[vsnet-chat 180] Re: V vs v

[bas@lowell.Lowell.Edu (Brian Skiff)]

     I can't speak for the AAVSO (I'm not even a member!), but it seems that
in the past they began using magnitudes corrected to the scotopic response,
which I think still appear on a number of charts, but then abandoned them
in favor of standard photoelectric V when they could get them.
     From a practical standpoint, the problem is actually measuring stars
acccurately in the "system" of the human dark-adapted response.  Because
of variations in eye sensitivity among individuals, and also observing
circumstances (bright surroundings versus "true-dark" sites), color
sensitivity varies considerably from person to person.  I'm assuming that
what most variable-star organizations have done is get standard V mags for
the comp stars when possible, and hope for the best.
     As more and more people begin using CCDs instead of doing visual
estimates, well-calibrated magnitudes (and colors!) will be needed.  The
best expression I've seen for the "visual" color response is:

mv = V + 0.2(B-V)

so that, for example, a star with a B-V color-index of 2.0 will appear about
0.4 mag fainter visually than in the standard V system.  I understand that
the coefficient can vary from around 0.15 up to as large as 0.35.

\Brian

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