[daip] FLATN query

[David Titterington]

Dear AIPS,

We've recently started using the 31DEC10 version of AIPS, having upgraded
from 31DEC08, and have found the changes to the FLATN task reported in the
June 30 2009 AIPSLetter, in particular the use of the EDGSKP parameter:

  EDGSKP......Omit EDGSKP (or if EDGSKP < 0, -EDGSKP-1) pixels on all
              sides of the input images.  This is to avoid various
              edge effects from damaging the result.  If EDGSKP >= 0,
              the inscribed ellipse of X radius NX/2-EDGSKP and Y
              radius NY/2-EDGSKP is cheked and all points outside it
              are given weight 0.0001.  (EDGSKP=0 -> 5 for the ellipse
              but the whole image is used otherwise.)  To include all
              pixels set EDGSKP=-1.

We typically use FLATN to stitch together multiple pointings of raster
observations made using the Arcminute MicroKelvin Imager (AMI) here in
Cambridge UK, setting the primary beam via PBPARMS.  Typical input values
are:

  PBPARM        0.1         1    -0.3251      0.4592    -0.02648   *rest 0    
  
  NFIELD        1 
  NMAPS        61
  IMSIZE      512         512
  COORDINA   *all 0
  COOREF      256         257
  ROTATE        0 
  REWEIGHT      0           0  
  WEIGHTIT      0 
  APARM      *all 0

We find that unless we set EDGSKP=-1, to avoid using the inscribed ellipse
weighting option, pixels outside the primary beam cutoff in the output map
are not blanked to zero, and can typically contain 'noise' significant
enough to make viewing the data of interest (which are 0.1-10 mJy) difficult.
Noise maps made with OPTYPE='NOIS' are particularly affected.

We'd like to report this issue in case is hasn't already been seen, and
there is an easy fix.  In the meantime, the workaround of setting EDGSKP=-1
solves the immediate problem for us.

Many thanks, and best regards.

David Titterington,

Mullard Radio Astronomy Observatory,
Cavendish Laboratory,
JJ Thomson Avenue,
Cambridge CB3 0HE, UK.

Tel: +44 1223 369103
Email: djt at mrao.cam.ac.uk