regarding the pointing spec and offset pointing, min has been looking at this in detail lately. he seemed to think that given 50 12-meter antennas (a reasonable number, given our latest estimates), you could easily do a very accurate pointing calibration in less than 1 minute. he was considering using the pointing determination scheme currently used at OVRO, which is several repeated triangles at the HP point. several others of us pointed out that the continuous circles or other schemes may actually be better, and min might look at these other schemes. min's analysis is similar to, but differs from, robert lucas' analysis in MMA memo 189.
one thing that might be of use here is to have min actually calculate the estimated accuracy in the pointing determination if we are willing to allocate 1 minute out of every 15 minutes to pointing calibration.
- regarding the fast switching, we briefly discussed how many FS cycles might be undertaken by the array, which concerns the projected lifetime of the array. harvey pointed out that FS cycles don't actually stress the structure very much, since the travel distance is so small.
- again, on fast switching, we briefly discussed whether we need to have a spec on the time required to actually reach the final pointing spec (i.e., rather than 2 seconds to get within 3 arcsec, have the spec be X seconds to get within 0.7 arcsec).
[peter pointed out to me yesterday that once you get "close" (like to within 3 arcsec), it is only a couple of antenna oscillations beyond that to reach the required pointing, so that if the resonant frequency is 10 Hz, then it is only a couple of tenths of a second to get the final bit of pointing accuracy. see figures 8-10 of torben andersen's MMA memo 231.]
- regarding OTFM, a brief discussion resulted in the general agreement that the spec'ed 0.5 deg/sec rate was sufficient.
- regarding OTFIM, a longer discussion seemed to reach no conclusions. there is still some confusion among us as to how OTFIM measurements will actually be undertaken. this is all tied up in how the total power measurement can be done. if the total power measurement is done separately, in OTFM mode, then the 0.05 deg/sec is probably sufficient. if the total power measurement is done at the same time as the interferometric measurement, then the 0.05 deg/sec is not fast enough, and you probably need the same 0.5 deg/sec rate as the OTFM mode has. in the case that the total power measurements are done in OTFM mode, you still probably want the measurement to have the same SNR as the interferometric measurements, which implies that it might take of order twice as long to do a mosaic than if you collect the total power at the same time as the interferometric data.
- regarding slewing to a new source, brief discussion met mostly with the telephone equivalent of blank stares, implying to me that the spec'ed rates are probably OK.
John Lugten had some comments about this:
1. I think our pointing specification has been gradually eroded away from 1/30 beamwidth.
For a 12 m antenna at 300 GHz one calculates a primary beam FWHP = 18.5 arcsec and 1/30 FWHM = .62 arcsec if the dish illumination is chosen to be that giving best aperture efficiency using the frequency- independent illumination scheme of Padman et al (1987). At BIMA, we chose to underilluminate the dish to increase the primary beam FWHM by about 7%, which increases the beam solid angle by 14%, and costs only 1% on the aperture efficiency. This would give 1/30 FWHM = .66 arcsec for the MMA. I think a strong case can be made for this choice. Another possibility which apparently is being studied is that of using ``shaped'' optics to increase the aperture efficiency. I think that would be a poor choice, but it would result in 1/30 FWHM of less than .6 arcsec -- perhaps significantly less.
Also, unlike the surface accuracy specification where we budget about 10 microns for setting error, we make no allowance for measurement error in the reference pointing position. This measurement error comes from 2 sources: electrical noise due to finite integration time and anomalous refraction. I presented these numbers as part of my SPIE talk in March for a 10 meter aperture, and have updated them for the 12 m aperture. Following Lucas' memo I calculate for a 200 mJy source (on average 2.1 degrees away), Tsys/freq = .5, 50 antennas, 15 sec integration time on each point of a 5-point, an electrical noise pointing error of .16 arcsec. If one is less optimistic on system temperature, taking Tsys(K) = freq(GHz), then I get a positional accuracy of .32 arcsec. For the anomalous refraction contribution, using the Holdaway/Woody memo which gives .90 arcsec median jitter at 50 degrees source elevation, assuming the same 5 x 15 sec total integration time for our 5-point (and that the atmospheric pattern speed is about 12 m/s, giving one independent sample of anomalous refraction every second), I calculate .16 arcsec pointing error. Combining even the more optimistic system temperature noise contribution with the anomalous refraction contribution gives .23 arcsec pointing uncertainty.
Combining all this, I would argue that to get 1/30 FWHM performance at 300 GHz, we really need to allow the antenna contractor at most 0.6 arcsec pointing error in the offset pointing mode. If one believes in illuminating for best aperture efficiency with unshaped optics, or in shaped optics, or is less optimistic about system temperatures, then the contractor pointing accuracy spec should be more like .5 arcsec.
2. I don't think it makes much difference what scheme one uses for pointing up, 5-point, 4-point, 3-point or continuous circles. For all cases we allow minimal time for overhead in moving (i allowed 15 sec to slew to calibrator and back to source, and all moving between the 5 points, for a total of 1.5 minutes for each 5-point). Lucas points out that one gains sensitivity by having 1/5 of all antennas on source during each of the 5 integrations of a 5-point, which I think argues that one does not save significant time by skipping the center position as in the OVRO triangle method.
3. As Harvey says, for fatigue life due to cyclic loading, the lifetime is rapidly reduced as the magnitude of the cyclic part of the stress is increased compared to the endurance stress (which is the allowable stress for 10,000,000 cycles for most materials). For our maximum antenna acceleration of 24 deg/s/s the edge of the dish accelerates at about 1/4 g, which probably does not stress most structural elements close their endurance limit since most of the mount structure is designed by stiffness requirements, or perhaps by survival wind for the dish itself. However, much of the drive train probably will be designed by fatigue lifetime requirements.
John brought this subject up for discussion. It was generally agreed that we need to establish a table of what sorts of dynamic range and image fidelity might be expected with the proposed 0.7 arcsec pointing specification, and what those might be for specifications of 0.5 or 0.6 arcsec. It was thought that these estimates could be obtained from existing simulations; John will talk to Mark and develop these.
The whole question of specifications for submillimeter science was discussed, as the array was extended to submillimeter wavelengths a few years ago after the Tucson Science workshop. The MAC subsequently developed some of the science arguments for the submillimeter, but few if any of the specs have been changed in view of this submillimeter science. This would be a good topic for MAC discussion.
MAC Meeting - Wootten noted that Wednesday 13 Jan at high noon EST is the appointed hour for the nex MAC meeting.
Configuration studies -- Kogan, Helfer
Leonia reported on the use of his newly developed AIPS routines for simulating observations of Cass A. He took Perley's 1.5 GHz image and simulated observations of it using circular and optimized (for minimal sidelobes) configurations. These were also made with simulated noise. Clearly, sidelobes were much suppressed by the sidelobe minimization aray configurations. Leonia pointed out that the typical size of a 'picture' divided by its resolution cannot be better than the maximum size of the array divided by the size of its central hole. We discussed the directions which these studies should proceed along. It would now appear that 48 x 12m will be the dimension of the LMA (=MMA/LSA). We decided that future simulations should use this dimension. We should set up a strawman set of configurations and move toward simulations of it, including imaging using mosaicing. Site testing -- Radford
The meeting at URSI was the MMA ImCal group's first milestone. We did not get to discuss this topic.
Eurotasks -- Wootten
What MMA Imaging and Calibration tasks might the Europeans aid us in?
1. Investigation of 183 GHz phase correction systems. This would be a continuation of the current program, in which one 183 GHz radiometer is situated at each end of the 12 GHz interferometer baseline at Chajnantor.
2. Investigation of total power imaging modes. The continuing 'DREAM' mode and related modes at the JCMT, as well as characterization of 1/f noise in SIS receivers, could help us to understand problems associated with sensitive total power observing modes.
3. Soil sampling.
Fall 99 Meeting - Wootten
Note that the 12m '30th birthday meeting' will be of substantial interest to our group:
6- 9 Jun 'Imaging at Radio through Submillimeter Wavelengths' meeting in Tucson
The scientific meeting in the Fall of 1999 to showcase the science that the MMA/LSA will be able to perform in the 21st century will be held, probably, for two days with the venue the Carnegie Institute of Washington, 14th and P St NW across from AUI (Andrew's former home). This will probably be after Labor Day but before October. We have not yet been able to get dates from the fellow who schedules the auditorium however. The MAC will work as the US Scientific Organizing Committee for this meeting, along with the newly appointed European 'MAC': Karl Menten (Chair), Roy Booth, Stephane Guilloteau, Richard Hills, Ewine van Dishoeck, Malcolm Walmsley, Peter Shaver (sec'y). I will cc: Ishiguro. Dick Kurtz was appointed as PM of the LSA; nominations are being sought for Project Scientist.
Anyway, I propose to get a list of available dates and nominate this whole bunch as the SOC, letting people decline to serve rather than offer to do so.
----------------------------
----------------------
----------------------------------------------------------------------
-------- Action Items 11Jan99
------
-------- Travel
------
T. Helfer: A. Wootten: 20 Jan (Tucson LSA MMA) J. Mangum:
M. Yun:
B. Butler:
S. Radford: