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Its unlikely its between the filters and the CCD chamber window (which is antireflection coated fused quartz on a FLI Proline). It would show in other images on the AP scope which it hasn't.
It is more likely to be between the filters and the corrector.
When 16803 cameras first hit the streets from Apogee U16M and FLI Proline 16803 (the first 2) there was a lot of internet posting about reflections visible in flats.
It boiled down to reflections between the filters and the correctors/flatteners.
The chip is so large that basically every scope needs a corrector/flattener for it to work. This then showed up the high level of reflectance off of the then various filters.
What happened is you had most of the filter manufacturers come out with new antireflection coatings on their filters. Apogee added an aperture mask to their camera as well. Most blacken the edge of their filters with matt black paint to further reduce reflections. Square filters can have uneven edges that cause reflections.
As Mike has not had trouble with his antireflection coated Astronomiks Mark 11 filters I suggest his O111 or S11 or even Ha are not antireflection coated and hence the reflections.
If you used O111 S11 and Ha filters that are coated this most likely would go away.
It is almost certainly reflections off the narrowband filters (which look like mirrors) back and forth off the rearmost corrector lens.
By the way is the corrector AR coated and if so is the rear facing element AR coated?
Not really that big of a deal really. If it were in your LRGB images then it would be.
Greg.
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