[CCD-astrometry-photometry] RE: [Aavso-photometry] photometry variation with measurement aperture

[arne]

Walt Cooney wrote:

> I did not include some of the other info you asked about.  The maximum 
> pixel
> values for the comp star and variable were around 5000-6000.  I have a 
> 16 bit
> AP-7p so I was well short of half well depth but not rediculuously 
> underexposed
> either.  The net count which I presume to mean measurement aperture 
> minus sky
> ranged from 60,000 to 90,000 for a measurement aperture of 6 radii.
> "Background" as MIRA calls the average sky background was around 1500.

Walt further replied that the background was "per pixel."  Since the 
peak pixel values
for the comp and variable were 6000-ish, this means that sky becomes 
very important once
you are more than a couple of pixels from the star center.  However, 
while a mismeasurement
of sky would affect the results for a single image, this should be 
random over the entire
image set.  Since each image has a systematic trend (brighter variable 
with smaller aperture),
my guess is that the comparison star has a companion.  As you shrink the 
measuring
aperture, the comp star companion is removed, making the comp star 
fainter and the
variable effectively brighter.
  Note, however, that Walt's light curve does improve as he moves to 
smaller apertures,
indicative of less sky background and therefore better signal/noise.
Arne