[daip] forwarded message from Chris Phillips

[Eric Greisen]

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From: Chris Phillips <Chris.Phillips at csiro.au>
To: Eric Greisen <egreisen at cv3.cv.nrao.edu>
Subject: CLCOR bug (maybe)
Date: Mon, 25 Nov 2002 16:32:32 +1100 (EST)

Hi Eric,

I have been trying to use CLCOR to fix a source position problem and think
there maybe a problem with it.

- - I am using 31DEC02, but not updated for about 2 months.

- - I have some single baseline (just to complicate the analysis of the
  problem) merlin data at 6.7 GHz.

- - The phase calibrator was observed (not my fault!) 10" offset. I know the
  observed position and the correct position (from the VLBA calibrator
  list) so should be able to correct for the error.

- - The observed position was 19:22:34.0631 15:30:15.582

- - The correct position should have been 19:22:34.6992	15:30:10.032

- - I ran CLCOR with CLCORPRM 0 0 0 0 +9.1943, -5.550

- - When I have done this the position in the SU table for this source is
  	19:22:34.7327	15:30:10.017
  (which is also the centre of the map if I run IMAGR with no shift).

- - If run IMAGR on the shifted data and compare it to IMAGR with a
  RASHIFT and DECSHIFT the same as above on the original data, the maps
  are identical.

(As this was the calibrator, I am running IMAGR on uncalibrated data (& no
selfcal), but Merlin seems to be phase stable enough that this works, I
can image the calibrator source, though only just)


So it seems to me that CLCOR does the right thing with the data, but gets
the position in the SU table wrong. Maybe I am missing a subtle effect,
but the difference is relatively large (~0.5").

Complicating matters even more, there seems to be a 0.23" in Dec in the
position listed in the VLBA calibrator list, but I am assuming this is a
separate problem.

Does this indicate a problem with CLCOR to you, or is it just me?

Cheers
Chris


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