[Aavso-photometry] transformations

[arne]

Steven Orlando wrote:
> Arne,
> 
> You mentioned catalogues, which current catalog is the best for 
> photometry? Which catalogues currently being developed will the best for 
> photometry?
> 
> I know that for some catalogues, like Tycho-2 and 2-MASS, there are 
> conversion formulas to convert to the Standard BVRI system.
> 
There is no perfect catalog out there; all are compromises when it
comes to photometry.  You pick one from column A, the next from column B.
The two all-sky catalogs that are best are Tycho2 (basically B&V down
to perhaps 10th magnitude) and 2MASS (JHK, all sky).  All professionals
use 2MASS for their NIR photometry, and its great.  For optical work,
though, it is much harder.  I'm using a dozen or so catalogs for
the comparison star database, with logic to select between them
based on their limitations.  We hope to rectify that situation in
a few months with the Ellijay survey.  There are other future surveys
underway, or nearly so, such as PanStarrs and SkyMapper, with the situation
improving in a few years.

So if you are currently looking for a V magnitude of a star, I'd:
(1) use Tycho2 and conversion formulae if the star is brighter than 10th
(2) use SDSS and conversion formulae if the star is fainter than 14th
(3) use ASAS in between

Note that the latter two catalogs do not cover the entire sky,
and have other limitations, so this simple solution will fail quite often.
Arne