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Astronomik H-Alpha Clip-In Filter for DSLRs Reviewed

Astronomik clip-in filter for EOS cameras. Credit: Alan Dyer

 

What would it take to be able to shoot deep-sky objects any clear night of the month, regardless of Moon phase? The answer — the cost of a narrowband filter like the Astronomik H-alpha tested here.

By shooting through a narrowband filter, you block all extraneous light except those narrow wavelengths emitted by nebulas. Moonlight and urban light pollution are mostly shut out. It is possible to get good images even under conditions that would render unfiltered color images a fogged-out mess. (See Robert Reeves’s review of the Radian Triad Ultra filter for an example of a color filter for use under such skies.)

The Astronomik 12nm H𝛂 filter allowed capturing the nebulas around Orion with a waxing gibbous Moon in the sky. This is a stack of 24 four-minute exposures with the Canon Ra at ISO 1600 and Canon RF28-70mm lens set to 50mm and at f/2. Credit: Alan Dyer

A Hydrogen-alpha (H𝛂) filter is particularly effective, as that’s where many nebulas, made mostly of hydrogen, emit most strongly, at a red wavelength of 656 nm (nanometers). By contrast, many unwanted sources of light (both natural and man-made) peak in the yellow, green and blue bands of the spectrum.

However, unlike dual-, tri- and quad-band filters such as the Radian, which can produce color images, a single-band H𝛂 filter lets through only deep-red light, and so yields an image that is all red. To be usable on its own, it has to be converted into greyscale monochrome.

But I like monochrome astrophotos; they carry on the timeless tradition of B&W images by classic photographers such as Ansel Adams and Edward Weston.

Single-band “line” filters, such as the Astronomik H𝛂 tested here, are required equipment for those who shoot with specialized monochrome astro cameras.

As a clip-in filter, the Astronomik sits inside the camera body in front of the sensor. Once a lens or camera adapter is clicked into place, the filter is held securely. Credit: Alan Dyer

But my interest was in using the Astronomik filter to shoot nebula-rich fields along the Milky Way with a “one-shot color” camera, the Canon EOS Ra, the astronomy-modified version of the Canon R. The Ra (now discontinued) is designed to record a greater level of the deep red H𝛂 light from emission nebulas. I would not recommend using a stock DSLR or mirrorless camera with an H𝛂 filter because they roll off their sensitivity to deep red.

I tested the model that clips into the bodies of Canon R mirrorless cameras. Astronomik also sells clip-in filters for Nikon, Pentax and Sony cameras, as well as standard 1.25- and 2-inch sizes for screwing into camera adapters, and also unmounted versions for insertion into filter wheels.

Other companies – KaseOptolong, and STC Optics – make clip-in nebula filters for Canon R and other brands of cameras, but not single-band H𝛂 filters.

Choosing a Filter

I bought Astronomik’s 12nm (nanometer) bandwidth version. Astronomik offers narrower 6nm versions in all formats, but at a considerable premium in price (€445 for a 6nm vs €285 for the 12nm model I tested). Astronomik recommends these more selective filters only for use with advanced cooled CMOS cameras under light polluted skies. For my Canon camera under rural skies, I bought the wider version.

Each Astronomik filter is said to be factory tested, with my unit labeled as having 96.5% transmission. Not having a spectrophotometer, I could not check this claim.

Perhaps making the buying decision more difficult, Astronomik also offers a more expensive “MaxFR”version (€142 to €590) of each of their 12nm and 6nm line filters. These shift the bandpass to better accommodate the steep light cones from ultra-fast systems such as Celestron’s RASA astrographs and Starizona’s Hyperstars.

I used my lower-cost standard filter with some lenses at f/2 and found no issues. Perhaps the MaxFR version would yield better light transmission and shorter exposures with fast lenses, but I’m happy with the cheaper standard 12nm version.

While Astronomik does have dealers in North America, I ordered my filter directly from Astronomik in Germany without issue. Shipping by FedEx was surprisingly economical and quick, though I did incur import taxes and clearance fees.

Another example of a wide-field H𝛂 image, this is a stack of 24 six-minute exposures of the northern Milky Way with a Canon RF28-70mm RF lens at f/2, taken on a very transparent night with an 8-day-old gibbous Moon lighting the sky. Credit: Alan Dyer

Compatibility

The advantage of a clip-in filter is that the one filter can be used with a variety of lenses and telescopes. I wanted to use it with my Canon RF lenses.

The disadvantage is that it can be used only with that camera model. While the clip-in filter for my Ra does fit other Canon R mirrorless cameras, it will not fit inside Canon DSLRs.

Any filter placed between the optics and camera sensor will shift the focus point. For example, with my Canon RF28-70mm f/2 lens I found it reached sharpest focus on stars with the lens set not to infinity, but to as close as 3 meters. However, the focus shifted inward much more at the 28mm focal length than with the lens set to 70mm.

That focus shift can cause major issues with some lenses whose optical performance can worsen significantly when used with clip-in filters. In some wide-angle lenses I’ve tested with clip-ins, the aberrations at the corners blow up, though to an extent that varies from lens to lens.

With telephoto lenses and telescopes this focus shift causes much less, or no, ill effect. Even so, I would caution users expecting to use this or any clip-in filter with wide-angle or even normal focal length lenses. They might not work as expected, and users then blame the filter. The problem is actually the lens being used in a way it was not designed for.

Focusing and Framing

No matter the optics, using any filter between the lens and optics (as opposed to over the front of a lens) requires focusing with the filter in place. With a filter as dense, dark and deep red as the Astronomik H𝛂, that’s a challenge. It is hard to see anything through it, even with mirrorless cameras which have much brighter “live view” images for focusing than do DSLRs.

The camera has to be aimed at a bright star to focus. Even then, with the small star images from wide lenses, focusing aids (see my review here) like Bahtinov masks and diffraction gratings dim the image so much as to be useless. Even with a fast telescope, I found a Bahtinov mask was just useable on the sky’s brightest stars.

Raw Image Comparison: Out of camera (top) H𝛂 images look very red. The raw images from DLSRs and mirrorless cameras can be converted into monochrome (bottom) by switching camera profiles in Adobe Camera Raw (shown) or Lightroom. Credit: Alan Dyer

The preview images will look very red. Setting the camera to a Monochrome picture style turns the previews black and white, which helps in seeing the field. However, doing so does not turn the DSLR or mirrorless camera into a monochrome model! The raw files still retain all the color information, and only the red pixels record any signal regardless of the camera setting. Thus the issue with exposure times described next.

Exposure Times

Using an H𝛂 filter with a one-shot color camera, which has its own set of red, green and blue micro-filters over each of the sensor’s photosites, has the disadvantage that only one-quarter of the photosites, the red ones, record any signal. H𝛂 images therefore tend to be noisy, requiring lots more exposure time, or many more stacked exposures, (or better yet, both) for the best results.

As the inset shows, this is a blow-up of a single 8-minute H𝛂 frame used in the M35 composite below. It shows the high level of noise from having only the red pixels recording any signal. Credit: Alan Dyer

With the Astronomik 12nm filter, I generally increased exposures by at least a factor of four over an unfiltered image (by doubling exposure time and doubling the ISO), if not more. I also tried to shoot as many sub-frames as the night would allow. I would expect that the 6nm version would require even longer exposures, and more of them.

Moonlight in Monochrome

This or any filter will work best under dark, moonless rural skies. However, I used the Astronomik H𝛂filter mostly on clear moonlit nights at my rural site, to take deep-sky images on nights otherwise useless to me.

It worked very well, but I found the best results still required very transparent nights with no haze or humidity, and with the Moon well away from the field of interest. Having the Moon just outside the field will still introduce ugly gradients.

 

A demo image taken in cloud and with the Moon off frame in Taurus, with the Ra and 28-70mm lens at f/2 and 34mm and at ISO 1600. A single image taken Feb 8, 2022. Processed only in Camera Raw with Adobe Monochrome profile. Credit: Alan Dyer

Even under rural skies, I found natural and artificial skyglow can still introduce gradients when shooting fields at low altitude. That would be even more true under light polluted skies when shooting toward urban light domes. Don’t expect miracles. It is still best to shoot targets when they are as high as possible, and only on the clearest nights — though as astrophotographers know all too well, those tend to be the nights when the Moon is up!

This is a stack of exposures of the region around M35 in Gemini, through a broadband Astronomik CLS filter for a normal image, blended with a stack of images through the Astronomik H𝛂 filter, contributing most of the red nebulosity. Haze this night added the glowy stars and artistic soft focus effect. Credit: Alan Dyer

 Optical Quality and Use

Unlike with some filters (see my test of several dual-band filters), I did not see any unwanted halos around bright stars, though with a deep red line filter I would not expect halos in any case. The filter worked well in all respects with the lenses and fast refractor telescopes I used it with.

I used the filter primarily to take monochrome images for their own sake. However, it is possible to use this or any H𝛂 filter to take images to be combined later with color frames, with the H𝛂 image contributing enhanced red nebulosity. I found this did work, as demonstrated above.

This shows single images with the CLS broadband and H𝛂 filters, blended in Photoshop to create the final color composite presenting the best of both views. Credit: Alan Dyer

However, as shown below, I preferred the results when I shot unfiltered images on dark nights and blended those with dual-narrowband filtered images, usually shot the same night for consistent framing.

With the contribution of the green Oxygen-III emission and the greater bandwidth of such filters (like an Optolong L-eNhance or IDAS NB1), I found they recorded even more nebulosity, with the unfiltered image layer retaining natural star colors. But such a combination is practical only under dark skies.

This shows two blends — bottom left: images with an Optolong L-eNhance dual narrowband filter blended with unfiltered images, and bottom right: The same unfiltered set blended with Astronomik H𝛂 images, for a less dramatic result. Credit: Alan Dyer

However, your mileage with any filters, this one included, will certainly vary, depending on your sky, equipment, targets selected, and your processing workflow.

As well as it works, I would not consider an H𝛂 filter like the Astronomik an essential item in an astrophoto kit, certainly not for use with one-shot color cameras of any type.

However, for a relatively modest cost, the Astronomik H𝛂 filter can expand your shooting opportunities into otherwise unusable nights and produce beautiful monochrome images that stand on their own as worthy of showing off to the world.

Plus:               Halo-free images that record nebulosity well under bright skies

Minus:           Clip-in filters can exacerbate lens aberrations

Retail: €285 without European VAT (about $315 U.S.) for 12nm EOS R clip-in version tested; Prices for other sizes and cameras start at €110 for 1.25-inch filters.

Website: https://www.astronomik.com/en/

About Alan Dyer

Alan Dyer is an astrophotographer and astronomy author based in Alberta, Canada. His website at www.amazingsky.com has galleries of his images, plus links to his product review blog posts, video tutorials, and ebooks on astrophotography.

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