[Aavso-photometry] photometry variation with measurement aperture

[Walt Cooney]

Folks,

I could use a little advice.   I am doing some standardized photometry on DK CVn
and have found I can move the magnitudes up or down by a few hundredths
depending on what measurement aperture I use.  Clearly that's bad for
standardized photometry.  Aperture size appears to be an optimization between
S/N for seeing the trend behavior vs. producing data with good absolute
accuracy.

I'd like to understand why it matters.  If you carve off the top of the light
curve distribution function, I don't see why is should matter whether you carve
off and measure more or less as long as you carve off and measure the same for
each star.  It can't (?) be a problem with few pixel statistics because that
would show up as random noise.

I'd appreciate some thoughts on this.

Clearest skies,
Walt Cooney