Strange artifacts in picture of IC405

I agree with the others who described the likely cause. I have run into this issue before with my system as well. The only solution I have ever found is to take a second set of exposures composed differently so that the reflections are either absent or in a different location, then substitute some data from the second set of exposures for data from the first set using pixel math and a mask.  Works great, but requires an entire second set of data.  The only other solution I can think of is to see if you have the same issue in all channels. If not, you might be able to substitute, for example, some green data for your red just in the area of the arc, again using pixel math and a mask. You'd have to do some sort of linear fit, but that shouldn't be too hard.  This avoids the need for a second set of data but will only work if the reflection is visible only in a single channel.

- Jared