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

[Brad Walter]

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.  

-----Original Message-----
From: Walt Cooney [mailto:waltc at cox.net] 
Sent: Tuesday, December 28, 2004 6:19 PM
To: Brad Walter; aavso-photometry at mira.aavso.org
Subject: RE: [Aavso-photometry] How can I beat the FLIP?


Brad,

An east/west gradient across your flats can give you a gem-flip
discontinuity in a light curve.  Are you taking twilight flats by any
chance?  Take a twilight flat and divide it by a dawn flat and see if
you get a gradient. If you are taking light box flats, divide flats
taken with the box rotated 90 degrees and 180 degrees.

Clearest skies,
Walt Cooney


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