[daip] CALIB feature request.

[Craig Walker]

The VLBA flux scale calibration depends on the use of gains measured by 
the VLBA staff.  Those gains are not always correct.  The receiver might 
be new and good data might not be available.  There might be a 
systematic pointing problem in effect.  The focus/rotation settings 
might be wrong (recent experience at HN).  There might be snow in the 
antenna, which can affect gain and not Tsys.  There are many such 
mechanisms and a significant fraction of observations have one or more 
antennas subject to such issues.  The ANTUSE parameter is the mechanism 
(along with the elevation limit) to keep such antennas from degrading 
the absolute calibration.

But it is not always obvious when one antenna is deviant.  The only way 
I know is to look hard at the SN tables, and there is no good way to get 
an average value of the gain, constrained to be above the elevation 
cutoff.  I suspect that most users do not pay such close attention.  I 
had a case recently where one antenna was far off and the reason was not 
obvious - it did not show up in the Ts plots which look quite reasonable 
other than having high values.  I didn't get suspicious until I noticed 
that the source flux density was rather different from what I had been 
seeing in other similar observations.  LISTR showed that the gains for 
that antenna, which was in the ANTUSE list, were very different from the 
others.  When I removed it, the flux scale got much more reasonable.

I would like a mechanism for such situations to be obvious to the user. 
  You may have other ideas, but the simple one I can think of is to have 
CALIB write out the average gain for each antenna used in the 
normalization.  The would be the average gain above the elevation cutoff 
for each antenna in ANTUSE (or all if ANTUSE=0).  It would probably be a 
value for each IF.  This adds to the output, but it still will not come 
close to what IMAGR puts out while going through major cycles.  It could 
be optional if, say, VLA users don't want to see it.  I suspect that 
having such output will noticeably improve the flux calibration of the 
VLBA just because users will have better feedback and will start paying 
closer attention to the ANTUSE selection.

Cheers,

Craig




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     R. Craig Walker            Array Operations Center
     cwalker at nrao.edu           National Radio Astronomy Observatory
     Phone  505 835 7247        P. O. Box O
     Fax    505 835 7027        Socorro NM 87801   USA
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