[Aavso-photometry] Differential transformation

[Radu Corlan]

On Tue, 28 Dec 2004, Chuck Pullen wrote:

> 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!

Chuck,

The idea is that the tranformation coefficients are not affected by 
extinction (to the first order at lest), and they can be fitted from 
different fields using only differential photometry (so no asumptions are 
made on the extinction). You need to have stars of different colors in 
each field, but by combining different fields, the spread need not be as 
large as when using a single field. Reduction is a bit more involved, but 
that's why we have computers ;-).

Radu

> 
> 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
> 
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Radu Corlan       Snail Mail: Bucuresti sect. 1, 
rcorlan at pcnet.ro  str. Argentina nr. 28, Romania

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