[Aavso-photometry] BVRI vs. griz

[arne]

Tom Krajci wrote:
> Why did the pro's make the switch from BVRI to griz?
> 
> Is there an astrophysics advantage?  (Better, more accurate 
> classification/ID of stars because the bandpasses don't overlap?)
> 
> Is it a better match for typical CCD response?
> 
> Other?
> 
The pros switched because SDSS happened. :-)
Seriously, SDSS used these specific filters for discrimination
between stars and galaxies, since it was a large-scale structure
project and needed to place its spectroscopic fibers efficiently
on galaxies.  In addition, Jim Gunn was the primary architect
for SDSS, and he designed these filters many years earlier;
people tend to use what they know.

Since SDSS measured some 230 million sources, it is the "king"
as far as photometric surveys are concerned.  For stellar work,
you just need broad-band magnitudes of some sort in order to
classify objects, so SDSS works essentially as well as Johnson/Cousins.
Since much of the big telescope time is used for galaxies,
they have Sloan filters at those observatories, and it is
more convenient to stick with that filter set than to change
them out every time someone wants to do stellar work.

There is still a lot of professional Johnson/Cousins imaging
taking place, so it is by no means a complete changeover in
the professional world.  The trend is certainly towards
Sloan, however, especially now that inexpensive filters
are available.  APASS will measure stars in both Johnson
and Sloan filters as a transition survey (though only for about
50 million objects).
Arne