[vsnet-chat 2044] Re: Supernovae estimates using USNO SA2.0

[aah@nofs.navy.mil]

A-xxx and SA-xxx are exactly the same as far as positions
and magnitudes are concerned.  SA is just a subset of A.
  There are differences between A-1.0 and A-2.0.  While
the astrometry is definitely better with A-2.0, it is not
clear that the photometry improved.  Wait for Brian's
modification of A-2.0, as I hope he will be able to get
the photometry on a consistent system.
  The only systematic difference that I could see between
reporting SNe magnitudes with SA rather than USNO-A is that,
for reasonably dense fields, Dave used an algorithm for SA
that selected stars within a specific magnitude and color
range if a choice was possible.  That might slant CCD-unfiltered
magnitudes using SA comparison stars, as compared to random
stars chosen out of USNO-A. That should be a very small
effect compared with the zero point and color errors
inherent in either catalog.
Arne

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