[daip] [!11084]: AIPS - IMAGR: coordinates differ from NVSS more than expected, especially in declination

[Robert Gray]

Robert Gray updated #11084
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IMAGR: coordinates differ from NVSS more than expected, especially in declination 
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           Ticket ID: 11084
                 URL: https://help.nrao.edu/staff/index.php?/Tickets/Ticket/View/11084
                Name: Robert Gray
       Email address: roberthansengray at gmail.com
             Creator: User
          Department: AIPS Data Reduction
       Staff (Owner): -- Unassigned --
                Type: Issue
              Status: Open
            Priority: Default
                 SLA: NRAO E2E
      Template group: Default
             Created: 03 October 2017 04:49 PM
             Updated: 03 October 2017 04:49 PM
           Reply due: 05 October 2017 04:49 PM (2d 0h 0m)
      Resolution due: 29 June 2020 12:00 AM (999d 7h 11m)

Hi!
 I’m using IMAGR to image and search for single-channel features (12.7 kHz) in data from the Allen Telescope Array, and notice some apparently systematic differences between NVSS source coordinates and the coordinates reported by SAD for 13 MHz channel 0 sources over the ATA half-power 2.5 degree field of view.  
 With that relatively narrow total bandwidth, I did not expect chromatic aberration to be a significant problem, and it’s my understanding from the Explain file that IMAGR images each channel separately, and THEN averages the results (when NCHAV is set to a value like the 825 I’m using, the center channels out of 1024) so that it should not be a problem.  Is that correct?
 Comparing the half dozen ATA sources with SNR>6 with their NVSS counterparts, I find that the RA difference is scattered from 2 to 60 arcsec, averaging 25, which is roughly consistent with some prior work (Croft et al 2010 ATA 20 cm Survey which found positions matching to 20”).  But, declination seems to vary fairly systematically with distance from the center, with the ATA value ~100 arcsec higher than NVSS near the center, and ~150 arcsec lower than NVSS 1.25 degrees away from the center near the edge of the half-power field.  These declination differences are inside the typically 350x150” synthesized beam, but are a lot bigger than the 20” number cited by Croft et al, and seem systematic, and I’d expect better from 3-hour averages.  I’m using IMAGR defaults with IMSIZE=1024, CELLSIZ=18, ROBUST=5.
 Any light you can shed on this would be much appreciated.
 Best wishes,
 Bob Gray

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