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I have a bunch of twilight flats that have been preprocessed (dark, bias, overscan etc.) and need to remove the additive fringing from the (multiplicative) twilights. I normalized each one by dividing by the mean and then combined them using ccdproc in astropy. I then divided the normalized flats by this "new" flat and viewed them in DS9 but I still cannot see the fringing. I know it's there because I've seen it clearly before. I could use some helpful tips. TIA

PS I also tried in old skool Iraf and don't see anything obvious.

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    $\begingroup$ Note that the fringing amplitude can change depending on the strength of telluric emission lines. A twilight flat will not necessarily show the same fringing pattern as fields taken in the middle of the night. Though maybe this is not too problematic in the i-band. You can always median stack all your night time images to see the fringe pattern, after scaling by the median sky background in each frame. $\endgroup$ – ProfRob Jun 6 '20 at 22:58
  • $\begingroup$ I've just asked What does “additive fringing from the (multiplicative) twilights” mean? $\endgroup$ – uhoh Jun 7 '20 at 0:18
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Fringing is normally very difficult to see in twilight sky flats with broadband filters (the i filter is 100 nm wide).

The fringing is caused by thin-film interference at particular wavelengths defined by spectral lines (OH lines in the i-band) in the the light from the sky.

At twilight, the sky background is completely dominated by scattered sunlight, which does not contain these lines and therefore the fringing pattern is weak and/or may have different characteristics to the fringing obtained in dark, night sky observations when the background (at red/IR wavelengths) is dominated by narrow spectral lines

The usual way to define the fringing pattern is by median stacking night-time observations (moonless) after scaling for the strength of the sky background (or the exposure time).

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"oilslick" fringing pattern - subtle but there

I was finally able to make the fringing visible by block averaging after taking the steps above. If anyone else has any helpful tips I'd love to hear them!

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