Dualband filters have opened the world of convenient narrowband imaging to One-Shot Colour (OSC) camera users. Askar are hoping to shake things up further with the release of their Colour Magic D1 and D2 filters which, when used in combination, allow for full SHO imaging.
SHO imaging for OSC users
Narrowband imaging used to be the preserve of astrophotographers using mono cameras and three key filters: Hydrogen-alpha (Ha), Oxygen III (OIII), and Sulfur II (SII). However, a few years ago the introduction of dualband filters hugely increased the versatility of OSC cameras. These new filters, such as the Optolong L-eXtreme allowed OSC imagers to efficiently capture Ha and OIII wavelengths using just one filter. While not quite as effective as a mono camera plus three dedicated filters – SII, Ha, and OIII –the results using these dualband filters proved to be surprisingly effective, while being much cheaper. I’ve used an L-eXtreme extensively – click here for lots of examples, and check out my article on OSC vs Mono from a City. There was always a catch, however: the lack of SII data. Dualband imagers had to decide whether to create bi-color images, or use software solutions to create a pseduo-SII channel.
Enter Askar, who have just released a new set of 6nm dualband filters aimed at OSC imagers, currently available 2″ mounted. The first of these, called the D1, is fairly standard, capturing Ha and OIII data. The D2 is what could be a game-changer, capturing SII and OIII. By combining data from these two filters, Askar claim that the results are very similar to using a mono camera and full set of filters.
Processing
Being an OSC imager myself, I was very excited to put these filters through their paces. I already had eight hours of Askar Colour Magic D1 data on the Elephant’s Trunk Nebula taken for my dualband filter shootout article. I took another eight hours using the D2 filter, all with my lovely new Askar 130PHQ refractor (review here).
After integrating the two stacks in PixInsight and applying a quick autostretch, it was evident that the two filters were indeed collecting different wavelengths of light.
I then followed the first part of a short processing tutorial provided by the manufacturer. The basic steps are to integrate the data from each filter separately, and ensure that the two resulting images are aligned to one another.
The D1 integration is then renamed “HO” (i.e. Ha and OIII); while the D2 integration is renamed “SO” (i.e. SII and OIII). Each integration is then split into its constituent RGB channels, which are promptly renamed:
HO_R renamed to H
HO_G renamed to O1
HO_B renamed to O2
SO_R renamed to S
SO_G renamed to O3
SO_B renamed to O4
So, the red channel from the D1 filter is taken to be the H channel. The red channel from the D2 filter is taken to be the S channel. The four remaining channels – called O1, O2, O3, O4 – are integrated into one image, which is renamed O. Hey presto, we have SHO channels from two filters.
There are different ways of reaching this step, and what I’ve outlined is a simple approach just to convey the general idea. Once you have the SHO channels, you’re into the workflow of mono imagers.
Target one: The Elephant’s Trunk Nebula
There are as many different processing workflows as there are astrophotographers, so I simply aimed to produce a colourful image to bring out the data that these filters captured. I was pleased with the result, in particular with the blues that often prove elusive when processing regular Ha OIII dualband data.
There were some challenges, such as overly purple stars, that I did my best to deal with. Experienced mono imagers would no doubt take all this in their stride, but OSC imagers may need to learn a few new tricks. (I found PixInsight’s “CorrectMagentaStars” to be useful).
As mentioned earlier, regular Ha OIII dualband imagers have the option to create a pseudo-SII channel during processing. This is quite simple in PixInsight thanks to a one-click process described here. How would an Ha OIII image processed using this technique compare? Fortunately, using that technique as a foundation I had previously created an image from a 22-hour integration from my Bortle 8 city centre using purely Ha and OIII data (specifically, 11 hours from an Askar Colour Magic D1, and 11 hours from an Optolong L-Ultimate 3nm. Click here for a whole post about that image, or use the slider below to compare:
A quick straw poll among fellow astroimagers, as well as non-astro friends, revealed a split in opinion over which image is better. Personally, I prefer the pseudo-SII version, which may be because the image was slightly higher quality as a result of the cleaner 3nm Optolong L-Ultimate data.
Target two: The Heart Nebula
The Heart Nebula has plenty of OIII, so I decided it would make a good second target for my tests. I collected 12 hours of data using the Askar Colour Magic D1, and then another 12 hours using the D2.
I found this easier to process than The Elephant’s Trunk Nebula, perhaps due to the increased levels of OIII. I was very happy with the colour palette I managed to bring out, with the shades of blue in particular being something I’ve always associated with SHO mono images. The left part of the nebula is almost rainbow-like. I consider the result to be quite remarkable, especially considering this is from a city centre. Click here for a whole post about the image.
I then tried making an Ha OIII version with a pseudo-SII channel with data just from the Askar D1. The result is respectable but lacks clarity, likely due to the fact that it’s half the integration time. Try as I might though, I just couldn’t reproduce the glorious colours of the Askar D1 D2 combination. You can compare the two using the slider:
Post-test thoughts
It’s clear that Askar have produced something very impressive with their Colour Magic D1 and D2 filters. Study the Heart Nebula picture in particular and remind yourself that it was taken using an OSC camera!
Of course, it’s important to consider cost. The Askar Colour Magic D1 and D2 filters can be bought individually for £320 each, or as a pair for £620. Optolong have recently released their L-Ultimate Ha OIII dualband filter, which with a pricetag of £389 is more expensive than the individual Askars, but has 3nm bandpasses compared to the Askar D1 and D2’s 6nm. Check out my Dualband Filter Shootout for examples of how 3nm is superior to 6nm.
This presents OSC imagers with a dilemma: buy the Askar D1 and D2 filters, which allows for SHO imaging but at 6nm; or save £231 and opt for an Optolong L-Ultimate filter, be content with bi-color or pseudo-SII processing, but with the superior 3nm and the simplicity of using one filter instead of two. There’s not a clear-cut optimal answer, and it’s down to individual imagers to decide what’s the best route for them.
Note that Askar have their own 3nm Colour Magic Ha OIII filter (£595) that they’re sending me to review. Askar say they’re not currently planning on releasing a 3nm SII OIII filter. I think this is a shame, and it feels like Askar have come so close to causing a seismic shift for astroimagers. Imagine these D1 and D2 filters, but at 3nm and priced competitively. That would elevate them from impressive to truly incredible.
As it stands, the Askar D1 and D2 filters are still interesting propositions that could siphon imagers away from mono set-ups. If you’re a purist and want the absolute highest image quality, scientific accuracy, and flexibility, then mono still is – and likely always will be – the answer. But with filters like the Askar Colour Magic D1 and D2, you can choose OSC and get close for a much lower cost.
On a personal note, I’m going to keep experimenting with the D1 and D2. I want to do more tests pitting them against the L-Ultimate in particular. Once Askar send me their 3nm D1-equivalent filter to test, I’d like to try combining that with 6nm D2 data.
These Askar filters open up a lot of possibility, and I think that’s a brilliant thing in this hobby. Let’s have more imagers trying different things! I’d love to see a mono imager try these filters and see what the results are — there’s no law that says they’re exclusively for OSC cameras, after all.
Extra note, 11 Jan 2023
It’s been brought to my attention that some of these filters may perform better than others. See this Cloudy Nights forum thread, and this more general video from Cuiv about equipment lottery. I don’t have the required kit to test the reality of my filters against their specifications, but going by eye alone I think they’re OK. It’s worth bearing this “lottery” in mind if you do want to buy these filters though. First Light Optics have a robust returns policy, and if you’re buying second-hand it’s always a good idea to ask for example images.
Extra extra note, 11 Feb 2023
I’ve found that the Askar D2 filter combines very nicely with the Optolong L-Ultimate, which is like an enhanced version of the Askar D1. So, going forward for narrowband targets, if an object has a decent amount of SII in it, I’m going to collect data using my L-Ultimate and D2. If the target doesn’t have much SII, I’ll just use the L-Ultimate.
For example, this photo of The Soul Nebula combines Optolong L-Ultimate and Askar D2 data for SHO (and no filter for RGB stars).
Nothing will motivate me to create more content quite like money!
Great !! Do you have any comparison between Askar D2 with IDAS NB3?
That’d be interesting, but I think the IDAS NB3 series has been discontinued? In any case, I don’t have one to compare.
Great tutorial! One question; We are always trying to get the most from our channel data. But we usually have been discarding the B channel with dual-band filters because of the higher noise and lower SNR. Do you really think adding the B channels actually raise the SNR of the Image Integration? From the looks of it the G channels look very strong but the B look pretty weak… Have you tested a few methods to determine the best?
Thanks for the excellent articles and reviews!
James
Ah now this is a very good question! I did actually try making the O channel just from the two G channels, but PixInsight’s ImageIntegration tool said it needed a minimum of three images to work. So then I just followed the tutorial and threw the two B channels into the mix too. I think there’s potential in refining the whole process and I’m certainly open to axing the B channels. How then to get one more channel in there to make the O though? The two Gs and a synthetic third one somehow?
Right I understand Image Integration needs 4 channels to work… humm? Maybe I’m overthinking it or maybe PixelMath would create a stronger Oiii channel somehow. I just don’t like the idea of adding excessive noise with the B channel.
I just purchased one of the Antlia Triband RGB filters and the advertising says it creates a stronger B channel somehow. There is a strong signal band peak way lower than the Oiii (perhaps just above UV) that would perhaps contribute to the B channel? It’s interesting filter somewhere in-between a LP and NB. It’s been crazy rain in Northern California of course haha.
James
Let me know how it works out for you! Another suggestion has been made to me that we could just make duplicates of the two G channels to trick PixInsight into thinking there are four. I haven’t had time to give this a go yet.
Hi! I’m also using L-ultimate and I’m planning to get a color magic D2. However the filter thickness may be different. Does it mean you need to refocus and adjust the backspacing again when u swap to D2 ?
Thanks
I use a Petzval-like refractor, which means that backfocus isn’t a concern for me. Even with other types though, I doubt that you’d need to modify your backfocus spacing. Regular fousinging is a different story, and I would recommend refocussing. Are you using an Electronic Autofocusser? They make things a lot easier. I actually set mine to automatically refocus every hour during an imaging run, to compensate for focus shift due to temperature variations.
Thank you for the reply Lee!
I’m also using a Petzval, will need to adjust Back focusing only if I slap a 0.7 reducer. But like what I mentioned, I doubt it will be effected as it’s only a filter change 🙂
Ok great. I’ll get a EAF then😅
My compliments for this great review which I read a bit later. Regarding the mixing between Optolong and Askar filters, the combination between L-Ultimate and D2 is with two different band (3nm VS 6nm). So what about using L-Extreme (instead L’Ultimate) which has bandpass closer to the D2 one?
Max
Hi Max, thanks for your comment. You could pair the Askar D2 with an L-eXtreme and it would work fine, but the L-Ultimate gives a cleaner signal, and no halos, so is the better choice — but more expsneive!
Tanks gor your fsdt reply. Since I am Edge 8 user and have both 0.7 and 1.9 (Hyperstar) reducers which filters do you think are better at these focal ratios considering I use also a ZWOASI 294MC Pro? My bortle sky is often 7-8, sometime 3-4. Thanks again.
Max
Hmm, it might be hard to find one filter to fit all your needs. For fast systems like when you’re using your Hyperstar, you need fast variants of these filters. An example is the Optolong L-eXtreme F2 High Speed. I’m not sure if any manufacturers currently offer fast variants of an SII/OII dualband filter.
HI Lee, what would be your opinion on the ‘Antlia ALP-T Dual Narrowband H-a, H-b, SII & OIII (5nm)’ as a replacement of the Askar D1&D2.
The Antlia has the extra H-beta channel.
My setup is Celestron Egde 11HD + 0.7 reducer (F/7) + ASI 2600MC OSC
Thanks
I haven’t used these so can’t really give an informed opinion. But considering the lack of H-b sources, my instincts tell me that Askar’s approach of doubling-up on the OIII (Ha/OIII & SII/OIII) is the better idea. However, I’m willing to be proven wrong!
Great write up thank you, what Chanel would you put the Hb into would it just be the same as the s2 or would you split the chanels accordingly?
Many thanks
Hi Brad, none of these filters capture Hb, but if I did have some to throw into the pot I’d use it as part of my blue channel.
Hi Lee,
I’m having problems with “ensure that the two resulting images are aligned to one another”.
I’ve got stacked images from D1 and D2 filters.
I first tried using the “autocrop” for the 2 stacks, but ImageIntegration process would not work.
Then I tried using the non-cropped stacks out of wbpp, and even if ImageIntegration did accept the O1-4 images, the resulting image was not good (double-vision like result).
I suspect caused by miss-alignment.
how to ensure the 2 resulting images are aligned to one another ?
thx for the great articles !.
Note1: I follow your pre-processing guide, which so far has worked well for single images. it’s first time I’m trying the HSO using D1 & D2 filters.
Note2: Even with the badly integrated O1-4 as O, the resulting LRGB combination came-out with a lot of green, which makes me think the process I followed in the SharpStart Video may not work for me ..
You should use the StarAlignment process on all your integrated stacks. That will align them. You may need to run the same DynamicCrop on all of them as well, to ensure there are no black borders.
finally got the change to use StarAlignment after watching one of your videos. working well. thx a lot!
How would the IDAS NBZ UHS filter and the D2 filter work.
My plan would be to use the DB Extract script to separate the channels and the use Narrowband Normalization
I believe those filters should work well together. You can see an example processing tutorial on this page: https://urbanastrophotography.com/index.php/2024/07/11/b161-ldn-1121/
Wow!!! Amazing images!!! I’ve been a daylight photographer for years but am just getting into Astro landscape and DSO photography. My struggle has been whether to go OSC or mono. This answers my question. I love the process you took in the last image where you combines the SHO image with RGB stars. I was thinking about this as I read your blog and then you did it in the end. I love the color variation of my landscape star shots and would really like to keep this in my images.
Thanks Earl, I’m glad you found that useful. I’ve actually been doing some astro landscape work myself recently (albeit not from the city), and I might write the experience up as an article.