I'm a beginner astroimager, currently using a DSLR for DSO imaging, but I've already come across one issue in my photos that was bothersome enough to me that I spent some time coming up with a way to overcome it. The problem is: stars that are crisper (sharper) than they should be, because their cores, or sometimes their whole disks, have saturated. I suspect this problem happens more frequently with DSLRs, because from my reading it appears that astro CCD camera pixels have a deeper "well".
In some instances only a few of the brightest stars in my image have saturated and therefore look too crisp; in other cases, they may become too crisp as a result of some steps in my processing (which I should have probably avoided in the first place, but regardless, I may have the effect there and want to get rid of it).
Here is an example of what I'm talking about. This is not my image but one I found online that is particularly egregious.
As you can see, most of the bright stars are so saturated that their whole disk is at maximum white. I never get the problem to this extreme, but this is a good example - if we can fix this one, we can probably fix anything.
Here are the steps in PhotoShop:
- Select / Color Range
- Make sure dropdown at the top says "Sampled Colors"; if not, change it.
- Make sure dropdown at the bottom says "Greyscale"; if not, change it.
- Click on the center of one of the blown-out stars. Just the brightest stars will remain, indicating they are selected.
- Change "Fuzziness" slider to include more or fewer stars. For this example I put it at 180, almost the maximum.
- Hit OK
- Use Lasso tool, hold down Alt and draw around any DSOs to remove them from the selection
- Select / Modify / Expand by 4
- Select / Modify / Feather by 2
- Hit Ctrl-H to turn off the "marching ants" so you can see the results of the following steps more easily.
- Filter / Blur / Gaussian Blur. Set Radius - larger radius does more smoothing. I used 1.5 for this image since the crispness is so extreme; for others I might use 1.0 or even 0.7 for a milder effect.
- The previous step makes the stars seem less bright, so to compensate for this bring up Curves (Ctrl-M is the shortcut) and apply a curve that looks like this:
- Move the single anchor point on the curve further away from or closer to the diagonal to get the amount of brightness boost you want. Tip: before starting this whole process, duplicate your image and do everything on the duplicate. Then you can easily switch back and forth to the original and see how the brightness of the "after" stars compare to the "before". (You could also do this by copying your image to a new layer.) I generally let the modified stars be slightly dimmer than they originally were, but only a little.
- Hit Ctrl-H to turn the marching ants back on, then select somewhere in the image to get rid of the selection. (Otherwise, you're likely to be confused when subsequent processing steps don't work.)
- You're done!
Here is the result:
Hopefully you'll agree that this is an improved, more natural-looking image. If this is too soft for your taste you can apply the fix less aggressively, by doing the effect in a separate layer then setting its opacity to something less than 100%.
Hope this is useful to someone.
p.s. Is there a way to make inline images in a post appear in a larger size, so that readers don't have to click on them?