Imaging galaxies offers up its own unique challenges, but it can also be extremely rewarding. Just the thought of imaging galaxies, that is, collecting photons of structures millions of light years beyond is itself almost incomprehensible. My mind reels at the very thought that I am taking images of an object that far away made up of billions or trillions of stars, some of which will be harboring life that is possibly looking back at us, perhaps even taking images of our galaxy.
Well, where do you begin? First, you must think that if these objects are so distant, they are often very faint, and they are. Except for a few local galaxies like M31 and M33, these are often very faint objects that will require long integration times to collect light, and because of this are often void of color, so what do we do?
First off, it is a balancing act. If your sub-exposures are too long, you risk collecting too much light at the core and exceeding the well capacity of your camera. What that means is that you have reached the end of the sampling period, and your data is not as good as it can be. If that is the case, you just need to cut back on the length of your subs.
As far as gain settings, I shoot everything at unity gain. In my case, this means leaving the settings that my ASIAir Pro defaults to, but you can look it up for your camera and make the appropriate setting adjustments in your imaging software for your specific camera. On DSLR cameras, this is known as ISO, and the equivalent terminology in that case would be optimum ISO.
So, let’s say we have chosen to do 2-minute subs (120 seconds), we have determined this is fine as far as well depth, and we are going to do an imaging run. Be sure to turn your dithering on, you want all the help you can get where it is available. I would recommend doing a minimum of 4 hours worth of subs, and from that point on if you want to improve your image, double that time, and then double it again, so 8 hours or 16 hours.
In collecting lots of light to work with, you have sampled plenty of data to begin to stack your image and process it. Do not forget all those calibration frames, and once again just use all the tools at your disposal to build the best image possible from your data.
Be prepared to see a grey image at the end of your stacking. This is something I had to realize too, because we are often used to seeing some color to a nebula or cluster fresh out of the stacking aspect. To obtain any color from your galaxies, you are going to have to boost the saturation, likely more than you have had to prior, and that takes some getting used to.
As you begin to process your image, a few things that I will do includes my initial stretch with levels in Photoshop, and I will look for any gradients in my image. If you have an off greenish cast to my color, I will use the HLVG plug in to clean that up. And then it becomes a process of using the sampling tools in Photoshop and making curves adjustments to look at the balance between the core and the spiral arms. As long as you have kept your data collection below the limits of your well depth, you can clean that up pretty decent.
While many suggest using the Andromeda galaxy as a learning point, remember that this is one of the more challenging galaxies to actually image because it is so bright, but don’t let that stop you from going there. There are plenty of fine examples online to give you an idea of what can be done. Good luck and have fun.
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