[vsnet-chat 1356] Re: Graham E-region standards

[aah@nofs.navy.mil]

Berto Monard asked about transformations from (V-R) to visual magnitudes.  I did some
calculations awhile back for Janet Mattei using the CIE response curves for photopic
(bright adapted eye) and scotopic (dark adapted eye) eyes, along with Johnson/Cousins
curves and the Gunn-Stryker spectrophotometric atlas.  The results were:

     P = V -0.001 -0.066(B-V)       = V -0.010 -0.073(V-R)
     S = V -0.006 +0.382(B-V)       = V +0.031 +0.448(V-R)
     
These relations are reasonably good for colors < 1.5.

Whose eyes are scotopic and whose are photopic?  Beats me, and I certainly don't want
to get into any arguments -- I'm not any kind of expert here.  The normally quoted
transformation is v = V +0.2(B-V), which falls inbetween these relations.  The main
point is that (B-V) and (V-R) both give similar transformations, and so both should
be ok for determining visual magnitudes from photometric magnitudes/colors.  The (V-R)
fit is somewhat worse since you are extrapolating to the bluer visual magnitudes (especially
the bluer dark adapted eye) rather than bracketing them, but R is certainly easier to
measure with most amateur CCDs than is B.

Arne

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