Comparison of ZWO 7nm Oiii (2018 version) versus Baader 6.5nm Narrow Band OIII Filter – CMOS Optimised (2021)

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

ZWO 7nm Oiii 36mm filter

The ZWO filter gives us a very obvious halo around the bright star.

ZWO 7nm gives a bright halo after 20 minutes of integration

Baader 6.5nm Narrow Band OIII Filter – CMOS Optimised

Baader 6.5nm Oiii

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).

Baader 6.5nm – Very dim halo after a 20-minute integration.
Baader 6.5nm – Even after 80 minutes integration the halo is still faint and remains fainter than the halo produced by the ZWO filter 20-minutes of integration.

Finally for comparison here is a closeup of the ZWO Ha subs (80 minutes integration). There is no halo apparent.

ZWO Ha 40 subs stacked. No halo to worry about.


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.

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The Crescent Nebula NGC6888

This is was my first image of the 2021 season after a long break. It’s the result of two nights of roughly 1hr total for LRGB, 2hrs Ha and 5hrs Oiii. 

I’ve been practicing with methods to combine the images…

LRGB subs were 30sec @ gain 0, Ha and Oiii susb were 120sec @ gain 200. ASi1600mm pro and filter set, scope was the OO250 newt. Even at 30 seconds and 0 gain I still manage to clip some stars.

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The Wizard Nebula NGC7380 in HOO

NGC7380 The Wizard Nebula

This is the Wizard Nebula. I’m told it is supposed to look like a wizard in a pointed hat and gown with open arms. I imaged this over two nights under very bad weather conditions and a bright moon.

I attempted and abandoned some S2 – the signal was weak and too many subs were trashed. At the end of the two days I applied more than my usual rigor to the frame selection process and ended up with just 100mins each of Ha and O3.

Stacking just the best subs didn’t change the star sizes but the fainter ones were sharper, over-all contrast, range and noise was improved (so spending the time up-front to select good frames is really worth it). O3 was still weak but I decided not to push it too hard.  I applied the halo reduction technique for the O3 described here it works surprisingly well.

I’ve combined the signals as R=Ha, G= 0.3*Ha +0.7*O3, B= O3. The central blue area of the nebula was enhanced using an O3 range mask and just increasing the blue saturation a little. This was quite tricky to process and easy to overdo.

After imaging this nebula, I’ve purchased a replacement O3 filter. For future narrow-band projects I’ll be swapping out the notorious-for-halos ZWO 7nm O3 filter for the Baader 6.5nm (new for 2021) O3 filter that is supposed to have much improved antireflection and halo reduction coatings. I plan to do a side-by-side comparison in a mini-review just to see if the Baader filter is worth it in my setup.

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The Cocoon Nebula IC5146

After cleaning and correctly(!) fitting my LRGB filters I decided to go a for a bright broadband target. Clouds, moon and poor seeing did not help but I managed about 45 mins each for RGB and Ha – shot with the ASI1600mm. This is tight cropped as I combined Ha from two sessions and the framing wasn’t very well repeated. (I really must work out a way to index my camera rotation.)

This is the first full imaging session since changing the RA and DEC boards of the CEM70G. The resolving power (Dawes-limit) for the scope is about 0.45 arc seconds. At this focal length the ASI1600mm pixel size is 0.65 arc seconds. This camera is basically working at the limits of the scope. If you pixel-peep this image even with the poor seeing you can tell the CEM70G is now getting the most out of the combination. An OAG may give me fewer reject subs but I am very happy using my 60mm guidescope +ASI290 guide camera.

Aperture 250mm /9.84 inches
Maximum Magnification :500x
Resolving Limit (Dawes): :0.46″ arcseconds
Native Focal Length / F: 1200mm / F4.8
Ideal Resolution : 0.67″ – 2″ arcseconds
Resolution 0.65″ arcseconds per pixel
SamplingSlight Oversampling ✅

Camera Chip FOV 0.84° x 0.64°

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The Andromeda Galaxy M31

This is the result of 3hrs and 50 minutes of 5min subs @ ISO800 using my resurrected Canon 550d, Optolong L-pro, Altair 70mm EDT-F on my trusty HEQ5pro.

I’m pretty happy with picking up the star bridge between M31 and M110, I noticed that disturbance even carries on past M110 if I over-stretch it.

More integration time would smooth out the outer regions and perhaps even pick up some of the blue/pink cloudy zones but I may leave it there to focus on M33 which I have never imaged before.

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Now take my Soul, IC1848 the Soul nebula in SHO

Right next to the Heart ( IC1805 The Heart Nebula – SHO) you will find the Soul.

This was trickier for me to process than the Heart nebula. Although it was clear, conditions were not great and I had few gremlins to deal with. Overall, it meant fewer hours of data so far captured (7hrs total, 2hrs each for Ha and O3, three hours for S2). This shows in the noise and processing artifacts visible in the enlarged image as I attempt to get something close to the Heart image in detail and vibrancy.

Details: ASI1600mm pro, Altair Astro 70mm EDT-F on the HEQ5.

Here is another attempt at processing the data, this time I just used the subs from the second night as they were better framed, so this represents about 3.5hrs of S2, Ha, O3, processed as SHO in a pseudo-Hubble palette.

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IC1805 The Heart Nebula – SHO

This is my first imaging season using narrow band filters on a mono camera. I’m enjoying the freedom to image and not fight moonlight or light pollution from the city.

The Heart nebula (IC1805) is in the constellation of Cassiopeia. This is the result of 6hrs of 120s subs in S2, Ha and O3. Imaged using the ASI1600mm pro with a gain of 200. Processed in PixInsight with the Hubble palette SHO.

A nebula around a star or is it a galaxy?

Visible at the top of the heart image is what looks like a small blue galaxy or planetary nebula. At bit of research reveals this is WeBo 1 (PN G135.6+01.0) a planetary nebula. It has a good signal in Ha and Oii. Perhaps imaging with my 250mm Newt will reveal more of its structure.

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The NorthAmerican Nebula NGC7000 in SHO

This image is the result of 13 hrs of imaging. I still have much to learn about image processing but I am quite happy with this result. The S1, Ha and Oiii subs were 120s @gain 200 to avoid burning out too many stars.

NGC7000 in SHO – this is the same data but an improved processing technique

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SH2-101 The Tulip Nebula

The Tulip Nebula after some very bad processing in PixInsight.
It goes to show that all the tools in the world are no good if you don’t know how to use them!

I imaged this nebula over 2 nights, several weeks ago in August but I had a terrible time processing the data. I was convinced I had some issues with my flats and then my subs. Then I was sure I had some light pollution leaking into the camera. I was at a loss for a solution but then I happened to read a post over at Forum | Backyard Astro that suggested to me the problem was how I used the Weighted Batch Processing script. I had recently updated PixInsight and was just using WBPP script with its out of the box default values.

That was a stupid newbie error, correcting the configuration of WBPP for the flats and light processing completely resolved the problem. As a result, I went back through all my recent images and began a methodical reprocessing.

And here is the result of more care and attention in processing the image.
This is 6hrs total, 300s subs @ Gain 200. 2hrs each for Ha, S2 and O3. Imaged using my trusty Orion Optics 250mm Newtonian with the ASI1600mm Pro.

I don’t want to go through that pain again so I have written down the steps and will publish what has worked for me.

One thing you will notice if you pixel peep the image is that the stars are vertical short lines. Guiding was close to perfect for this target so I believe the problem is caused by the RA motor drive board of my CEM70G which is a 2020 vintage, later mounts have improved RA and DEC boards. At least I hope that is the root cause as I’ve ordered replacements from the iOptron factory and I dont really want to use an OAG. [The last I heard the factory airport was in lock down due to covid so I don’t know when to expect them.] Because of the star shape it is a little more difficult to use my star reduction technique that worked so well for my IC1396 image. I shall have to see how others do it.

One thing I have learnt as a result of writing my steps down is slavishly following a process does not work well for all images. Sometimes you have to deviate from the path and experiment too. But still, a written down process is a good place to start from.

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IC 1396 The Elephant’s Trunk Nebula

This is a narrow band S2, Ha, O3 image taken over 6 hours (2hrs each) in one night.

Imaged with my Altair Astro 70mm ED-Triplet and my ASI1600mm pro camera.

This image is a reprocess of the same data with slightly less green removal and a different crop.

Finally worked out that Automatic Background Extraction was causing my vignetting in processed images and not poor flats or something else in the data. I’ve had another go and produced an almost full-frame image (minus the few pixels for dithering) with better colour control and handling of the stars:

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