ALMA Atacama Compact Array Tiger Team

Teleconference, 20 July 2001 1415 UT

Draft Minutes

Participants: P. Cox, R. Crutcher, S. Guilloteau, F. Gueth, M. Gurwell, M. Holdaway, J. Mangum, Koh-Ichiro Morita, J. Pety, A. Wootten,

1. Progress Reports. Crutcher suggested that written material be posted a reasonable time before each meeting to allow participants time to review it. Let's try to have all written material in at least 24 hours before future meetings.

Also a protodraft of Al's summary of the ACA documents is available.

Each Friday a progress report will be posted to http://iram.fr/~alma along with relevant emails (email address alma@iram.fr) for keeping up with progress. At IRAM, a number of tools have been created to automate a number of computations. Pety can now run a large number of simulations. Gueth has been working on phase calibration, including using the WVR. The amplitude error part of the code is still needed. All is ready to begin simulations next week. Images were discussed. These include

M51 HST image

Two versions of the images can be found at:

ftp://ftp.tuc.nrao.edu/mholdawa/

M51HA.MODEL256

M51HA.MODEL512

Cluster Image

Tatematsu's example is on Morita's WWW site

Protostellar Core image

Michiel's examples of a protostellar disk buried in a core include: HCO+ 1-0 , HCO+ 4-3 and C18O 1-0 fits images. One of the science cases involves bright cores in extended weak halos such as these.

Debris Disk image

Crutcher noted that one type of imaging problem was not included among these-that with low contrast but structural detail on small scales. Polarization work abounds with images such as these. Wootten suggested a comet or the elliptical galaxy image discussed in Cambridge. Holdaway suggested that the bright portions of the M51 image might be filtered so as to leave a low contrast detailed image. It was generally agreed that this was suitable.

Holdaway discussed his recent written contribution on Surface Errors, Taper Misalignment and Pointing Errors. He makes several assumptions about ALMA/ACA operations, detailed in his memo. Guilloteau made two points. 25 micron surface errors may be too pessimistic for the 12m antennas-he suggested using 20 microns. For the ACA, the 10 microns suggested in the Baars memo may be too optimistic-perhaps 12 microns would be a better number. Note that if the ACA operates in concert with four 12m antennas we will be able to select the better antennas for this chore. This was generally agreed. A second point was that gravitational deformation should be properly accounted. Holdaway used a calculation from Lamb for zenith structural deformation with the best fit parabola taken out, ten microns. Although a small fraction of the total, because it is systematic, it contributes a noticeable amount. Guilloteau noted that we now have real designs and prototype data which should be used; this number might be 8 microns. These are of the same order but if numbers are available from the Vertex or EIE designs those would be the best numbers to use.

Stephane introduced the question of how to deal with anomalous refraction. Holdaway suggested deriving this from the phase screen model, with small enough integration times to do this for each integration. Guilloteau agreed noting that we would probably end up with seeing limited observations; this term will probably dominate error terms. Morita noted that ASTE won't provide data on anomalous refraction on the site in the near term.

Crutcher offered time on the NCSA supercomputers which include a large LINUX cluster. Holdaway noted that his software is archaic non-LINUX software. Guilloteau noted that the IRAM software operated under LINUX. For the present exercise, the existing computer power seemed sufficient.

Crutcher brought up the evaluation process discussed at the ASAC meeting. Ewine suggested that a team of Crutcher, Welch, Cox and Yun evaluate-that is, make scientific measurements from-the simulated images to determine their suitability for the science purposes of ALMA. Holdaway suggested that this process could begin now, with the perfect images, for instance convolving the model images with a clean beam and analyzing them. For example, determining the arm interarm ratio of flux in M51. This way, when simulated data come in the models will at least be understood in terms of the science they are capable of delivering. This was thought a good idea. Wootten will assemble and distribute the images.

It has been proposed that after ALMA construction has ended, the SMA be resited to Chajnantor to serve as the ACA. This has a number of political and operational ramifications as well as scientific ones. Funding mavens will likely see this as an ideal way to keep costs down so we may expect the EACC to request an opinion from the ASAC which will in turn request us for evaluations. Wootten suggested this be included as an ACA strawman thought the specifications are fairly different from those for the ACA (1" pointing primarily; surface is measured at 13 microns, subreflector 350mm). Crutcher asked whether details of this suggestion-cost estimates, etc-are being actively pursued. Gurwell replied that this was really just at the exploratory stage now.

Guilloteau noted that we still need to make sure that uv data may be exchanged between packages and proposed an exchange within the next few days. Holdaway noted that he had sent a UV FITS file to Monger. For the total power work image data seems fine, from which pointing or 0,0 point input might be provided.

2. Next teleconference

The next ACA Tiger Team teleconference will take place on Friday 3 August at 14:15 UT. Wootten will distribute details.