[daip] problem with 31DEC10 aips and WSRT dataset ?

[Neeraj Gupta]

Dear Eric,

   I write this mail to request some help to understand an issue
that I have come across while analysing WSRT dataset with AIPS
31DEC10.

   This is one full-synthesis L-band dataset with 20-MHz BW
split into 1024 channels.  Analysing it with AIPS I find that
position of all the sources away from the phase-centre are
systematically offset by 5-10" wrt the FIRST catalogue.  The
position of radio source at the phase centre however seems to
match well.  All the offsets are in about the same direction
as seen in one of the plots attached (see contour plot - AIPSIMAG.png
- for one of the sources where crosses are positions from FIRST).
Therefore it is difficult to explain these offsets by some error in
time or frequency.   The WSRT beam is about 40" in the N-S
direction but the offsets are significant as they are seen more or
less for all the sources that are away from phase-centre.

To check this further I imaged the same dataset with CASAPY
and found that the same offsets are reproduced when I make the
contour plots in AIPS with KNTR (see CASAIMAG.png).  The
image produced with MIRIAD however shows no offsets
(see MIRIADIMAG.png).

However when I see these same images through casaviewer,
I find that the MIRIAD and CASAPY images are well aligned
wrt each other whereas only AIPS image is offset.  See the
images MIRIADinCASA.png, CASAinCASA.png and AIPSinCASA.png.
In all the three cases, contour plot correspond to MIRIAD image that
matches well with the FIRST catalogue positions.

In short,  image produced with MIRIAD (which as NCP projection)
appears correct in both the AIPS and CASAPY.  On the other hand
image produced with AIPS appear to be offset both in the AIPS
as well as casaviewer.

I also imaged using AIPS the dataset that was calibrated in MIRIAD
to create the MIRIAD image and AIPS still produces the offsets.

Therefore, at this point it is not clear to me if this the problem is
related
to viewer or the dataset or even specific to WSRT itself.

I am also attaching two FITS files with this mail:

NGC3226SC.FITS:  This is dataset that was calibrated in MIRIAD.
Imaging this in MIRIAD produces an image (also attached as
NGC3226MIIRIAD.FITS) that shows no offsets wrt FIRST.  The same
dataset when imaged in AIPS shows offsets for off-centre sources.


I will greatly appreciate any information or help that you can provide
on this issue.

cheers,
Neeraj
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