My first foray into astrophotography was in the 1990s using film. I photographed the Andromeda Galaxy many times, usually using a Nikon F3 camera and a Nikkor 400mm f/2.8 lens. I thought what I captured back then was pretty good. But my images definitely couldn’t compare to those published from professional observatories, or even other amateur astrophotographers using large telescopes. I wanted to create an image of M31 that would rival the best of the published images I’d seen, but I figured the cost of the necessary equipment would forever be beyond my reach.
Fast forward to the mid 2020s, where digital cameras have replaced film and image processing software and techniques developed for major space-based telescopes like Hubble and Webb are available to the public. Creating world-class images of galaxies and nebulae is now within reach of anyone willing to devote the time and effort (and, admittedly, a chunk of change).
I’m proud to present to you the image of the Andromeda Galaxy that I’ve dreamed of creating for the last 30 years:
A total of 20 hours, 45 minutes of image exposure time went into the creation of this image.
The Goals
I had several goals I wanted to achieve with my image:
- Capture the blue tones of the outer reaches of the spiral arms, where hot, new stars are being born.
- Capture the warm tones of the galaxy core, where older stars exist.
- Prevent the brightness of the core from overwhelming the details there.
- Resolve individual stars in the galaxy.
- Capture Hydrogen-alpha (Ha) light from nebulosity in the spiral arms.
The Equipment
For this project, I was going to use the following equipment:
- Askar 103 APO f/7 telescope (a refractor telescope with a lens diameter of 4 inches)
- Askar .6x reducer/corrector (turns the 700mm f/7 telescope into a 420mm f/4 telescope)
- ZWO 2600MC Pro camera (an astronomy-specific camera with an APS-C-sized sensor
- ZWO ASIAir Plus astrophotography computer
- Sky-Watcher EQ6-R Pro telescope mount
- ZWO 2-inch filter drawer
- ZWO CAA (Camera Angle Adjuster that rotates the camera to achieve the same composition on the target night after night)
- Sky-Watcher 50ED guide scope
- ZWO 120MM guide camera
- ZWO Electronic Auto Focuser
- Pegasus Astro Powerbox Advance Gen 2 (to provide power to all the devices)
- Dew straps on both the main scope and the guide scope
- Optolong L-eXtreme filter (to capture Ha light)
Here’s what all that looks like when assembled and waiting for nightfall:
My Askar 103 APO telesope all kitted up and waiting for night to fall. Yes, there is a veritable rat’s nest of cables.
The Process
I began planning my M31 project earlier this year by using a free planetarium software package called Kstars to determine when the galaxy would be available in the night sky above my backyard. This application allows me to use an image of my backyard as the terrain in the program, so I can get accurate timings for when deep sky objects would appear over the trees. Using this software, I learned that around mid-July, M31 would rise above the trees in the northeast around midnight.
Starting my project when the galaxy would be above the trees around midnight meant that a clear night would give me at least 4 hours of exposure time before the sky begins to brighten at twilight. I knew I would need several nights to get enough exposure time for the image I had in mind, though I had no idea exactly how many hours of exposure time I would need.
Frustratingly, a combination of rain, clouds, and smoke from Canadian forest fires meant that the first clear night after July 15th didn’t come for over a week. July 23rd cleared out quite nicely by nightfall. Since Andromeda would not clear the trees until after midnight, I started the night by imaging several hours of a nebula cataloged as IC1396 and known as The Elephant’s Trunk Nebula (this is another project that is still underway. Keep checking for when I make a blog post about that image). After shooting a couple of hours’ worth of images on The Elephant’s Trunk, I moved the scope over to M31 and got an hour of exposure time before clouds started to move in.
And this pattern would hold over the next few weeks as we would have a clear night, then several nights of clouds, smoke or rain, then another clear night, and so on. Eventually, I managed to accumulate 12 hours and 20 minutes of exposure time in all. About half of these images were shot for 120 seconds each, while the rest were shot for 300 seconds each. Why the different exposures? Because I wanted to ensure that I was capturing data on the galaxy in a way that I would not be overexposing the core. I could see the histogram on each individual image as it was captured and I knew that I was nowhere near overexposing the highlights with the 120 second exposures and even the 300 second exposures were good, but I knew the mix of data was the best way to ensure that I had detail throughout the contrast range.
After each night where I collected data, I would process that night’s images, plus the previous nights’ images, so I could see how the additional data from each session would improve the details in the total image. After I had accumulated just over 12 hours of exposure time, I could see that I had achieved most of my intended goals: I had blues in the outer arms, warm tones in the core, detail in the core, and I could see individual stars and star clusters in the galaxy itself. All that was left was to gather the wavelength of light specific to emission nebulae throughout the universe: Hydrogen Alpha (or Ha) light.
Do you remember learning about atoms and the energy levels of electrons? I do, but I certainly don’t remember much beyond the fact that when an electron falls from a higher energy state to a lower energy state, a photon is released. The only way I remember that is that I came up with a joke using this phenomenon: I’m so lazy, I’m glowing! (Cue rim shot: ba da disss.)
Anyway, Hydrogen-alpha light is emitted by gaseous nebulae in space when the gas is being irradiated by a nearby star, usually one or more hot, young stars such as a blue giant star. The radiation from the star will cause the electrons in Hydrogen atoms to jump to a higher energy state, and when they drop back down to their lower energy state, they release a photon with a wavelength of 656.46 nanometers. I didn’t know that number by heart; I had to look it up on Wikipedia.
Capturing Hydrogen-alpha
So, you may ask, if 12 hours of exposure time wasn’t bringing out the Ha already, how do you do it? The answer is through the use of a filter known as a Dual-Narrowband filter. There are many types of these filters made by various companies. The one I own is from Optolong, and it blocks almost all light except for two bandwidths: that of Hydrogen-alpha and doubly-ionized Oxygen, known as OIII. Ha is on the red end of the spectrum, while OIII is blue-green-ish. For this project, I was only interested in the Ha light as I was already picking up the blue colors in the outer bands and didn’t need more.
So what does this Ha light look like in the Andromeda Galaxy? Here is what the 8 hours and 25 minutes of exposure time created:
Combining Ha with RGB
Notice all the red patches in the spiral arms of the galaxy image above? Those are the Ha areas I was interested in merging with my 12 hours of broadband (unfiltered) exposures. The process for doing so took me quite a while to learn, and I spent days experimenting with different ways of doing it until I found a process that worked and produced the image I had in mind.
The software I use to process my astrophotography is called Pixinsight, made by a company called Pleiades Astro. Many of the scripts and processes available in this software were developed by the scientists working with professional observatories and NASA.
The process involved working with the Ha/OIII exposures to process them to a point where I had brought out the Ha data I wanted to merge with my RGB data.
I then separated the Ha/OIII and RGB images into their individual R, G and B channels (making greyscale images containing data from the red, blue and green data of the full color images). The full RGB images were preserved for use later. I then discarded the G and B channel images from both files, leaving me only with R (red) channel data from the Ha/OIII image and the full 12 hour image.
At this point, a nifty process called Photometric Continuum Subtraction was used. This process compares two greyscale images to determine the differences between them and creates a new image with ONLY those differences in data. In other words, it compared my Ha red channel data with my RGB red channel data and spit out a new image with JUST the Ha blotches. Here’s what that looks like:
Once the narrowband Ha data were separated from the broadband red-channel data, a nifty script called CombineHaWithRGB was used. This script knows that the narrowband image file you provide it is to be added to the red channel of the RGB image you provide and it does it in a way that can minimize the effect to not influence the non-red parts of the image.
After that step, all that was left was to tweak the color and contrast to create the final image and, as they say, “Bob’s your uncle!” Here’s that final image once again:
It’s easy to get carried away when processing the color in astro images, and I worked to keep the Ha from being overly saturated and as natural-looking as possible. I think I succeeded!
And of course, I couldn’t end this post without showing off some of the details I was able to capture in my image, so here are three examples.
Resolved Stars:
Preserved Core:
Which blob of light is the actual core of M31? That would be the one the red arrow is pointing to.
Subtle Details:
There are dozens of such galaxies in my image.
In the interest of brevity (ironic, considering the length of this blog post), I skipped many of the details of how my final image was created. If you are interested in the details or need help processing your own astro images, feel free to use my contact form to reach out!
