[daip] CLCOR position shifts

[Leonia Kogan]

Mark,

Can I start with the history.

Until the last change of CLCOR(~year ago), CLCOR had modified the CL
table in accordance of the given shift of RA and declination but had
not change the SU table.

~Year ago we decided to make the relevant change of the SU table.
There was a discusion here which coordinates: apparent or epoch (J200 usually)
should be modified. Finally we decided to modify the apparent coordinates 
and recalculate the modifyed apparent position to J2000.

So the apparent coordinates should be shifted precisely by the amount
given at CLCORPRM(5,6) and the J2000 coordinates correspond to the 
modified apparent coordinates and should be different of the epoch 
coordinates given at the initial SU table.

Leonia


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>From reid at cfa.harvard.edu Tue Sep  3 12:44 MDT 2002
Date: Tue, 3 Sep 2002 14:44:28 -0400 (EDT)
From: Mark Reid <reid at cfa.harvard.edu>
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To: lkogan at cv3.cv.nrao.edu
cc: Andreas Brunthaler <brunthal at mpifr-bonn.mpg.de>
Subject: CLCOR position shifts
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Leonia,

Andreas Brunthaler and I are doing an astrometric VLBA program
to measure proper motions of galaxies with respect to background
quasars (BB132, BB142).  We need to shift the interferometer phase
center in some cases, but are confused by what happens.

I had assumed that since the (u,v) coordinates are in J2000
and therefore the maps are in J2000, that any shifts done
would also be in J2000.  However, the "apparent" coordinates
seem to be updated, while the J2000 coordinates remain unchanged.

Is this correct?  If so, doesn't this seem inconsistent?

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