NRAO/ALMA Imaging Group Meeting, March 1 2005

present: Mark Holdaway, Sanjay Bhatnagar, Kumar Golap, Ed Fomalont, Al Wootten

Reconstituted Minutes (ie, not in the order the meeting happened):

Sanjay is working on a pointing self-cal algorithm. Currently a primitive pointing error solver works, and Sanjay is working on the code that corrects the data for the pointing errors in Imager. (This is a huge step forward.)

Kumar is currently working on mosaicing, combining total power and interferometric data.

Mark's vision is to be able to simulate all relevant imaging errors with AIPS++: thermal noise, realistic atmospheric phase errors, atmospheric amplitude fluctuations, pointing errors, voltage pattern errors, illumination offsets, and probably some other errors we haven't written down yet (eventually we will get to polarization). The big results we will get from this vision are: 1) we should be able to say what effects will be most limiting for imaging as a function of frequency and atmospheric conditions, 2) we will be able to exercise our soon-to-be-written self-cal algorithms and determine how to use them iteratively (ie, the errors are not orthogonal, so the iterative solution for different effects -- ie going back and forth between deconvolution and the different sorts of self-cal -- may be problematic).

(Ed thinks that sooner or later we will have a Single Grand Solver. He's been hanging out with geodetics folks too long?)

At some point, we realized the obvious: in order to get a good model for whatever sort of self-cal you are doing, we will need to get the (error free) total power + interferometric imaging under control first.

We will have at least two different regimes to worry about: a) weak source -- self-cal is not possible b) strong source -- self-cal is possible Thinking now, it seems there may be several regimes: c) source too weak to use self-cal, but with images limited by the various errors nonetheless d) enough SNR to accomplish one sort of self-cal, but not another kind (HEY! We need to develop some rules about what sort of pointing corrections you can solve for as a function of SNR and source structure.)

Misc comments:

A zeroeth order approach to these errors: flag antennas with the worst pointing (or other error) if we cannoy correct for the errors.

For single pointing imaging: doing a small mosaic may actually help with the pointing self-cal. We'll find out.

Need to consider snapshot simulations in addition to long track simulations.

Kumar was interested in knowing what sort of dynamic range is possible. Mark said you can have a 10 Jy unresolved quasar and easily have a thermal noise level of 10 uJy -- ie, 10^6 potential DR

Ed volunteered himself & Steve to do simulations at a future date. (( Ed: how about this: I simulate the data, and you reduce it? you'd become the world's expert on running pointing self-cal beofre it is really needed. ))

Ed still thinks non-random residual phase errors will dominate the imaging process. We will look into this: ==> Ed will investigate phase errors as a function of cal-target separation at the VLA (Ed: I know that I investigated the RMS of the residual phase errrors -- it is possible that I didn't look at the average phase error!) The 65 micron antenna position spec leads to 1deg phase errors ( for a typical cal-target setup, at 950 GHz ??? ); calibrator position errors can lead to systematic phase errors. What else? Thermal effects in the fiber optics? Fiber optics torsional effects?

Mark didn't mention this, but he's got atmospheric simulation code in Glish (uses the full AIPS++ data structure for the MS, gain tables, etc, should be easily integratable for the whole burrito simulations.

Our next meeting will be March 15, 4pm EST, 2pm MST (when do you guys go onto Daylight Savings Time?)

Take care,


-- AlWootten - 07 Mar 2005
Topic revision: r1 - 2005-03-07, AlWootten
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