I'm just trying to understand the halo problem with the ZWO ASI 1600MM-C.
Previously I've observed halos around brighter stars when using the ZWO narrowband filters. However, last night I noticed a pattern as per the attached image. (This is a single 5 min sub simply stretched heavily in PI with no other processing - so sorry about the excessive noise - and shown at about 3:1 size).
What is odd is the Airy disk pattern. Previously the halos have been pretty much disks with no internal structure. If anything, this is worse as it's kind of distracting to the eye. I'm guessing this may have happened as the seeing seemed quite good last night (FWHM down to a smidge over 2" for some subs) so the structures survived the 5 min exposure - that'd be right …. you can't win in this game!!
I have seen quite a bit of debate on the halos and whether better filters might fix it, but there doesn't seem to be consensus. And given this Airy pattern I'm not too sure this is necessarily the same issue. And there's possibly the other issue of people seeing patterns from the microlenses in their images - which I've yet to see.
My questions are
- what is causing this?
- would Astrodon filters fix/mitigate the issue?
- any processing tips to reduce the effect?
I've seen lots of discussion about the lack of AR coating on the sensor glass, but the problem has only (so far!) reared its head when using narrowband filters, so it seems to be either the filter or an interaction of(reflection between) the filter with e.g. the AR-less glass. Hence my question of whether Astrodons might help - but I've seen inconsistent reports from people who've used Astrodons. Perhaps because it depends on other factors like f ratios, filter distances, etc. Any ideas please??
Thanks in advance for any advice/comment.
Paul
P.S. I think that single star is about mag 7. The other image is mag 5 or so. This is taken with an RC8 at focal length about 1200mm after reduction (f6 effective).
The filters are the ZWO 1.25" mounted ones.
P.P.S. Sorry. That was a bit verbose!
