Date: 4 Feb 2003
Time: 4:00 pm EST (2:00 pm Socorro, 2:00 pm Tucson)
Phone: (434)296-7082 (CV SoundStation Premier Conference phone 3rd floor).
Past minutes, etc on
MMA Imaging and Calibration Division Page
Notes from the meeting are in this font!
Here is a direct question... What is our answer ? I guess we have 3 = choices
- 240 GHz (or 260 GHz)
- 280 GHz
- 350 GHz
>From pure sensitivity reason, 260 GHz would be the best if receivers = keep their promise... Note that it is the brightness temperature of the = emission which matters, not the flux. In the estimates, I assumed an = emissivity going as the frequency, as appropriate for circumstellar = disks/protostellar envelopes.=20
Perhaps 280 GHz (i.e. the low end of the Band 7) is a good compromise...
The real question is which is more important: noise and stability = (=3D=3D> 260 GHz) or beam squint (=3D=3D> 280 GHz or more) ? Also, band = 7 may be done as 2 4 GHz IFs with band centers separated by 12 GHz, i.e. spanning 16 GHz total, while band 6 gives a continuous 8 GHz bandwidth, = which is better for the performance of the quarter wave plate.
Can we first commit to a deadline for Hans ?
------------------------end SG comments------------------------
We need to consider spectral line coverage also, for which perhaps Crystal has some ideas. She submitted a list some time ago. Is it still current? Our discussion of 24 October 2000 is still relevant?
On 26 September 2000, Crystal provided a list of lines of Zeeman interest--is this still up-to-date?
Crystal sent an updated note . The consensus was overwhelming that focussing on a wavelength with best stability and low noise was preferable. Crystal noted that the most important spectral lines favored a window at 240 or even 230 GHz. There was some discussion that the CO lines may be of interest, in which case lines with low to moderate optical depth are preferred. Since a quarter wave plate type of widget has about a 10 per cent bandwidth, this suggests the lower frequency. Probably moderate beam squint is not dominant, this can be cancelled with proper technique. As an aside, Troland and Crutcher reported that at Lband the GBT beam squint is unmeasurable, below 0.1%. The specification was "as low as possible."
Since a single design has been the ALMA plan since before the ASAC came into existence, it has not considered in detail the scientific implications of an array of heterogeneous antenna design. We have from time to time discussed problems associated with two designs, from the differing polarization properties (a particular concern, see below), to having three or more beam patterns to keep track of for mosaicing (design 1, design 2 and the cross design), to the differing systematic errors which would ensue, resulting in different phase performance of the antennas, as well as different systematic pointing errors. Scientifically, polarization and mosaicing science would suffer considerably, I think.
In the April 2002 Report, the ASAC said: April 2002 Report, paragraph 1 of section 5 of the ASAC Report: "The ASAC heard detailed reports both on the progress of the prototype antennae under construction as well as on the plans for their testing in Socorro, NM. In particular, it was gratifying to hear that the North American antenna is expected to be assembled and ready for testing this summer. The program for evaluation of antenna performance seems satisfactory although it is regrettable that interferometric holography will probably not be possible in practice. The ASAC is worried by the tight schedule for testing the European and Japanese prototypes which are each scheduled to arrive in Socorro in April 2003. It seems extremely important that the delivery schedule be adhered to in order to allow testing for all three prototypes to be concluded by the end of 2003. The ASAC would also like to underline the importance to the project of the choice of a single antenna design subsequent to an objective comparison of all three prototypes. This presumably would allow cost savings and increase ease of maintenance which, in the end, will increase the productivity of ALMA. The ASAC also notes that the choice of a single antenna type is particularly required to guarantee good performance for polarization measurements."
The gist of the telescope design discussion followed a suggestion from
Rick Fisher--that the discussion should be 'What are the scientific
benefits of two designs?' for which he proposed there is no answer, for
the specific case of one aperture size. There was little support for
a heterogeneous array design.
Large Configuration: Progress Report.
Conway has developed the antenna pad specifications.
The Configuration Group has delivered the configuration plan to the Site Group, and surveyors are at work at Chajnantor. For photos, please see this Configuration realization page.
JUN 22-27, 2003 2003 IEEE AP-S INTERNATIONAL SYMPOSIUM AND USNC/CNC/URSI NATIONAL RADIO SCIENCE MEETING
JUL 13-26, 2003 Twenty-Fifth General Assembly of the IAU
JUL 22/25, 2003 IAU Symposium (221): Star Formation at High Angular Resolution, Sydney, Australia. Tyler Bourke is a co-chair of the scientific organizing committee. Good opportunity to show ALMA capabilities.
SEP 22-26, 2003 4th Cologne-Bonn-Zermatt-Symposium on The Dense Interstellar Medium in Galaxies