Possibly my favorite part of the hobby of astronomy is the continual improvement of a wide range of skills needed to produce images. The image processing stage makes all of the patience of hardware acquisition, maintenance, fine-tuning, and operation worth it. I would like to take my favorite targets, the Deer Lick group and Stephan’s Quintet, and focus on my journey through learning astronomical image processing.
2019
I first imaged this target around November 2019 using my SkyWatcher Flextube 300P Dobsonian, newly-acquired Tom Osypowski equatorial platform, and a Nikon D5300 DSLR. I was able to get two minute exposures with this setup, though it looks like I was not able to keep many. Even though the equatorial platform was capable of being guided, the results were not really consistent. That could have been mostly operator error.
Back then, I used Siril to stack and process the images and may have done some touch-ups using GIMP. Flats may not have even been a thought. I really had no idea of color calibration, and I think I just adjusted color balance based on the histogram peaks as I was stretching. With such short total exposure, denoise was often heavy-handed, almost turning the images into watercolors. I believe I was trying to get results that I saw from others, whose images contained much more exposure time, with only processing. Stretching was hard MTF, with no regard for star color.
Though today the results seem laughable to me, the ability to capture details in distant galaxies from my backyard using modest commodity equipment was mind-blowing. I got to see this entire world of worlds for myself.
I present to you Purple Deer Lick, ca. 2019 (colorized),
2020
In the beginning of 2020, I upgraded from my Dobsonian on a platform to a real equatorial mount and mountable Newtonian. Not only did this allow me to get extended exposure times on targets, it let me do it from inside the house through the majesty of local area networking. No longer did I have to sit in freezing temperatures adjusting platform tracking speeds and resetting the telescope. I used APT to control the telescope and let it slew and image targets all night long. In April of 2020, I acquired a ZWO ASI2600MC, my first cooled dedicated astronomy camera. This was a significant upgrade to the DSLR, both in noise and in quantum efficiency, as well as losing the horrible lossy compression used by Nikon cameras that would result in concentric colored rings.
Processing still included Siril and GIMP. At least I finally learned how to do color calibration in Siril, so things were no longer purple. I learned a bit about the subtleties of denoising, not completely butchering the images. And I was able to combine multiple nights of data together into a final product.
It at least looks like galaxies now!
2021
In April of 2021, I finally bought PixInsight. What a life changer that was. Not only is it full of tools specifically geared to astronomical image processing, it allows you to open and work on multiple images at once. It has superb masking tools and denoising algorithms that beat the socks off anything I could do in GIMP. And it is extendable, letting enthusiasts write their own tools within it.
I imaged this target all over again in November 2021. I had learned some things about flats, though my method at the time was still not perfect. I also learned to deal with backgrounds a bit better using dynamic background extraction. Fixing the background lets you stretch harder, showing more dim details. I was able to display a little bit of the integrated flux nebulae in the background of this image.
Deconvolution was a strong desire back then, but it was extremely difficult to do without introducing glaring artifacts. I forewent it in this image. Star color preservation was still not even a thought. Seems like I kinda went back to that purple feel.
2022
In 2022, life changed for me again. Russell Croman released neural network-based tools for PixInsight. Long gone were the days of spending hours minutely adjusting sliders in TGVDenoise and Deconvolution. With a little CPU horsepower, I had star removal, sharpening, and noise reduction in minutes. And the results were remarkable.
Color calibration was still tricky. I would try to use the photometric color calibration tool, but it would mostly not produce desirable results. The green channel would often still be too strong, and the reds would by muted, killing any hydrogen regions in galaxy images. I mostly still used the manual color calibration tool by selecting the core of the galaxies.
At this point at least, star color preservation was on my radar. I would use the masking and blurring techniques to try to bring some color to the cores of saturated stars. And being able to remove the stars and process them separately made a big difference. And my background reduction techniques were improving.
2023
I decided to give the data from 2021 another go. With the new spectrophotometric color calibration tool, updates to the Xterminator tools, and some tricks I’ve learned in background extraction and star processing, I knew I could still do better than last year. It’s a great time to be an amateur astronomer! And as much as I love color, being gentle with saturation makes all the difference between an image of space and a cartoon. Gentle star reduction also helps the targets pop through.
