[Aavso-photometry] How can I beat the FLIP?

[Arne Henden]

Brad Walter wrote:
> Thanks for the suggestions Walt. I thought about the flat issue. I have
> not been rotating my light box. However, I made sure that I had the
> light box on the same orientation for both sets of flats and I have
> measured the center and 8 positions around the perimeter of the light
> box with a good hand help spot light meter and I can't detect a
> variation. Also when I pump up the contrast as far as it will go, the
> only gradient I see is from the moon. Since I rotated the camera 180
> when I made the flip, N, S, E & W  are in the same direction with regard
> to the chip, and the moon glow gradient was in the same place although
> it got worse as the night progressed.  
> 
Explain this more.  When you flipped, you rotated the camera
manually with respect to the telescope?
   This error sounds like flats to me.  You can further check by
obtaining the difference in magnitudes between pairs of stars on
your frame, comparing the results from flip/nonflip.
   The best way for precision photometry with a refractor is to
defocus.  That spreads the light over more pixels, permits higher
signal/noise in a single frame, permits longer exposures to average
out scintillation, and uses more pixels so that pixel-pixel variations
have a smaller influence.
Arne