[Aavso-photometry] Differential transformation

[Geir Klingenberg]

OK, that makes perfectly sense. I will experiment a bit with the
transformation calculations to investigate the effect of the different
methods.

Thank you all for your help

Geir


On Thu, 30 Dec 2004 08:54:50 -0700, Arne Henden <aah at nofs.navy.mil> wrote:
> Geir Klingenberg wrote:
> > Arne and Chuck - I'm sorry I keep on nagging but I'm not sure I get
> > it. How does extinction affect the results as long as I don't mix
> > stars from different fields (that is for v1 - v2 both star 1 and 2 is
> > from the same image)? I understand that there can be extinction
> > effects between the top and bottom of an image but in that respect
> > there's no difference between using one or many fields.
> > 
> The point is that each field is unique as far as the extinction
> difference from top to bottom.  If you are only looking at Landolt
> standard fields, and only when they transit at your site, and they
> transit at airmass 1.2 or such, then you have consistent error in each
> frame, the size of the error depending on the size of your field.
> However, if you look at some standard fields at airmass 1.0 and others
> at airmass 2.0, then you can get some quite large errors.  The fact
> is that the full equation includes extinction, and the difference
> equation needs to also include extinction.  You can use a mean extinction
> coefficient to remove the effect to first order.
> 
> > I agree with Radu that there are pros and cons here. On one hand it is
> > important to get stars with great color variation and also high SNR.
> > But that is difficult in crowded fields and I get only a handful of
> > "good" stars from M67 and NGC 7790 due to my large FOV. So it makes
> > sense to pick the best from many fields.
> > 
> If you have a large FOV, there are several Landolt "fields" that include
> numerous standards.  For example, look at Selected Areas 110 and 111.
> 
> > On the other hand one should account for extinction effects. But the
> > error should not be too large if one stays at low arimasses. And it
> > might be that this is a lesser evil than getting extinction
> > coefficients with large errors due to a not-so-photometric site.
> > Errors in the extinction coefficients will of course in turn affect
> > the transformation coefficients.
> > 
> I think the differential approach has validity, just don't apply it
> blindly and use true Standards.
> Arne
> 
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