Earlier this year purchased a ZWO ASI1600mm Pro + 36mm 7-position filter-wheel complete with the LRGBNB filter set.
The ZWO filters represent very good value and I have had a lot of fun with them. However, of the set the Oiii is not the best performer, it produces distracting halos around bright stars and rings around dimmer ones.
I have learnt a technique to reduce the impact of halos during image processing but it is better not to have them in the first place if they can be avoided. Its asking a lot of budget filters so I had a look around, prepared to spend a lot more. But I noticed that the Baader filters had recently been updated with improved anti-reflection halo reducing layers (even claiming halo free). At £143 from FLO they are a similar cost to the ZWO filters when bought individually. I decided to give the Baader filter a trial to see how it lived up to the claim.
I fitted both the Baader and ZWO Oiii filters to the filter wheel for a side-by-side comparison. The optical train was ASI1600mm Pro, Filter-wheel, Lacerta KomakorrF4, Orion Optics 250mm F4.8 Newtonian. Autofocusing between filter changes was done with a ZWO EAF.
The camera gain was set @ 139, sub-frames were 120s. I took 34 subs with the ZWO Oiii and selected the best 10 frames to stack. I took over 100 frames with the Baader. Using the same quality selection criteria I had 38 frames, from which I took the first 10 for stacking for this comparison. I used the N.I.N.A autofocus routine with each filter change. All subs were taken during the same one-night session. Conditions during the night varied but because I restarted a few times, the conditions evened out for the two filters.
To conduct the test, I chose a new target for me but one with a reasonably bright star in the ROI. I chose the Bubble Nebula. Below are the stacked images with automatic stretch applied:
ZWO 7nm Oiii Narrow Band Filter
The ZWO filter gives us a very obvious halo around the bright star.
Baader 6.5nm Narrow Band OIII Filter – CMOS Optimised
The Baader filter also has a halo but it is very faint. Additionally, the star appears better defined suggesting less scatter overall. The fainter stars appear brighter than in the ZWO image. (More testing would be needed under stricter conditions to verify this).
Finally for comparison here is a closeup of the ZWO Ha subs (80 minutes integration). There is no halo apparent.
Conclusion
The new Baader 6.5nm Oiii filter offers a significant improvement in halos over the ZWO 7nm. There are still halos present in my particular setup, but the Baader halo is over 4 times fainter that the ZWO.
There is also the suggestion of less light being scattered, giving brighter stars and better definition in distributed objects like galaxies and nebula.
The Baader Oiii filter wins over the ZWO.