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

[Neeraj Gupta]

Hi Eric,

   Sorry, I forgot to respond to your question.  Yes  I used
uvw provided in the WSRT data to image.  According to the
header the three baseline axes are marked as:

AIPS 1: Rand axes: UU-L  VV-L  WW-L  BASELINE  TIME1

Since there is no suffix I guess AIPS assumes that it is SIN projection.
This is not correct and should be NCP.

cheers,
Neeraj






On Thu, Feb 24, 2011 at 9:21 PM, Eric Greisen <egreisen at nrao.edu> wrote:

> Neeraj Gupta wrote:
>
>> 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.
>>
>
> I suspect the issue to be the following:
>
> Have you simply used the u-v-w provided in the WSRT data?  If so, they are
> computed on a special NCP projection rather than -SIN.  Were you to change
> the image header of an IMAGR output to say RA---NCP and DEC--NCP rather than
> RA---SIN, DEC--SIN which it now says, I suspect that the source coordinates
> will line up.  Alternatively, run UVFIX on the data before imaging in AIPS
> and then make images.  They will not line up with the others, but the source
> coordinates will.  I suspect it is all a matter of imaging geometry.
>
> Eric Greisen
>
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