[evlatests] Deep Imaging of 3C345

[Craig Walker]

I like measuring the RMS using the whole field, including the source, 
but using the histogram fit in IMEAN.  That ignores the source and is 
less dependent on where you put boxes.  Of course, it does depend on 
field size.

Cheers,

Craig


George Moellenbrock wrote:
> Rick-
> 
> I think your reply deserves wider distribution.  Sorry if this is 
> nit-picking...
> 
> On Fri, 12 Feb 2010, Rick Perley wrote:
> 
>> In this case, BLCAL was run and applied.  I can easily image without applying 
>> it (but am now working on an X-band dataset -- higher priority).
> 
> No problem.  Hmm, I actually was expecting that it was BLCAL-less and the 
> magnitude of the dynamic range quote was due mostly to measuring rms well 
> away from the mess in the center.  Note that 3C345 is polarized, so the 
> variable polarization-induced closure errors won't be soaked up very well 
> by a constant BLCAL.  Maybe a BLCAL so-biased by source polarization 
> contributes in some complex way to the zoo of artifacts you noted.
> 
>> The 'dynamic range' in the vicinity of the strong source is probably down 
>> from what I quoted by a factor of a few.  I'm not sure any such quote is 
>> useful -- the rms one gets is hugely dependent on where you put your box.
> 
> For a complicated source like 3C345, I agree.  On/near-source errors will 
> be quite variable.  But these really are the meaningul errors, especially 
> for observers looking at interesting things (not "dots").
> 
>> Probably more important is the peak artifact height -- this is about 1 mJy, 
>> which is 1/10000 of the peak in the map.
> 
> That's a 35-sigma (!) artifact if your original report of dynamic 
> range is meaningful and to be taken seriously....  (Nit-picking?!)
> 
>> There is a larger artifact than 
>> this in fact -- but it's two cells from the core, and probably represents 
>> errors associated with the real structure of 3C345 (a jet, very well known). 
>> You can't see it in the image of course, as it's buried by the clean beam.
> 
> Yes, complex sources are tricky.  Interpretting the info I suggest/request 
> is probably easiest for the purest of dynamic range limit tests (truly 
> 'dot' sources), but _whenever_ you're moved to quote dynamic range, this 
> additional info (and noting dozens-sigma artifacts!) is indispensible, 
> regardless of (or perhaps because of?) the effects of source complexity.
> 
> In general, I wonder if reporting some combination of realized/expected 
> rms (with some indication that it varies significantly for complex fields) 
> and the level of systematic artifacts above the realized rms (and as a 
> fraction of peak) is more useful than the rather blunt "dynamic range" 
> quote, which I find pretty misleading.  It is just not that useful without 
> lots of qualification. Clearly, that you can find a spot that implies 
> 350k:1 is the least interesting thing about the image.  (At best, is it 
> merely an indirect indication of the highest _possible_ dynamic range you 
> might achieve if you can sort out the systematics correctly?  And a fairly 
> poor indication since it is probably an upper limit...)
> 
> In any case, just trying to tease out a clearer picture of what you've got
> there...
> 
> -George
> 
> 

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     R. Craig Walker            Array Operations Center
     cwalker at nrao.edu           National Radio Astronomy Observatory
     Phone  575 835 7247        P. O. Box O
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