color limits

[Arne Henden <aah@nofs.navy.mil>]

Michael R. correctly pointed out privately that my prior emails
indicate systematics in the Ic transform, but also indicate
-0.2 < (Bt-Vt) < 1.8 limits for all of the B,V,Rc,Ic transforms.
I know that the Ic systematics do limit the color range
over which the transform is good, but I did not indicate
what that limitation was.  Rather than introducing a
new limit at this stage, I would suggest using Michael's pared
Tycho file for photometric calibration as it currently
exists.  I will redo the transforms next week for another
iteration on the photometric reference catalog.
Arne