[Aavso-photometry] Differential transformation

[Arne Henden]

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