MMA Imaging and Calibration Group

Agenda for meeting Tuesday, 7 Aug 2001 at 4pm EDT.

Date: 7 Aug 2001

Time: 4:00 pm EDT (2:00 pm Socorro, 1:00 pm Tucson)

Phone: (434)296-7082 (CV SoundStation Premier Conference phone).

Past minutes, etc on MMA Imaging and Calibration Division Page

Minutes

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News. -Wootten

News from my point of view.

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Meetings Last Week-Wootten

The ALMA Configuration Group had a Thurs telecon; minutes under construction. We expect a near-final configuration to be available before the Nov-Dec site campagn season, followed by a site campaign, followed by the CDR.

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The ALMA ACA Tiget Team teleconference was held on Friday. Minutes are under construction. http://iraux2.iram.fr/~alma/ has details. Highlights include the first imaging simulations using CLEAN/GILDAS; Stephane reports:

m51ha Original Halpha image, with a lot of photon noise - Better rendered by ALMA + ACA + SD

m51 trimmed version of m51ha, with no noise and limited emission area. - Better rendered by ALMA + SD

cluster K.Tatematsu image of protostellar cluster Smooth, extended structures with several peaks. - Better rendered by ALMA + ACA + SD

hco43 M.Hogerheijde image of protostellar condensation. Lot of smooth, extended structure, with just one peak... - Better rendered by ALMA + ACA +SD, but median fidelity does not exceed 30... - ALMA + SD does a poor job on that (fidelity 12)

m31 m31 HII region from Holdaway/Morita - Better rendered by ALMA + ACA + SD

turb "Typical" molecular cloud, with random distribution with known power spectrum from J.Pety. Lots of power in the short spacings. - ALMA + ACA + SD makes a decent job on that one

Al distributed images to ASAC science team and has gotten no response.

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Payne reports that the edge taper issue was discussed in the Receiver Meeting.

On 1 Aug, James said: "On the question of PO calculations and edge tapers I have suggested that the value of the edge taper is not a real specification. The reason for this is that the edge taper directly affects only the edge diffraction at the secondary mirror (and for a full analysis the derivative of the field has to be known also). What is important to ALMA is the sensitivity which depends on the efficiency and the added noise. To calculate these the full beam produced by the optics needs to be known. If the beam is a true Gaussian then the edge taper uniquely defines the whole beam. This is also true if the a perfect J0 horn aperture is imaged on to the secondary mirror. However, in a real system there are various intermediate stops which truncate the beam. These produce edge diffraction rays which interfere with the nominal beam. This can lead to in-phase addition at the centre of the secondary and out of phase at the edge, giving a larger edge taper. Similarly the reverse may be true, giving a smaller edge taper. These do not necessarily change the efficiency.

If PO is going to be used for the feed analysis then the results MUST be stated in terms of efficiency rather than edge taper. Note that this does not necessarily mean that a PO analysis has to be done through the whole whole antenna (though it would be great if someone could do this). For example the spillover and the phase of the beam across the secondary could be easily calculated.

The analysis I did was a simplified model which assumed circular symmetry and perfect phase transforming mirrors. Even with this degree of simplification I had to take a lot of care in the evaluation to ensure that this model was correctly calculated to the fraction of a percent level. Clearly the model itself will be in error at this level or greater, but it does point out that a lot of care has to go into verification and evaluation of the results. I have quite a lot of confidence in the results based on comparison with measured data in the past. Note that it takes into account all the aperture stops, including the holes in the radiation shields and the dewar windows. Where the beam was reflected back past the feed horn the field was calculated to ensure that there would not be a significant perturbation due to the horn. Most of the effects which are not directly included in the simple model were estimated by other means."

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Please see the summary of the ALMA-GSMT meeting.

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Upcoming Meetings - Wootten

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.

The next ACA meeting will be on 10 Aug. Correlator meeting happeningnow at Nobeyama. And of course, the ASAC meeting on Thursday.

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ACA Report -- Holdaway, Wootten

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Calibration PDR -- Wootten

Wootten's draft report is available but he still hasn't gotten comments into it. 

Travel

Oh, always

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