[Aavso-photometry] Effect of aperture size

[Jim Roe]

arne wrote:

> A little more detail?  Are you saying that the differential magnitudes
> like (V-C) were offset by 0.3mag between the 6pixel radius and the
> 9 pixel radius aperture sizes?  This should not happen.  If you mean
> the instrumental magnitude like Vins or Cins, then those will change
> with aperture size since the star profile extends quite a distance from
> the peak pixel; usually larger apertures will give a brighter magnitude
> as you include more of the starlight.  If you mean that the (V-C) 
> differential
> magnitude differed by 0.3mag between apertures, then generally that can
> only happen if you include companion stars within the aperture, or
> if you are changing other things like the sky annulus simultaneously.

I went over the data and (V-C) in both cases was essentially the same. 
The only difference was that all of the instrumental magnitudes shifted 
by the same amount (0.273 mag fainter).  The only thing I could imagine 
that would cause this would be a different background level subtracting.

As I merely increased the aperture radius, the guard width stayed the 
same so the outer annulus increased in radius AND area.  Going back to 
the images and decreasing the guard band width to put the outer annulus 
at the same location eliminated the discrepancy.  HOORAY!  But what does 
that say about the uniformity of the background over pixel changes?  It 
seems to be uniform - I did the flat field corrections.  My images 
drifted a bit over the 2+ hour period and, at one point, I re-centered. 
  If there were a flat field problem, one would think it would show up 
more severely than a consistency to 3 mmag in the results?

AND that doesn't solve my essential problem in that how do I choose 
where to put the background sampling annulus for the rest of my 
measurements now and in the future (avoiding companion stars is 
obvious)? :-(  If I try several settings and get different results, how 
could I choose?

Still puzzled in Missouri

Jim Roe [ROE]