Processing the images
After I have accumulated all my data frames, it’s time to process them. I capture everything in camera RAW format to preserve as much detail as possible. During raw processing, which I do in Adobe Camera Raw, I clean up each exposure and make any adjustments that will benefit the image (which is the topic that would require several articles to thoroughly explain). Once each file looks as good as I can make it, I save it as a 16-bit TIFF.
Remember how we dithered the frames earlier? This is a technique borrowed from CCD processing to increase the signal-to-noise ratio. After processing the raw images, I upload all the individual image files into RegiStar, which is software that allows you to easily combine and align multiple images into a single composite. I align each filter set of images based on the stars, which appear slightly displaced between each exposure due to our dithering technique. Now, any constant background noise becomes random, and if you average all that noise together, it largely cancels itself out. The result is a master frame for each specific filter that has more and cleaner data than any single shot. I do this for both the RGB frames and the Hα frames.
Next, I use RegiStar to take my three main files — the master RGB frame, the star flare frame, and the master Hα frame — and perfectly align them with each other. (I have detailed my methods for processing in Adobe Camera Raw and Registar on a DVD that is out of print, but I can burn one for you if you are interested. Contact me through www.astrophoto.com.)
Now it’s time for Photoshop. I open the master RGB frame and make any additional corrections I think are beneficial, including some star reduction. When it finally looks the way I want it to, I add the star flare frame on top of the RGB frame as a layer and align the two using the Move tool. I change the blend mode to Lighten, which tells Photoshop to take anything from the star flare frame that is lighter (the flares) and add it to the master RGB frame. Like magic, featureless stars in the RGB frame now have both color and proportional size. You can control the degree of this blend by making the flare layer lighter or darker — a neat trick. I save this new RGB-flare master as a Photoshop file with layers before flattening it to simplify the next step.
Here’s where the real fun begins: adding the Hα data. Following the same process as before, I add the master Hα frame as a layer on top of the flattened RGB-flare image. I again change the blend mode to Lighten and watch as the RGB-flare image acquires all that beautifully red Hα light. Like before, you can control how much Hα light you blend in by making that layer lighter or darker. You can also use a Burn/Dodge tool to selectively add or subtract Hα data from your final image (which is also possible using layer masks, but the process is a bit more complicated).
When you are satisfied with the final RGB-flare-Hα image, flatten all of the layers and enjoy your new Hα-enhanced view of the cosmos. I hope this helps you get the most out of your fantastic Canon Ra camera!