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Re: computing I-mag from V-mag, how to?

Subject: Re: computing I-mag from V-mag, how to?
From: Paul Schlyter
Date: Sat, 18 Mar 2006 09:43:13 GMT
Newsgroups: sci.astro
In article <1142650864.146480.44650@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Yegor <yegorplam@xxxxxxxxxxxxx> wrote:

> We are interested in star I magnitudes.  Hipparcus catalog has
> V-magnitudes.  Is it possible to compute I-magnitudes from
> V-magnitudes?

No, you need to know more than just the V magnitude.  Different
stars with the same V magnitude can have quite different I
magnitudes.

> Is that what the V-I mag field is for?

Yep -- that's the extra information you need!

> If so, how to use it?

Remember your high-school algebra?  Let's mess a little with that
to find out the answer to your question:

   V - I  =  (V-I)

   I - V  =  -(V-I)

   I  =  V - (V-I)

Got it?
 

> Also, what is the ratio of red stars to blue stars (or a good web
> resources to find out)?

There are many kinds of rations - what kind of ratio are you
interested in?

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