Re: Photometry

[Arne Henden <aah@nofs.navy.mil>]

Stupendous Man wrote:

> 
>   Right.  If you _are_ interested in changes of a single star, then 
> I would bet large amounts of money that you could take the measurements
> Tom has made (selecting them from a database, say), and improve
> them by
> 
>      A) selecting magnitudes for surrounding stars in each image
>      B) building an ensemble of these surrounding stars
>      C) calculating a magnitude difference (target - ensemble)
>                for each image
> 
>   This will (I believe) yield much more precise differential measurements,
> at the expense of moving them off the standard scale in some small way.
> 
Yes; this is what I did with the Mark III processing suite, and is
also the technique we used with the FASTT variables.  What happens with
wide-field cameras at sites like Tom's is that you rarely get really
photometric nights.  While the all-field calibration gets a rough zeropoint,
you can get extinction variation, primarily due to clouds, across a 4-degree
field.  In addition, you may not do a good job of flatfielding/postcorrection,
so night-to-night variations may be important since the star will rarely
end up at the same location on the chip.  Bottom line:  if you want
precise photometry of some variable, use local comparison stars.
   I've promised an improved Tycho -> BVRI catalog, and have stated several
times that ARNE, ASAS, etc. will end up giving reasonable quality photometry
over the entire sky.  You should be prepared to redo the photometric
calibration at some future time, which is why I keep harping on keeping
the astrometry and photometry calibrations separate.
Arne