[daip] CLCOR vs IMAGR for shifts

[Mark Reid]

Eric Greisen wrote:
> Mark Reid wrote:
>> Eric Greisen wrote:
>>> Mark Reid wrote:
>>>
>>>> Anyway, it appears that CLCOR is still broken.
>>>>
>>>> Mark
>>>>
>>> Not that I know of.  It took data from you to establish this fact 
>>> before.  It will take more data if you really think this is the case.
>>>
>>> Eric Greisen
>>
>> The problem you fixed earlier involved VLBA data.  That fix seemed to 
>> have worked.
>>
>> The recent problem is with VLA data.  I don't know if this problem is 
>> related to the older one.
>>
>> I don't think you need to take more data (if by take you mean 
>> observe).   I would bet that any VLA-A data with a strong compact 
>> source would suffice.
>>
> 
> You have not understood me.  I know nothing about what you are doing - 
> are you reducing q band or 4 band data?  It took you providing us your 
> calibrated data set before we could find the problem last time.  And in 
> finding that problem we did re-examine the code very closely and found 
> nothing more to correct.  What parameters are you actually using for the 
> imaging and CLCOR?
> 
> Note that I will be unavailable until March at this point.
> 
> Eric Greisen

I thought I supplied the basics of the observations in my message of Feb 
13, where I indicated the program name, the VLA configuration, the 
polarization setup, and the CLCOR and IMAGR parameters I was using (see 
below).

I am not sure what level of detail you need, but here is an example line 
from the JObserve file:

V-111         15 54 38 17 45 40.0385 -29 00 28.173C    QQ       3333T 

//DS 2AC         3 

//LO     51.6   13.0      3510      3510              0000 

//FISF          124.861800              224.861800   43122.080000 
43122.080000

Since you are out until March, I'll experiment with other data sets and 
see if the problem is "universal" or related to the details of AR678.

Mark

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     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.
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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
Cambridge, MA 02138, USA     Web  : www.cfa.harvard.edu/~reid
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