[vsnet-chat 2163] Re: EV Aqr again

[aah@nofs.navy.mil]

Bruce's sequence and comments about the field
photometry file for evaqr:
 http://ftp.nofs.navy.mil/pub/outgoing/aah/sequence/evaqr.dat
should drive home the reason why I observe a field
three or more times during an interval of a month or more
before I consider the field photometry reliable.  Here are
the three nights of data for the GSC star:

yymmdd     HJD       V      B-V    verr   bverr
981018  51104.6813 12.351  1.500   0.002  0.001
981019  51105.7201 12.340  1.509   0.002  0.002
980622  51351.9233 12.640  1.442   0.002  0.003

The first two measures showed no variation outside of normal
photometric error (the 2millimag error above is unrealistic
based on a single datum).  However, the third measure was eight
months later and the red star had obviously changed
in magnitude.  If you just observe a field once and consider
that you have enough photometry to derive a sequence, you
risk both a zero point offset (depending on your nightly
calibration) and the inclusion of variables.
Arne

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