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Some years ago I posted on "AAVSO-photometry" a question about the "right" color index to use in the equations of the trasformation coefficents of BVRcIc filters.

This was the reply: 12/28/08….. Arne to aavso-photometry group “RE Transformation Coefficients-Color Index’s 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

Since that time I'm using the following equations for transformation (simplified without extinction and zero point not important now):

B(std) = b(instr) + T(b)*(B-V)

V(std) = v(instr) + T(v)*(B-V)

Rc(std) = r(instr) + T(r)*(V-Ic)

Ic(std) = i(instr) + T(i)*(V-Ic)

Now I have another question: given the above formulas, which are the best definitions of transformation coefficients?

Tb = slope of (B-b) plotted versus (B-V) should be ok;

Tv = slope of (V-v) plotted versus (V-Rc) or versus (B-V)? Why?

Tr = slope of (R-r) plotted versus (Rc-Ic) or versus (V-Ic)? Why?

Ti = slope of (I-i) plotted versus (V-Ic)?

Again, if I'm using BVIc filters (without Rc), then:

B(std) = b(instr) + T(b)*(B-V)

V(std) = v(instr) + T(v)*(B-V) or (V-Ic)? Why?

Ic(std) = i(instr) + T(i)*(V-Ic)

and, in this case,

Tv = slope of (V-v) plotted versus (B-V) or versus (V-Ic)?

Maybe to much questions... Sorry, but I have found different solutions on this topic and I am bit confused. Thank you very much in advance for any help!

Massimiliano (Max) MMN

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Last Updated: May 20, 2014 - 12:38pm

Last Updated: May 20, 2014 - 12:38pm

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