[daip] Re: CLCOR position shifts (fwd)

[Leonia Kogan]

Andreas,

You wrote:

>The J2000 coordinates are the same in the SCHED output and the SU-Table. 
>The apparent coordinates are different from the SU-Table.

I think this is the answer.

Can you edit your original SU table (ofcource copiing it before) by TABED
to get the apparent coordinates identical with SCHED output?

Now

Socorro correlator use only epoch coordintes. FITLD (in AIPS) calculates 
the apparent coordinates and put them into SU table. Approximately  a year
ago, we found a mistake in FITLD's calculation of the apparent coordinates.

So if you FITLDed your data long time ago you could have a mistake in
the apparent coordinates at the SU table.

Leonia
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>From brunthal at mpifr-bonn.mpg.de Wed Sep  4 08:53 MDT 2002
Date: Wed, 04 Sep 2002 16:53:08 +0200
From: Andreas Brunthaler <brunthal at mpifr-bonn.mpg.de>
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To: Leonia Kogan <lkogan at zia.aoc.NRAO.EDU>
CC: reid at cfa.harvard.edu, daip at cv3.cv.nrao.edu
Subject: Re: CLCOR position shifts (fwd)
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Leonia Kogan wrote:
> Mark,
> 
> Andreas wrote:
> 
> 
>>It could also be that the initial apparent positions in the SU table are 
>>wrong and CLCOR calculates in the first shift the J2000 of this wrong 
>>coordinates.
> 
> 
> I think this one of the posible solution of the problem.  Andreas can
> run CLCOR with the opposite sign of the correction and see if the both
> aparent and epoch coordinates will return back to the origional ones.

I tried a shift with the opposite sign, and the apparent coordinates 
return to their original values. The epoch coordinates don't.

> Can Andreas look at the SCHED output and see the both right coordinates there?

The J2000 coordinates are the same in the SCHED output and the SU-Table. 
The apparent coordinates are different from the SU-Table.

> 2. I hope he followed the HELP file to uce correctely the COS(delta)
>    for the RA correction

As Mark mentioned, I applied a DEC-shift only.


Andreas


> 
> Leonia 
> ----- Begin Included Message -----
> 
>>From reid at cfa.harvard.edu Wed Sep  4 07:44 MDT 2002
> Date: Wed, 4 Sep 2002 09:43:54 -0400 (EDT)
> From: Mark Reid <reid at cfa.harvard.edu>
> X-X-Sender: reid at mjr.harvard.edu
> To: lkogan at cv3.cv.nrao.edu
> cc: lgreenhill at cfa.harvard.edu
> Subject: Re: CLCOR position shifts (fwd)
> MIME-Version: 1.0
> X-MailScanner: Found to be clean
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> Status: RO
> 
> 
> Leonia,
> 
> I got this message from Andreas Brunthaler.
> Apparently, even with the very awkward convention
> of shifting in apparent coordinates (when everything
> else is done in J2000 coordinates), there still are
> serious errors in the SU table.
> 
> Mark
> 
> 
> -------------------------------------------------------------
> 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  : cfa-www.harvard.edu/~reid                  
> -------------------------------------------------------------
> 
> 
> ---------- Forwarded message ----------
> Date: Wed, 04 Sep 2002 13:55:01 +0200
> From: Andreas Brunthaler <brunthal at mpifr-bonn.mpg.de>
> To: Mark Reid <reid at cfa.harvard.edu>
> Subject: Re: CLCOR position shifts (fwd)
> 
> Mark,
> 
> the J2000 coordinates get also updated, but not as they should. 
> These are the coordinates i get in the SU table before and after a 
> shift of -0.00409 arcsec in DEC:
> 
> before shift:
> 
>       RAEPO	     DECEPO	  |	RAAPP           DECAPP
> 6.7636903750E+00 5.9981378722E+01 | 6.7661974065E+00 5.9985040301E+01
> 
> after shift:
> 
>       RAEPO	     DECEPO	  |	 RAAPP          DECAPP
> 6.7637355776E+00 5.9981300756E+01 | 6.7661974065E+00 5.9985039165E+01
> 
> The DECAPP got changed correctly by -0.00409 arcsec. However, the DECEPO 
> changed by -0.2806776 arcsec, and the RAEPO by 0.1627296 arcsec. So the 
> J2000 coordinates changed in total by more than 0.32 arcseconds after a 
> shift of only 0.00409 arcseconds.
> 
> If I apply a SECOND or third shift, it changes all coordinates by the 
> right amount. The first shift is always wrong by this 0.32 arcsec.
> 
> Kogan wrote that the new J2000 coordinates are calculated from the the 
> new apparent positions. It seems that there is an offset of 0.32 arcsec 
> in this conversion. The first shift then calculates a wrong J2000 position.
> It could also be that the initial apparent positions in the SU table are 
> wrong and CLCOR calculates in the first shift the J2000 of this wrong 
> coordinates.
> 
> Andreas
> 
> 
> 
> 
> 
> 
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> 
> 




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