[Aavso-photometry] B-V from V-I

[James Bedient]

So the question I have: Is assuming dwarf stars and calculating U or Rc or I
like that an acceptable way to operate for routine, patrol-type photometry?

For example, I was working through some faint CVs last night, and I stuck to B
and V since I had V magnitude comps and B-V colors.  I could easily have run the
gamut through UBVRcI, but didn't, since I lacked a sequence.

OK, I know, I could bite the bullet and shoot standard fields, but that
resembles... work.  :-)
You all know how I hate that.

Jim

----- Original Message ----- 
From: "Arne Henden" <aah at nofs.navy.mil>
To: <aavso-photometry at aavso.org>
Sent: Sunday, February 20, 2005 12:10 PM
Subject: Re: [Aavso-photometry] B-V from V-I


> There are differences between colors of giants and dwarfs, but
> primarily at the cool end.  The giants have lower gravity and need
> lower temperature to match the same spectral type of dwarfs,
> so the colors of "cool" (say, G/K) giants are typically redder.
> Arne
>
> Michael Koppelman wrote:
> > Great, thank you.
> >
> > An implicit question hidden in my question was: should this really
> > depend on what kind of star it is? Do we start getting complications
> > from emission and absorption on some stars or something? Seems like a
> > blackbody should pretty well approximate most stars when talking about
> > these fairly large bandpasses.
> >
> > Thanks!
> >
> > Cheers,
> > Michael Koppelman
> > http://www.lolife.com/astronomy/
> >
> > On Feb 20, 2005, at 11:19 AM, Arne Henden wrote:
> >
> >> For dwarfs, the relation is approximately
> >> (B-V) = -0.07 + 0.88*(V-I)
> >>
> >> In other words, (B-V) is usually slightly smaller than (V-I).
> >
> >
> >
> >
>
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