Walter Leonhard Schramböck:
I had this problem with two of my dual narrowband filters: huge halos around some stars, although I noticed that the affected stars are not always the visually brightest in the image. I also noticed in the last series of photographs that it seems to be particularly bad for stars that shine strongly in the red spectrum. My suspicion: the filter coatings have a problem in the infrared spectrum, something unpleasant is happening that leads to this false light - whatever it is.
My two affected filter candidates are: the Optolong L-extreme and the dual narrowband filter from ZWO.
As a test, I installed a UV/IR-cut filter from Baader in front of the dual narrow-band filter from ZWO, and the halos were actually completely eliminated.
So if you also have this problem, just try it out and see if it helps.
I have to mention: this is happening at f2 with my Samyang 135mm Lens. Probably f2 is amplifying any halo-related filter-problems.
I released both filters from it's housing and installed both as a stack in the holder from baader because there is more space. UV/IR-cut filter at the scope-side, dual-narrowband at the sensor-side.
It worked perfectly well for me, what do you think? Does the light-transmission suffer significantly with this approach?
Picture shows the installed filter-stack.
Had a similar issue with my Samayng lens and used a similar approach but instead with a Hoya 55mm filter in front of the lens that also acts as an aperture to bring the lens closer to F/2.8 without diffraction spikes.
Worked well enough for me but had similar questions about stacking filters and could not really get to a satisfying answer yet.
