Too often I've run into sequences which seem to be internally inconsistent. By that I mean that the reported value of the check star (using VPHOT in all cases discussed here) - which is itself a sequence star - is often off by a half magnitude or more. I depends on the comp star(s) chosen which led me to produce the little table below from lasts night's measurements in the field of V642 Cas.
The values at the head of each column are the sequence stars used as a comp star when the others are used as check stars.
The rows are headed by the check star being measured and the values in the row are the (measured value - catalog value) for each comp star.
At first blush, it would appear best to use 110 and 112 as comp and check stars (in either order) but how are we to understand that either of these misses the value of 116 by a half magnitude if all of the stars have "good" photometry? Using more than one sequence stars in an ensemble leads to big uncertainties in any of the reported targets - which is what led me to this investigation as I was trying to figure out how to improve my reported errors (S/N is NOT a problem).
Jim Roe [ROE]