[daip] Re: CLCOR

[Eric Greisen]

Chris Phillips writes:

 > I don't really understand the meaning of the "!!!" here. I understand that
 > CLCAL changes the SU table, and I have a feeling it is doing it wrong
 > (though I may be missing some subtle point).

    We may all be missing something here - this has been checked and
rechecked many times.  The FITLD question is a VLBA one since the time
used by FITLD was one day off so the apparent position in the SU table
(has to be calculated by FITLD for IDI data) and so the CLCOR
computation did not match.

 > 
 > OK, I was previously looking at the source coordinates found through
 > LISTR. These do not seem to be the same as the EPOCH or APPARENT
 > positions in the SU table. The LISTR positions are surely important
 > because they end up being the coordinates of the centre of the map made by
 > IMAGR.

    LISTR uses the epoch positions straight from the SU table handled
in double precision - at least for its SCAN listing.  In fact, that is
the only place that coordinates are stored.  How did you get the SU
coordinates listed below?  If from PRTAB, do you have the version with
NDIG as an adverb so that, with NDIG > 3, you get enough digits
printed in the coordinates?  If you do not have this option, I don't
know how to get at apparent coordinates.  (I think I added this in
part because of questions about CLCOR.)

 > 
 > Anyhow what I have is:
 > 
 > "Observed" positions:

    By this you mean before CLCOR?  With data from merlin we may be
depending on the apparent/epoch conversion being similar to the one in
aips....

A thing to try is a CLCOR with absolutely tiny corrections, e.g.
0.0001.  Does the epoch position change by more than about this?  If
so there is a disagreement in the initial table.

ERic

 > 
 > LISTR: 19:22:34.0631   15:30:15.582
 > SUEPO: 19:22:34.0560   15:30:15.5880
 > SUAPP: 19:22:35.8800   15:30:10.5840
 > 
 > The VLBA position of the source is 19:22:34.6992 15:30:10.032, so I ran
 > CLCOR with CLCORPRM 0 0 0 0 9.1943 -5.55

  An "epoch" coordinate I assume...

 > 
 > This then gives the positions of
 > 
 > LISTR: 19:22:34.7327   15:30:10.017
 > SUEPO: 19:22:34.7280   15:30:10.0080
 > SUAPP: 19:22:36.5040   15:30:05.0040
 > 
 > These have arcsec delta's of:
 > 
 > LISTR: 9.6785, -5.5650
 > SUEPO: 9.7132, -5.5800
 > SUAPP: 9.0195, -5.5800
 > 
 > To me this looks wrong.
 > 
 > > When your data were fitlded?
 > 
 > It was FITLDed a few months ago at Jodrell Bank. The data has been loaded
 > into aips and exported as FITS files a few times at different places. (Ie
 > originally done at Jodrell Bank. Then partially processed at JIVE, now at
 > ATNF. All probably different versions of aips...
 > 
 > Cheers
 > Chris
 > 
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