[Aavso-photometry] Color Index for Transform

[Massimiliano Martignoni]

Dear Arne

----- Original Message ----- 
From: "arne" <arne at aavso.org>
To: "Massimiliano Martignoni" <massimiliano.martignoni at alice.it>
Cc: <Aavso-photometry at mira.aavso.org>
Sent: Sunday, December 28, 2008 2:19 PM
Subject: Re: [Aavso-photometry] Color Index for Transform


> Massimiliano Martignoni wrote:
>> Dear Friends,
>> recently I started CCD photometry of variable stars using B V R I 
>> filters.
>> I find the transformations coefficients (T) for each filter using the 
>> AAVSO (Cohen) procedure, after I apply (T) to my instrumental magnitudes 
>> using the well-known formula: M = m+T(CI).
>> First and second order extintion are not important for my question and 
>> are here omitted from the above formula.
>> My problem is about the "right" Color Index to use.
>> Are correct the following equations?
>>
>> M(B)=m(B)+T(B-V)
>> M(V)=m(V)+T(B-V)
>> M(R)=m(R)+T(R-I)
>> M(I)=m(I)+T(R-I)
>>
>> Thank you very much in advance for any help!
>> Sorry for my poor english...
>>
> Hi Massimiliano,
> First, congratulations for considering the use of transformation
> coefficients.  I transform all of my data, and I will be promoting
> transformation to all of the AAVSO observers during the next year.
>
> These equations are simplified versions of the ones normally
> used.  The normal equations include a zeropoint term plus
> atmospheric extinction.  Normally, when performing differential
> photometry, those two missing terms cancel out since two stars
> in the same CCD image have the same zeropoint and nearly the same
> atmospheric extinction.  So, let's assume that you know what you
> are doing and are just choosing a color index for your transformation.

Yes, of course...
I simplified the procedure because my main purpose was to point out the 
question about color index...
It is also true that, as I am using stars on the same CCD frame, terms 
about ZP and first order extinction could be omitted...

>
> The normal guideline is that the color index you choose needs to
> include the wavelength range of your filter.  This is to remove
> as many systematics as possible - always interpolate, never
> extrapolate.  Your chosen color indices will work; the ones
> for B and V are pretty much standard. I prefer to use (V-Ic) as the
> color index for Rc and Ic.  (V-Ic) has more change with star color
> and so results in a better transformation.  For example, an A0 star
> has (V-Ic) = (Rc-Ic) = 0.00.  However, for a G2 star, (B-V) is
> about 0.63, while (Rc-Ic) is about 0.33, or half as large.  The
> transformation coefficients determined using (V-Ic) are more
> accurate since fitting something with wide range + error is
> easier than fitting something with smaller range + same error.
> (V-Ic) is also more sensitive to interstellar extinction.
> Using (Rc-Ic) means you use Rc, a bandpass that has Halpha in it,
> a very prominent line that can be in emission or absorption,
> causing an error in transformation.  This is very obvious with
> novae, where Halpha emission can dominate the continuum
> in the Rc bandpass.
>
> That being said, my use of (V-Ic) is a preference, not a rule.
> (Rc-Ic) will work.
> Arne

Thank you very much for your explanations! I will follow your guidelines...
Have an happy new year!

Massimiliano