Star clusters can feel underwhelming compared to grand nebulas or galaxies. Besides the Pleiades, my favorite target of all time, open star clusters are usually pretty blasé to me.
That said, they don’t take 10+ hours to capture. Stars are bright, and it’s usually only a night’s worth of work (or less) and you’re done. I was looking for that when I decided to shoot the Messier 35 (NGC 2168) / NGC 2158 star cluster pair.
New targets are coming up, but are not quite in their optimal position to start shooting. I’m in this in-between phase and don’t want to start a new, longer project. Even though I wasn’t excited to shoot this star cluster, I think it turned out much better than I thought. Part of that is due to the tools and methods I used for post-processing, which I’ll explain below.
What is Messier 35 & NGC 2158?
Messier 35 and NGC 2158 are two stunning star clusters in Gemini’s constellation. During the winter months, they are accessible to astrophotographers in the northern hemisphere.
M35 is about 2,800 light-years from Earth and contains hundreds of bright, young stars. These stars are tightly packed in a spherical shape, creating a beautiful cluster that sparkles brightly. The cluster is estimated to be around 140 million years old, making it relatively young compared to other star clusters in the Milky Way galaxy.
NGC 2158 is a globular star cluster located in the constellation Gemini. It is estimated to be approximately 14,000 light-years away from Earth. NGC 2158 is estimated to be around 4.6 billion years old, making it a relatively old cluster compared to others in our Milky Way galaxy. Globular clusters like NGC 2158 contain thousands to millions of stars densely packed together in a spherical shape. They are believed to be some of the oldest objects in the universe, formed during the early stages of galaxy formation.
Capturing Messier 35 & NGC 2158
Planning Ahead
I first looked on Telescopious.com to see how it would appear with my equipment. Below is a screenshot of a rendering on Telescopious with my equipment specs dialed in. The field of view is very close to how my image ended up.
I strongly recommend using Telescopious if you have 2 objects in your frame, as it helps you figure out your composition. Another use for Telescopious is for planning a mosaic.
My Astrophotography Gear
For this project, I used my Astro-Tech AT115EDT telescope and Antlia V-series Pro LRGB filters on my mono ZWO ASI533MM camera. I shot about 1 hour of each filter.
Staying Focused
Focus is critical in every aspect of astrophotography, but keeping your stars focused and small with a star cluster is essential.
The rule is that the smaller the star, the more in focus you are. With the ASIAir, I use a ZWO EAF and refocus every 1 hour and after every filter change. I also check the progress every half-hour using the ASIAir’s “detect” button which displays the average star size. In other words, I’m fanatical about star size.
Although tools such as Blur Xterminator exist to help fix out of focus master files, there’s only so much that Blur Xterminator can repair. It’s important always to keep your stars in focus. If you don’t own a ZWO EAF or similar auto focuser, go buy one for $149. You will be thankful that you don’t have to go outside in the freezing Winter cold to refocus your telescope.
Post Production
Stacking Messier 35 & NGC 2158
When I finished M35 & NGC 2158, I had 240 subs at 1 minute each. This gave me exactly 4 hours of data. The breakdown is 60 subs for every filter. As mentioned above, I used LRGB filters to capture this on my mono camera.
I loaded all of these subs and their corresponding flats, dark, and dark flats into Astro Pixel Processor (APP). For settings in APP, I keep to the default settings for almost everything. There are only two things I change on the integrate tab:
- For integrate, I choose average and then quality.
- For filter, I choose adaptive rejection and then just keep the default settings.
Pinxinsight Processing
I opened all 4 master files for Messier 35 & NGC 2158 in Pixinsight. I followed a normal regimen of steps that I always follow for processing astronomy objects:
- Dynamic Crop to crop and align all photos in the same position
- Dynamic Background Extraction to remove light pollution
- Blur Xterminator (Devoncolution) to eliminate any bluriness that may have happened while I shot all my subs.
- Noise Xterminator to remove the noise from each master file.
- Channel Combination to combine the R, G, and B channels together and create a color image.
- Scripts > Image Analysis > Image Solver to solve the new RGB image which preps it for color calibration
- Spectrophotometric Color Calibration to color calibrate the new RGB image.
- Histogram Transformation of the RGB image and luminance (L) image so they become non-linear.
- Color Spaces > LRGB Combination to layer the luminance channel on the RGB image.
- Intensity Transformations to color adjust R, G, and B and add saturation and contrast.
- Local Histogram Equalization to sharpen the image. I usually mask the background before I do this so I don’t sharpen the background.
That’s about it! After that, I saved it as a 16-bit TIFF file and brought it into Photoshop.
The Last Stop for Messier 35 & NGC 2158: Photoshop
As mentioned above, the next step is Photoshop, where I use a plugin called Star Spikes 4 Pro to add diffraction spikes to the larger stars.
Let’s pause here. I know a plugin to create diffraction spikes that you get out of a reflector telescope like a Newtonian is controversial. Some people hate diffraction spikes and want to eliminate them from their photos while others love them. I fall into the latter camp. I like them so much that I use a plugin to imitate them. I should just buy a Newtonian Telescope so this way I don’t need to fake it any more!
In Photoshop, I open the image and then apply the Star Spikes Pro 4 plugin, a paid plugin that can be purchased here. The plugin allows you so many different customizations for diffraction spikes, it’s crazy. I didn’t know diffraction spikes could be so diverse.
Rollover the image below to see it with and without the diffraction spikes. You will understand why I added them. It makes the photo pop so much more, giving it a next level of excitement.
The only big downside to Star Spikes Pro is that it’s only available on Windows. Sigh. I’m lucky enough to have both a Windows and a Mac, but I know most people will have either one or the other.
In Conclusion
Messier 35 & NGC 2158 are an amazing star cluster pair for astrophotographers. While star clusters may be underwhelming compared to grander astro targets, there are ways to enhance them, such as using the Star Spikes 4 Pro plugin to add diffraction spikes and saturate the stars.
As an astrophotographer, I aim to make every image as beautiful as possible. Maybe I’m an “art astrophotographer.” I’m not looking to create pictures as close to reality as possible; I’m looking for beauty.
