Old 20-05-2018, 05:08 PM
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DiscoDuck (Paul)
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Airy disk halos

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.


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!
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Old 20-05-2018, 05:19 PM
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Atmos (Colin)
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The "halo" issue usually seen with the ASI1600 is different in that it is a diffraction pattern interaction with the micro-lenses and the lack of an AR coating.

What you're seeing here is different to that. I'd be more inclined to say that it is a filter issue and if that is the case then Astrodons would be more likely to fix the issue.
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Old 20-05-2018, 05:19 PM
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Camelopardalis (Dunk)
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Paul, I use the Astronomik 6nm filters and don't recall seeing this issue, but it's been a few months since I did any narrowband (QLD summer weather!). I'll see what I can dig up.

Regarding the lack of AR glass on the sensor, I've only seen that anomaly twice in several years of imaging with a 1600, most notably on Alnitak when imaging the Horsehead....but it doesn't show up with halos like you're seeing (although I use a refractor, so no diffraction spikes), but more of a square matrix effect. And certainly not on stars anywhere near the magnitude you're describing (but again note I use a small-ish refractor).

Incidentally, I find the cost of the Astrodons shipped into Australia prohibitive compared to the competition, but every pocket is different...
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Old 21-05-2018, 10:35 AM
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DiscoDuck (Paul)
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Thanks for the replies Colin and Dunk. Sounds like a filter upgrade may help. Plus the other benefits a better filter would give as well of course in terms of narrow bandwidth and better transmission.

Dunk, I agree the Astrodons are ambitiously priced. If I do upgrade, it'll be over a long period one filter at a time!
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