[Aavso-photometry] Banding in flats

[arne]

robertjmodic at att.net wrote:
> Does anyone know what causes the horizontal light and dark bands seen
> on the left side of this image?
> 
> http://chagrinvalleyastronomy.org/images/mflatsky1s-1-10Rc%20MinMaxClip.fts
> 
> This is a master twilight flat made with a Rc filter.
> 
> The banding is present all the time with all my filters and with light 
> box flats as well.
> 
> I read somewhere that the banding may be an interference pattern caused by
> the coverslip not being parallel to the CCD.
> 
> The reason I ask about the banding is that it does not always flat out 
> properly.
> I have already checked the scope for and eliminated any scattered light 
> in the system.
> 
I don't see any horizontal light and dark bands on the left side
of this image, at least at the default stretch of iraf/ds9.  This
looks like a pretty typical flat to me.  Usually interference noise
is out of sync with readout rates, and such external interference
will show up as diagonal banding.  Anything that mimics the pixel
structure horizontally or vertically usually is inherent in the chip
or the readout electronics.  If you want to post a jpeg from your
display, where you have the image stretched so that you can see
the pattern, that would be helpful to me, though I'm guessing that
it is at such low level that the eye can pick it out but normal
software, such as aperture photometry, won't notice it.
Arne