[Aavso-photometry] Re: Calculating mag errors for filter bands from color indices

[Arne Henden]

Wolfgang Renz wrote:
> But the point that caused my question was due to the fact of having to use
> the SDSS r' mags and TWO color indices to get the z' and u' mags. This
> for sure introduces enlarged errors and error estimates for the z' and u'
> mags that are basically and essentially not neccessary. In general two
> transforms using neighbouring bands should give worse mags and error
> estimates than a single transform skipping the band in between. And
> having to combine a mag and two already "worse" color indices and
> error estimates gives even more worse mags and error estimates.
> 
> Doing this might be OK with (V-Ic) and (Rc-Ic) color indices as the V and
> Rc band are close together and overlap considerably to give low (V-Rc)
> errors (for non-very-red and non-blue stars). But the SDSS bands are well
> separated to not show this issue.
> So I was wondering why they choose this way to present their measurements
> for the SDSS Standard Star System. As Standard Stars are of much higher
> importance for photometrist than for astrophysicists I would have expected
> that they do it the way thats best for photometry to create secondary
> standards and to make all-sky photometry.
> 
> Wouldn't it have been better to give e.g. u'g'r'i'z' directly or (u'-r')(g'-r')r'(r'-i')(r'-z')
> instead of (u'-g')(g'-r')r'(r'-i')(i'-z') for a Standard Star System ?
> 
The colors are still essential for transformation determinations,
which is the primary use of such Standards.
   Perhaps the best would have been to list the individual magnitudes
along with the color indices, but that has not traditionally been done.
If you look at the actual SDSS dataset at
http://www.sdss.org
you will see that you obtain magnitudes and not colors from their
full database.  In addition, even though you have to use two indices
to get z' for example, since you are using multiple standards for this
determination, the errors get averaged out.  Perhaps it is not the best,
but I think you have to make concessions somewhere.
Arne