So you’ve invested in the necessary equipment, learned how to use it, and gathered a night’s worth of photos of some deep-sky target. You’ve also shot the calibration frames that you will need. The subframes from your camera, with a washed-out sky and a dull, low-contrast rendition of your target, are a far cry from the colorful astrophotos you’ve seen before. That means it’s time to head to your computer to process the photos and make them come alive.
After moving the photos to a computer hard drive, I convert the RAW files to TIFF files. I do this by importing the photos into Adobe Photoshop Lightroom. Once in Lightroom, I sort through the photos and choose all of the ones I want to include in my stack. There are always some I exclude because of soft focus, a cloud blowing across, or poor tracking. This step can be done during the stacking process, but I find it easiest to do in Lightroom. If any photos need to be rotated, Lightroom is also the easiest place to do this. I used to adjust the exposure of the subframes in Lightroom, but I have stopped doing that because it changes the brightness of the hot pixels, making the dark frames inaccurate. At this point, I export the files that I want to stack. The RAW files are exported as TIFFs.
Next, I make sure all my calibration frames are oriented the same way (vertical or horizontal). Then I launch Nebulosity, the software I use for calibrating and stacking. I open my light frames and calibration frames; then Nebulosity creates master calibration frames and applies them to the photos I have specified.
When this is done, it’s time to stack the calibrated subframes. I select one or more stars in each photo, depending on the stacking method I choose. Nebulosity then stacks the photos automatically. The stacking method I use does not add the photos to each other; it averages them. This eliminates most of the random noise because it is just that—random. Therefore, it is averaged out. This leaves me with a photo that can be “stretched” significantly without being overpowered by noise.
After the photo is stacked, I save it as a TIFF file, then open it in Adobe Photoshop. The first thing I do here is duplicate the background layer and turn off one of the two layers so I have it as a backup.
Unless I have used real flat frames, the next thing I do is make a false flat. This is done by duplicating the stacked photo and blurring the duplicate until the stars are basically gone. Then I use the clone stamp tool to eliminate any stars, as well as the target object. When stamping out stars and my target, I try to match the final brightness to what it would be if the stamped-out object wasn’t there in the first place. (By carefully taking these objects out of the false flat, I avoid degrading the objects when I apply the flat to my original photo.) The flat is then subtracted from the original stacked photo. If done properly, this should leave the sky evenly illuminated and neutral gray. Sometimes this process makes the sky black. If it does, simply darken the false flat and try again.
After I have applied the flat, I darken the sky by using levels. One common mistake that many beginners make is to darken the sky until it is black. In real life, however, the sky is not 100% jet black. Also, if you make the sky jet black, you will clip data from your faintest stars and from the faint areas of nebulas or galaxies. Thirdly, a sky that is very dark gray (with RGB values of about 20, 20, 20) is more aesthetically pleasing than a black sky.
Soon after we began doing astrophotography, we purchased a set of Photoshop actions for astrophotos that was put together by Noel Carboni. I used to use these actions quite heavily. I don’t use them as much now, but I always use the “Enhance Star Colors” action, usually right after darkening the sky.
Many of my early astrophotos had large bloated stars by the time I was finished processing the photos. Star bloat is something that you need to watch closely as you brighten your target object. I have found one particular method to be very helpful in decreasing the size of your stars. Select your brighter stars, feather the selection slightly, and use the “Minimum” filter. You can then decrease the effect by using the “Fade Minimum” tool.
After making my stars smaller, I work at enhancing my target object. Star clusters do not require a lot of brightening, but nebulas and galaxies, which are my main targets, always need to be brightened. There are various ways to make your subject brighter. I often use a combination of several curves, levels, and/or Carboni’s actions to lighten my target. I brighten my subject in stages instead of trying to do it all at once with one tool. I have found that I need to be careful when adjusting brightness, or the sky and stars will start to look unnatural. I often use adjustment layer masks, but if you aren’t careful when using these, your target may get unnaturally sharp edges.
Where my processing goes from this point depends a lot on what my subject is. If my target is an emission nebula, I enhance the reds in selective color. This often does a very good job of bringing out the nebula, as well as making its color more natural.
If my target is a galaxy, I use contrast and sharpening to enhance the structure within the galaxy. I sometimes do this with nebulas too, but much less than with galaxies. I sometimes use selective color, vibrance, and/or saturation to enhance a galaxy’s color. When using vibrance and saturation, care must be taken to keep the galaxy looking natural. I usually don’t adjust the vibrance or saturation when processing nebulas.
I try to be pretty careful to keep my photos natural, and I avoid adding artificial details. I do, however, adjust the color of my target object to match the color in photos by other astrophotographers. Light pollution often affects the color in my astrophotos, so I don’t feel that it is unnatural to match the color that other astrophotographers capture.
A few objects, such as the Orion Nebula and Andromeda Galaxy, combine faint outer regions with bright core areas. The best way to photograph these objects is to take several sets of exposures—at least one set to keep detail in the bright core, and at least one set to pick up faint details. These sets are stacked separately, then combined using layer masks in Photoshop. This process should be done before any other adjustments are made in Photoshop.
Now you know my basic workflow. There are certainly other techniques and software that will also do a good job of processing astrophotos, and many astrophotographers get better results than I do. I’ve found that this procedure works well for me, though, and it should work for you too. Don’t go too fast, hit “Save” now and then, and remember that practice makes more perfect.
Both of these photos show the Flaming Star Nebula (IC 405). The photo above is an unedited 5-minute exposure. The photo below is the result of stacking 27 of these 5-minute exposures, then editing the stacked photo. All of the stars and nebulosity in the bottom photo are also in the subexposures, but some processing is required to erase the light pollution and bring out these details.