[Aavso-photometry] Effect of aperture size

[arne]

Jim Roe wrote:
> I measured a transit of GJ436 in B.  The light curve was beautiful 
> (IMHO), see
> 
> <http://brucegary.net/AXA/GJ436/gj436.htm#Light_Curves>
> 
> BUT, there was a discrepancy in the depth of the transit of some 3 mmag 
> (compared to earlier results by lots of observers) so I re-measured the 
> images using different aperture sizes (I use MaximDL). I have taken to 
> using two or more reference stars (ensemble) and looking at their 
> standard deviation over the run to estimate errors.  But I also compare 
> their average to the photometry to catch gross errors (like wrong star 
> identification - which I've done on occasion).
> 
> The original set used a radius of 6 pixels.  The standard deviation of 
> the two reference stars over a 2 hour plus run was 3 mmag and their 
> averages were very close to the photometry (I am not transforming the 
> data - no time for an extra filter during the run).
> 
> The results with a 7 pixel radius were essentially identical to the 6 
> pixel values.  But the 8, 9 and 10 pixel results were significantly 
> different - not in the standard deviations which were still 3 mmag, but 
> in the averages of the target and the reference stars.  These all 
> increased (in magnitude) by about 0.3 mag.  (The three results for 8, 9 
> and 10 pixels were essentially identical.)
> 
> The depth of the transit dropped by the discrepant 3 mmag but I'm 
> concerned about the 0.3 mag change in the averages (which put the 
> reference stars significantly off from the photometry).  For "normal" 
> reporting of magnitudes to the AAVSO, how would one choose between such 
> different results depending upon the aperture size used to reduce the data?
> 
> (Note:  the star images over the 2 hour plus time varied with seeing and 
> wind gusts but the air mass change was very little.)
> 
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.
Arne