Re: puzzle in I-band photometry on Data Disks 18

[aah@nofs.navy.mil]

Scattered light is a typical cause of such an effect, since
you combine additive and multiplicative corrections into
the flatfield.  Flats look flat, images look flat since
the sky background gradient is removed, but you have
changed the effective gain across the chip.
  Why this occurs in the I camera and not the V camera
is an interesting question.  My guess is a red light
source somewhere nearby.
Arne