Some of the most incredible DSLR deep-sky images ever produced, likely combined regular RGB data (A typical color image) with images using a Ha filter. A clip-in filter for your camera such as the Astronomik H-Alpha 12nm CCD filter makes it easy to start gathering Hydrogen-Alpha images with your existing DSLR.
When it comes to astrophotography from a city backyard, narrowband filters such as H-Alpha allow you to collect detailed photos of many of your favorite nebulae and galaxies. The resulting greyscale images can then be applied to any existing full-color data you may have for an even more powerful image than in true-color alone.
Narrowband filters are an astrophotographers best friend.
The Astronomik H-Alpha 12nm Clip Filter for Canon DSLR’S
This narrowband filter lets the h-alpha light of emission nebulae pass through to the camera sensor, and blocks almost all of the other light spectrum. What does this mean?
It means that city light pollution and moonlight are completely erased! This opens up the doors to imaging during the full moon, and from a city backyard.
No more waiting until the new moon phase for deep-sky imaging!
Emission Nebulas and Hydrogen-Alpha
Emission nebulas are clouds of glowing gas, and they emit light at a very specific wavelength. This is information is beneficial to astrophotographers as we are able to isolate this wavelength for photography. The light from an emission nebula is created when the atoms in the gas are ionized by the formation of hot young stars.
The dominant wavelength in a hydrogen nebula is the deep red portion of the spectrum known as the hydrogen-alpha line (656nm to be exact).
Why this is important for DSLR astrophotography
By using a Ha filter in your DSLR, it is possible to increase the contrast between objects in the h-alpha emission line and the skyglow background. The filter completely suppresses the emission lines of artificial lighting such as mercury (Hg) and sodium (Na). The particulars of the Astronomik ha filter can be understood in the graph below.
Images captured using a narrowband Ha filter have some pleasing characteristics.
At the top of the list is reduced star size. This further emphasizes the detail and contrast of a deep-sky image by allowing it to stand out from the surrounding sea of stars. Filtering out skyglow and light pollution means that longer exposure times of 5 minutes and beyond from the city are possible from the city. You will likely capture more detail in your subject than ever before.
Here is an example of what is possible with a modified Canon DSLR and a 12nm Ha filter.
Choices – 12nm or 6nm versions
The 6nm version of the Astronomik Ha filter has an extremely narrow emission-line filter. This version targets an even narrower portion of the visible spectrum, blocking nearly the whole remainder of the spectrum. So wouldn’t the 6nm version the obvious choice?
Despite the increase in contrast and more light-blocking power of the 6nm version, there were a few reasons I went with the 12nm.
A major step in my pre-imaging routine includes framing my deep-sky target to include as much of the object and surrounding elements as possible. BackyardEOS streamlines this task with the frame and focus feature.
With the 12nm Ha filter installed in my DSLR, this becomes a much more difficult process when framing targets without any significantly bright stars in the frame! The same scenario transpires when focusing the camera. Not only is focusing stars using live-view out of the question, but test exposures may need to be as long as 10 seconds before anything appears.
It’s not the end of the world, but it does take longer to get up and running. The 6nm would double these impacts, and increase this setup time.
Many backyard astrophotographers would disagree, wishing they had gone with the 6nm instead. Ideally, having both filters would provide maximum H II contrast while retaining stars in the image.
Does my camera need to be modified for astrophotography?
Absolutely not. The same benefits of an H-alpha filter can be realized with a stock DSLR. However, the amount of red light your camera will be able to record will be drastically reduced. If you are serious about your astrophotography, it is worth thinking about modifying your camera either yourself or by a professional. Here is a shot of the Eagle nebula using the Astronomik Ha filter and a stock Canon EOS 7D.
This is typically what a frame looks like taken using a Ha filter through a DSLR. This emission nebula could have been captured in greater detail using a modified camera, but the skyglow and wavelength suppression capabilities of the filter are still present.
I am a huge promoter of DSLR astro-mods. If your camera is used for astrophotography exclusively, why not modify your camera yourself?
If you have either a light pollution or h-alpha filter covering your DSLR sensor at all times, you are not required to install any extra replacement filters. I carefully removed the IR cut filter on my Canon Rebel 450D by watching this video from Gary Honis. This is known as a full spectrum “naked sensor” mod, and it was the best decision I ever made.
Processing H-Alpha frames taken with a DSLR
What about image processing, is it different than a regular RGB image? Yes, there are some differences in the stacking and processing procedures, but not many. It is still important to take as many exposures (light frames) as possible, to increase your signal-to-noise ratio. Capturing dark frames will also have the same noise-reducing qualities when stacking images using an H-Alpha filter.
This video tutorial should help you understand the process:
In a nutshell, the Hydrogen Alpha version of your astrophoto will use the Red channel as a luminance layer that you will then merge with your RGB version.
HaRGB in Photoshop – Adding Ha Data to an RGB Image
Hydrogen-alpha images are essentially black and white images. By combining them will a full-color RGB astrophoto it is possible to create a beautiful composite photo. Adobe Photoshop is more than capable of accomplishing this task. I’ll cover this processing method in an upcoming step-by-step tutorial.
Below you will find a simple example of what Ha-RGB processing in Photoshop can accomplish. For the example below – a very limited amount of interated exposure time was used. As I collect more data, I will update the graphic with a more finalized result.
Here’s the bottom line
If you own a Canon DSLR modified for astrophotography, a clip-in Ha filter should be on your shopping list. I waited far too long before making this decision. Combining H-alpha exposures to your existing RGB data can greatly increase the amount of detail and contrast in your astrophotos. This is especially evident in deep-red emission nebulae.
The clip-in versions offered by Astronomik are rather expensive but are a perfect fit for modified or stock Canon DSLR’s.
In the Summer of 2017, I began using the Astronomik 12nm Ha filter on a one-shot color CMOS camera, the Hypercam 183C. This time around, I used the 2″ round mounted version of the h-alpha filter. Although a camera with a mono sensor will perform much better through narrowband filters, a one-shot color camera can still benefit from these narrow wavelengths of light.
The Ha filter factor
The ability to image during the full moon and surrounding week results in much more time under the stars. No more wasted moonlit clear nights. If you live in the city, a Ha filter cuts through even the most severe light pollution. Astrophotography in the backyard becomes possible, and that’s a beautiful thing.