Past minutes, etc on ALMA Configuration Group Page
Attendees
Hasegawa, Sakamoto, Wootten, Webster, Radford, Woody, Holdaway, Morita, Viallefond, Conway, Guilloteau
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Minutes
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John circulated the protomemo he wrote a few weeks ago and sent to Frederik on the subject of irregular sampling. He discussed the convergence of beam-based and u,v-based philosophies of array configuration design. During email discussions, Woody had said:
"It is also well known that better UV coverage, i.e. more data, produces better images. But this is also reflected in the PSF and the size of the peaks, more antennas and baselines produce better PSFs. Again memo #389 quantifies this.
"Thus it comes down to whether you want to "
1. fully sample the UV-plane so that you can produce nearly ideal PSFs after reweighting the data and lose S/N or
"2. produce the best possible PSFs at a given resolution without requiring reweighting and apply modern imaging techniques that will hopefully reduce the effect of the PSF imperfections to a negligible level. "
Having worked with a small low sensitivity array, I always favor maximizing the S/N, i.e. option 2."
Frederic tried to put most of his work on a web page. He prefers complete sampling.
John pointed out that the choice is really between a linear deconvolution which must be developed but which will produce faithful images from complete uv sampling, but which might have to be weighted near the edge to taper the beam
or a nonlinear deconvolution which will want to extrapolate longer spacings to produce a faithful image. John is working to understand what critical conditions must exist at the edge of the uv plane for faithful reconstruction. He thinks that the imaging will be quite good with only half a point per uv cell on average but is pondering this. In fact, ALMA fills every uv cell out to quite large distances, around 2km. In weighting to produce the images, only a little sensitivity, 5-10%, need be lost.
Guilloteau noted that the PdBI was designed to completely sample. However, many pad locations are never used because of the pressures on observing time. If very many cases need complete sampling, those cases have not dominated the proposals received.
It was decided that the 7-10 db recommended in the PDR statements should not be taken too seriously. Webster noted that his design was about 14-15 db taper; in his experience once one got the sidelobes below 1% or so discretization owing to a finite number of baselines dominated; that level seems to be around 12 db or so. He noted that the number of constraints --14km extent, compact configuration, number of pads--already posed difficult constraints.
There was general consensus to get physical and design the array. The perceived gap between approaches seems to be shrinking.
Woody brought up the issue of short spacing abundance, discussed previously. Guilloteau favored inner mean sidelobes as close to zero as possible, which might mean a single or very few 15m baselines.
Discussion turned to the availability of the 4 x 12m committed to the ACA for short baseline work. Generally, they would be assigned to the ACA and incorporating them for a few measurements into ALMA proper might be operationally difficult. Woody thought that an observer might make a case for this in his observational proposal. It was unclear that this solved the difficulties. Best to do some simulations with the configurations we arrive at. Webster mentioned that Steven can probably still do that, and the IRAM package is also available. It would be best if a single array can handle a large fraction of the science without mixing configurations. The role of the ACA was briefly mentioned; Wootten offered to distribute the ASAC study and did that.
Radford applauded the movement to get on with it. Wootten mentioned that we should have a plan which could be investigated on the December campaign, which should include an engineer. Final ratification should occur in January, after 18 Jan at a meeting perhaps in Socorro. Butler applauds this pointing out that...The weather is nice, some parts of the antenna should be there, there are good facilities (and a facilitator) and accomodations, and there is the chance that we could get input from folks like Cornwell, Perley, Rupen, Kogan, etc... who have thought about configuration design some, including the smaller VLA configuration under design.
Next Meeting?
The next meeting was suggested to be held at 1415 UT on 30 October 2001.