[alma-config] Comments on Marks proto-memo

[Mark Holdaway]

John said:
> 
> ASSERTION 2) Reconstruction errors due to incomplete uv coverage
> are likley to be less than 1 part in 1000, hence 
> pointing and other arrors can become dominat.
> 
> At the Tuscon CoDR I showed some VTESS simulations
> of the  M51CO image for a 6hr track, for both 
> a donut-0.25 and zoom spiral array. To work out 
> the fractional errors I have divided the image with the model, 
> clipping below a cutoff of only 10% of the peak brightness,
> i.e. I looked at the fractional errors only in the
> brightest regions of the source. For the donut 
> the rms fractional error was about 0.7% and peak about 3%.
> For the spiral the corresponding numbers were about a factor
> of 2 less, and apears to be limited by the added thermal
> noise (I should rerun the simulations with lower additive
> noise to get a better idea of the fundemental ratio in 
> quality).  Now this is a just a single image and there might
> be something special about it, one would
> really like to see if we get the same results on a bigger
> set of images.  
> 
> Note that the above results  was for a long track 
> and the fractional reconstrcution errors for 
> snapshots (the main style when mosaicing) one can only 
> be expected to be larger.
> 
> --------------------------------------------------------------
> 

We also need to distinguish between ON-SOURCE errors and OFF-SOURCE
errors.

ON-SOURCE errors are much larger than OFF-SOURCE errors.  In mosaicing, at
least, the off-source errors are roughly of the magnitude of the on-source
errors times the rms PSF sidelobe level.  From mosaicing pointing error
simulations, we found the off-source errors were like 1 part in 500 or
1 part in 1000.  The ON-SOURCE errors were like one part in 20.

	-Mark