Quote:
Originally Posted by sjastro
I have never heard of a g-r colour index.
Steven
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Just the g magnitude of an object minus its r magnitude, just as in B-V or V minus R etc.
Color indices using B and V and R and I magnitudes were the traditional ones, developed from photoelectric photometry.
A strongly positive color index usually indicates a red object.
But since the Sloan Digital Sky Survey, the ugriz system has become
very popular in the literature;
Here is an example of the transmission curves of the u and g and r and i and z photometric filters ::
http://www1.cadc-ccda.hia-iha.nrc-cn...a/filters.html
The traditional UBVRI magnitudes and filters are briefly compared with the ugriz filterset, in this review paper:
http://www.astrohandbook.com/ch02/be...otosystems.pdf
(This is also a good general reference on various filters used in photometry)
Essentially, a color-magnitude diagram of a cluster or galaxy using B minus V for the colour index is often very similar in appearnce to that using g minus r for the colour index.
To follow the current literature, it is getting ever more necessary to get a good appreciation of u and g and r and i magnitudes ; they are not that different from the standard UBVRI magnitudes.