[vsnet-be 50] Re: Theta CrB Comparison Stars

[Thom Gandet <tgandet@mindspring.com>]

Erhan and Taichi,

    The table below gives the differential extinctions (in magnitudes)
between Theta CrB and each comparison star that has been used.  Theta
CrB was assumed to be at 3h30m hour angle, or ZD=43 the hour angle
was chosen so that the largest zenith distance of any of the stars
is just under 60

    The differential extinction is in the second column, and the
zenith distance in degrees is in the third column.  A pessimistic
extinction coefficient, k, of 0.5 was used; for k=0.2 (a good site),
the differential extinction is about half the tabular values.

    The fourth column lists the difference, V-v, between V and the
"visual magnitude" computed using the expression V-v = -0.21*(B-V);
column five gives the difference between V-v for Theta Crb and each
of the comparison stars ([V-v](Theta) - [V-v](comp)); nominal values
of V=4.14 and B-V=-0.13 for The CrB were used.

         Eps Her   0.3   59 +0.00  +0.03   
         Iot Ser   0.1   51 -0.01  +0.04
         Chi Boo   0.0   40 -0.01  +0.04
         Eta CrB   0.0   41 -0.12  +0.15
         Bet CrB   0.1   43 -0.06  +0.09

    The greatest differential extinction is for Eps Her, and the
largest color difference is for Eta Crb.  If Eps Her is used as
a comparison star, differential extinction corrections ought to
be applied to the visual estimates.


Cordially,
Thom Gandet


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                      Thomas L. Gandet, Director
                            PO Box 77021                   
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