If you’re looking for a way to really add some bang to your emission nebula, then hydrogen alpha data is probably the way to go. To that end we have this offering from Astronomik. A 12nm hydrogen alpha filter that can clip into a DSLR camera body. Isolating a singular wavelength of light – 656nm – and then letting the sensor capture that can really make a difference in your astrophotography. Taking your images all the way up to 11.
There are several reasons why to invest in a hydrogen alpha filter, there are also a lot of filters out there to choose from at different wavelengths. But for now, in this article, I’m going to talk about my findings having used the Astronomik 12nm Ha filter which was on loan from The Widescreen Centre.
The Astronomik 12nm Ha Clip In Filter
So immediately I want to address this filter’s specific party trick: the clip in. This means that it has huge versatility for DSLR astrophotography as it clips into the aperture in your camera body. Not only is this a neat and tidy solution to getting the filter as close to the sensor as possible and eliminating vignetting as best it can, it also means you can continue using camera lenses and telescope adaptors without needing multiple filters.
It goes without saying that the clip-in variant is unsuitable for 1.25” or 2” nose pieces. Don’t fear however as Astronomik have you sorted also and there are standard 1.25” and 2” versions, as well as Canon Full Frame, Nikon, Sony and other versions of this narrowband filter.
The filter I’m using here is designed for the Canon APS-C family. I used this filter in a astro modified Canon 600D. This enabled the camera to be extra sensitive to the Hydrogen Alpha wavelength and was able to fully capitalise on this filter.
Do you need to modify your camera?
Yes and no. Your camera is already sensitive to the red spectrum and I’ve heard of people getting good results using a non-modified camera. But for the best results, I recommend you do look into the modification. If you are using a DSLR solely for astrophotography then it really is the best thing you can do for that DSLR and for your astrophotography. There are drawbacks with doing so, but I won’t go into them here.
Suffice it to say that to get the maximum bang for your buck when using the 12nm Ha clip filter is to use it with a modified DSLR. You’re allowing the maximum amount of Ha signal in, whilst blocking everything else.
Advantages
So as mentioned earlier there are several uses and advantages to using this 12nm clip in hydrogen alpha filter. I can think of 9.
- You get smaller stars
- You isolate the signal from the background sky
- Increased Contrast
- You can use it with RGB images to make photos POP
- Beautiful mono images
- Combat light pollution
- Combat Moon glow
- Usable with majority of camera lenses
- Relatively affordable
The main feature that I think makes hydrogen alpha really popular is making HaRGB composite photographs. These are images where the red channel is blended with the Ha data, and then also the Ha is used as a luminance layer. Really drawing out fine details and giving photos another dimension and almost a texture. Also, as they ignore all wavelengths of light aside from the Ha, narrowband filters are great for light polluted cities.
The usable data taken with these filters are hidden in the red channel. Whilst you may have an ‘RGB’ image from your DSLR, really when you look at it the blue and green channels are practically empty. You want to go in there and draw out the red channel and edit that as its own image. This can then be shared as is, or blended into composites as mentioned above.
Use Your Lenses
One massive appeal to DSLR astrophotographers will also be the ability to
use your camera lenses with it – again as this filter clips into the camera body. Now, be very careful here. Only full-frame camera lenses can be used, as well as the telescope adapters. This is because crop-frame lenses sit deeper into the camera body as they project onto a smaller sensor. This means that the lens will hit the filter, and nobody wants that.
However you probably already own some full frame lenses that are perfectly capable and suitable for use with this filter in astrophotography. Also, if your lens suffers from chromatic aberration then it won’t be a problem with narrowband. That’s because we only image one particular wavelength, and we then take that wavelength out of the specific channel during post processing.
Yes, that also means you can use non-ED achromatic telescopes as well if you’re going to only be using narrowband filters with your DSLR! Save your money on ED glass and get more aperture!
That probably isn’t very good advice and you should probably not listen to it…
Transmission
The data that was 
entering my camera sensor looked good. It really seems like that >95% transmission that Astronomik boast about seems accurate. Now this was purely a “hey, that looks good” sort of test. As I don’t have the stuff needed to actually test its transmission, but yeah. I have no complaints!
Halos?
One thing you’ll see advertised some places, or asked about, is whether the filter has no halos.
I noticed NO halos when using this filter. Halos are what appear to be reflections around bright stars. They’re unsightly and, in my opinion, can ruin an otherwise lovely astrophoto. I turned this filter over towards the Horsehead Nebula in Orion, and fired away. The trick here was Alnitak.
Alnitak is a really rather bright star in Orion’s Belt, and has an apparent magnitude of 1.77. So it’s pretty bright. Especially if you’re about to take 3-5 minute or longer exposures. I felt Alnitak was a good stress test and is brighter than a star I compared it against.
With a different filter from a competitor (now I admit the filters aren’t directly comparable but I felt it still was a fair enough test) I noticed halos around Navi. Navi (y Cassiopeiae) has an apparent magnitude of 2.47. So that is dimmer than Alnitak.
As you can see in the example here, Navi – with a competitor filter that boasted ‘No halos’ and was a narrower bandwidth (7nm) developed a halo. Compared with the 12nm bandpass on Alnitak. I’m happy to say I never noticed haloing from the Astronomik filter.
What are some drawbacks?
Nothing is perfect in this world and the Astronomik 12nm Ha filter is no exception.
Buying the clip in variant is £180 at the time of this review. Whilst definitely cheaper than the £266 for the 2” version, it is £64 more expensive than the 1.25”. It hasn’t got that much more glass in it either than the 1.25” and it will have a standardized construction method as well. So where the additional pricing has come from I’m not entirely sure. I suspect that they’re hiking the price up purely because clip-in might be more popular than 1.25”.
I originally thought about giving the pricing 3/5, but when I think more about the lack of halos I have to bump it up to 4.
Another downside is just a downside about narrowband imaging anyway. You need to exposure for longer to get a decent exposure. This single image of the Horsehead Nebula was captured using a 300 second exposure at ISO 800 on an f6 telescope. This is still at least 1 stop under exposed to me. In order to get this to a middle exposure I’ll need to either image for 8-10 minutes or raise my ISO to 1600.
10 minute sub exposures on an uncooled DSLR is a recipe for disaster. Even in the winter. Not saying its impossible, but the thermal noise will begin to kick in as will read noise. I once met high temps and long exposures with a 600D. It did not end well.
Also if you’re tempting to upgrade to a dedicated imaging camera sometime, you won’t be able to take this hydrogen alpha filter with you. As you can tell, it isn’t designed to thread into nose pieces. For that you’ll be wanting a 1.25” or 2” filter.
Extreme Light Pollution Suppression
Unlike a standard light pollution filter, a hydrogen alpha filter is what’s known as a ‘narrowband filter’. As mentioned above, this isolates one particular wavelength of light that’s emitted. There are several benefits of this:
- Greater punch from emission nebula
- Highlight nebula within other galaxies
- Ignore light pollution
- Allows you to shoot through most Moon phases
Admittedly the primary uses of narrowband filters are adding that punch to emission nebula and the light pollution suppression. But we can always enjoy added benefits.
The extent that light pollution is ignored is also circumstantial. For my Bortle 5/6 skies I found the 12nm to be just fine. I have used a 7nm also which naturally gave deeper and more contrasty photos (but required longer subs or higher ISO). If you’re in Bortle 7-8 I’d suggest 6-7nm is probably what you’d need, and for inner city Bortle 9 I imagine 5nm or narrower would be the best choice.
I’m only speculating here by seeing what inner city astrophotographers use for their narrowband filters. I don’t have light pollution that heavy so I’ve not needed to go that narrow.
The Moon
One point I made just above was about the Moon. Hydrogen Alpha filters allow you to carry on imaging during Moon periods. The extent of this Moon filtration is also dependant on the bandwidth you choose.
For the Astronomik 12nm I found any Moon phase that was greater than about 65% began to affect my sub exposures. I also shot during basically a full Moon, and quite close to the Moon as well (to test this filter’s abilities) and noticed more and more washout.
Compare both of these images. They’re both shot with the same equipment (Canon 600D astromod, Skywatcher Evostar 80ED Pro, 0.8x Altair Lightwave Reducer and this filter) on the same mount. Both are 300 second exposures at ISO 800. Just one is a 65% Moon and the other is 93%.
However the data was still usable and I was able to quickly throw together this image.
To be fair to the filter, I think the Moon Filtration is more so in part of the 12nm nature and not anything that Astronomik has done in the production process.
Lens Choice
As mentioned this filter is great for astrophotographers who use DSLRs and lenses. Even popular cheap lenses like the legendary Canon EF 50mm 1.8 prime (the nifty fifty!) works well with this filter. Even though this lens suffers chromatic aberration, with the Hydrogen Alpha filter you’re only taking the red channel out. So you ignore the CA!
If its a full frame lens, take your pick. Just try and get it as fast as possible because the filter will bump the required exposure time up.
Summary
If you’re looking for a nice discrete Hydrogen Alpha filter and you use a combination of Full Frame lenses and telescopes then really the Astronomik 12nm Ha clip filter is a strong contender. Put this filter in your DSLR, put a 50mm prime lens, attach it to your telescope and put it on your tracking mount for example and away you go capturing stunning Hydrogen Alpha data.
The Astronomik filter is a great entry way to DSLR hydrogen alpha certainly. 12Nm bandpass is almost perfect, as it doesn’t block so much light that you’re really cranking up your exposures and sensor temperature, and it blocks a good amount of Moonglow and light pollution out.
I think you’ll be hard pressed to find a more versatile and beginner friendly filter. Still being able to see bright stars in the live view, for example, makes framing and focus that much easier and you’ll be up and running much faster than the 6nm version.
And if you prefer to get the 2” version and put it on a M48 nosepiece, then do just that as well. They’ll be made of the same coating as the clip in version.
Right now this filter has to go back home as I’m finished with this review. I would love to buy it but right now my budget won’t allow it! It’s a bit pricey when you look at competitor filters for example. But the Astronomik filter is a sound investment that – whether its the 1.25”, 2”, clip or any of the variants, I’m certain you’ll keep reaching back towards.
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- Build Quality: 5/5
- Transmission: 5/5
- Moon Filtration: 3/5
- Anti-Halo: 5/5
- Price: 4/5
- Versatility: 4/5
- Total: 4/5
Links
Standard Variants & Canon Clip In Filters
Canon EOS M, R, Nikon XL, Pentax and Sigma Variants
The Horsehead Nebula in Hydrogen Alpha
https://youtu.be/ZRod6KuVwtU