[daip] CLCOR vs IMAGR for shifts

[Mark Reid]

Eric Greisen wrote:
> Mark Reid wrote:
>> Eric and Craig,
>>
>>    I was working on some VLA A-config 43 GHz data (AR678:maser stars 
>> near Sgr A*) and seem to have a problem shifting with CLCOR again.  
>> Here are the particulars:
>>
>> I'm using 31Dec10 AIPS.
>> The data has 2 polarizations (RCP & LCP)
>>
>> We pointed at Sgr A* and used it to (self)calibrate the data.
>> Then we image with several boxes, one on SgrA* at the origin and the 
>> others shifted (IMAGR: RASH,DECSH) by ~10 arcsec to the positions of 
>> masing stars.   This has worked well both now and in the past.
>>
>> As a test, I tried shifting with CLCOR (ANTC: CLCORPRM(5),(6)) and 
>> then imaging with IMAGR.  This caused problems:
>>
>> 1) when I shifted a large amount (30" mostly in Dec) in CLCOR and then 
>> by -30 arcsec in IMAGR, the image of Sgr A* was highly distorted. 
>> Actually, the image resembled the VLA in shape!
>>
>> 2) when I shifted a modest amount (5.6" mostly in Dec) in CLCOR and 
>> then by -5.6 arcsec in IMAGR, the image of Sgr A* was fine.  HOWEVER, 
>> there was a residual offset of about 10 mas in each axis.  This is 
>> about 1 part in 500 of the shift...pretty large.
>>
>> I would think this could be simulated with almost any VLA-A data set 
>> that has a source with good SNR.
>>
>> Any ideas what is going on?
>>
>> Mark
>>
>> PS: the reason I was doing this test is that I'm worried that by 
>> shifting in IMAGR (which I believe does only pure geometry changes), 
>> that I will need to correct for differential precession, aberration, 
>> refraction, etc.  I believe that CLCOR does the full calculation and 
>> thus would show me the "truth" (if it worked).  Does this sound right 
>> to you?
> 
> You give CLCOR way too much credit!!!  IMAGR and CLCOR do only pure 
> geometric shifts.  IMAGR does correct u,v,w for shifted facets.  Doing a 
> shift with CLCOR will change the recorded source position - but it does 
> not recompute u,v,w in any way.  IMAGR assumes that the u,v,w recorded 
> with the data are correct - which with a shifted source would not be 
> true.  UVFIX is the only task in aips that considers differential 
> precession, etc etc
> 
> 30 arc sec seems pretty small for getting serious sky curvature 
> problems.  But at such southern declinations W errors are very serious.
> 
> Eric Greisen

Eric,

    That's good to know...indeed I thought CLCOR did more.
I'll try UVFIX.

Anyway, it appears that CLCOR is still broken.

Mark

-- 

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Mark J. Reid                 Phone: 617-495-7470
Harvard-Smithsonian CfA      Fax  : 617-495-7345
60 Garden Street             Email: reid at cfa.harvard.edu
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