[Aavso-photometry] Differential transformation

[Chuck Pullen]

Hi Radu - I think that Geir was trying to not have to do any 
extinction/airmass calculations. Yes, you can use stars from all over the 
sky for your transformation determinations, but if they are at different 
airmass you'll have to calculate your extinction, and have a truly 
photometic site and night.  Doing them from one field is nice for many of 
us who don't have the best mountain top environments!

Thanks for sharing your web site.  I'll check it out.

Chuck


At 22:33 12/28/04 +0200, Radu Corlan wrote:

>On Tue, 28 Dec 2004, Chuck Pullen wrote:
>
> > Geir - in an ideal world, your approach would be perfect.  However, for
> > determining transformation coefficients, you want a wide range of color as
> > well as stars of about the same magnitudes so they have about the same
> > signal to noise across the color range.  And, of course, the stars need to
> > be non variable, and calibrated against a known data set, such as Landolt
> > fields.  So, the odds of finding all of these requirements in your 15' by
> > 15' image for a given part of the sky are pretty slim!
>
>Chuck - one can use Geir's approach and combine stars of different colors
>from different fields, they don't need to be all on one field. If you have
>enough stars with good photometry (for instance some of Arne's sequences,
>which are spread across the sky) the errors reduce pretty well.
>
>I use that approach (see http://astro.corlan.net/gcx/html/node9.html ) for
>a description and some example graphs.
>
>Radu