REVIEW:
Askar Colour Magic E2 SII/OIII filter – E2 vs D2
First published July 2025
Last updated January 2026
Intro
Askar have recently released a new filter in their SII/OIII Colour Magic range: the E2. I’ve been using the previous version, the D2, for around two and a half years (see my review here) and have found it to be an excellent performer for narrowband imaging from light polluted skies. The E2 comes with a significant price increase, though, which raises an obvious question: is it actually worth the upgrade? To find out, I bought one and tested it head to head against my D2 to see whether the differences are meaningful in real world urban astrophotography use.
Askar’s latest offering
If you’re new to the concept of filters, I recommend you read my article “Which filter should I use with my OSC camera?” as a primer.
This new offering from Askar is their latest SII/OIII filter. It’s designed to only let through the wavelengths of light associated with SII and OIII sources. The intention is that the data gathered using this filter can then be combined with the more-common Hα-OIII filters available, and then during processing you can produce a genuine SHO image — normally the preserve of mono imagers — but using an OSC camera.
This is the third SII/OIII filter produced by Askar. Generally speaking, the main difference between these three is their bandpasses, measured in nanometres (nm). Narrower (i.e. smaller bandpasses) are better, because then you’re filtering out more “unwanted” wavelengths of light, and so should get a stronger signal through. This is particularly useful for urban astrophotographers, as we get the additional benefit of bypassing a lot of the light pollution that plagues us. The catch is price: the narrower the bandpasses, the deeper your pockets need to be!
| Filter | SII bandpass | OIII bandpass | Price |
| Askar C2 | 15nm ±3nm | 35nm ±5mn | £95 |
| Askar D2 | 8.5nm ±0.5mn | 6.5nm ±0.5mn | £320 |
| Askar E2 | 4nm ±0.3nm | 3.7nm ±0.3mn | £589 |
Note 1: if your telescope has “fast” optics — think around f/4.0 or below — then these filters can lose their effectiveness. Sometimes manufacturers make high-speed filter variants, but I don’t believe that Askar do this.
Note 2: it’s been long-known that filter manufactures can play a little fast and loose with their product specifications. I don’t have a way of testing the bandpasses for my D2 or E2 to see if they match with what they should be.
As mentioned, these filters aren’t really designed to be used in isolation, but rather in conjunction with Hα/OIII filters. Askar have released Hα/OIII versions: the C1, D1, and E1. So it’s possible to buy a matching pair. I haven’t done this with the E2 though, for the simple reason that I already have a 3nm Hα/OIII filter (my Optolong L-Ultimate). This costs £389, so I’m not about to spend £589 on an Askar E1 that has essentially identical specifications. Although I grant you it would be interesting to see if there are any differences in practice!
Warm-up test: Vega halos
To start with, I wanted to check whether the Askar E2 would cause any issues with halos. These are circles that appear around bright stars. Halos were common when using earlier-generation filters like the Optolong L-eXtreme, but improvements in filter designs have largely fixed the issue. Still, I don’t want to take this for granted, so turned my Askar 130PHQ telescope to Vega and took a series of images using my D2 and then E2.
Using such a bright star is an extreme stress-test for a filter. I was happy to see that halos were nicely controlled, and comparable with the D2. Considering that in general use I’ve had no halo issues with the D2, I’m confident that the E2 will be fine too.
My testing method
Now it’s onto the image tests. I had a grand ambition to gather lots of data and statistics in order to compare the D2 with the E2. I asked some of the great minds over on the Stargazers Lounge forum for advice and quickly realised that I’d have a very tough time conducting rigorous and fair tests. Ideally I’d have two identical systems set-up, but that’s out of the question. And I don’t have an electronic filter-wheel to alternate between the two filters over the course of a night.
So instead I decided to focus on what I could achieve: collecting data from the same targets and then taking a few simple but consistent processing steps to get the images in a state where I could compare them visually. This is not a scientific method and you should treat my results cautiously as a result! However, I think it’s also fair to say that if I’m going to spend so much on a filter then I want to see tangible improvements in the actual images it produces, not just numbers on a spreadsheet.
So with all that said and done, on with my tests!
Target one: the Elephant’s Trunk Nebula
First up was the Elephant’s Trunk Nebula. I collected 90 minutes of 3-minute subframes using the E2 and then D2. The target was gradually getting higher in the sky, so the latter filter — the D2 — gets a slight advantage given that image quality tends to improve the higher an object is in the sky. I then used PixInsight‘s SubframeSelector to whittle down to the best hour of data for each. I integrated these and performed some basic processing steps.
Use the slider to move between the two images. The Askar E2 looks appreciably better to my eyes.
When processing SII/OIII data, you tend to remove the stars and replace them with natural-colour stars taken using a broadband filter (like the Optolong L-Quad Enhance), or no filter at all. This means that I’m not really interested in the stars produced, so I removed them to better compare nebulosity:
When using an SII/OIII filter with an OSC camera, as I am, the SII component looks red, while the OIII appears as green or cyan. In these two close-up views I think that both channels, but particularly the OIII, are stronger with the E2.
Target two: the Pelican Nebula
The next test occurred the following night. I followed the same process but this time for the Pelican Nebula. One slight change is that I imaged using the D2 first, giving the E2 the “target higher in the sky” advantage.
It’s another win for the E2, with the close-up in particular showing more detail in the red (SII) channel.
Target three: the Lion Nebula
Time for the final round! This time I imaged the Lion Nebula over two consecutive nights, and then for each image I whittled down to the highest quality two hours of subframes. It’s important to note that sky conditions can vary significantly on different nights, so as discussed earlier this definitely isn’t a fair test, but I still think it’s interesting.
Conclusions
Despite my dodgy methodology, to my eyes the Askar E2 consistently out-performed the D2. Those narrower bandpasses certainly seem to have an effect, with the SII and OIII channels coming through stronger.
There’s no denying that the E2 is eye-wateringly expensive though. The D2 is still a fine choice for anyone wanting to try adding SII/OIII into their OSC images. The E2 is a premium product, with a price-tag to match.
It’s worth just touching on the whole OSC vs Mono debate, given how OSC imagers now have a really good SII/OIII filter in the E2. One of my first ever articles for this website was OSC vs Mono from a City, published almost exactly four years ago! To very briefly summarise my opinion: if you want the absolute highest image quality and price is no object, then Mono is king. But if you have some budget constraints, or perhaps want a simpler image acquisition workflow, OSC may be best for you, and in the hands of a skilled imager can produce images that are largely indistinguishable from Mono. I believe that integration time and processing skill have more of an impact than whether you’re using OSC or Mono. And ultimately it’s about whatever you’ll find most fun to use! Regarding price, the E2 is horribly expensive, yes, but if you wanted mono Askar 3nm SII and OIII filters, those would set you back £870 for the pair (compared to £589 for the Askar E2). Wouldn’t that make for an interesting head-to-head though…
After careful consideration I’ve decided that the Askar E2 will be my new SII/OIII filter!
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Askar should pay you a commission–you just sold another pair! Well, actually High Point Scientific owes you. I’ve been limping along with an H-O filter, the Hutech NBZ and though it does a nice job in my Bortle 8 skies of Southern California, it doesn’t give me nearly the signal I need when I take it into NarrowbandNormalization in PixInsight. Always a disappointment–but you’ve convinced me this will help. Excellent review and very helpful. Thanks for posting it.
Thanks Frank, I’m sure you’ll enjoy using the filter!