Figure 1 - Processed With Gradient Exterminator
I decided to do a test of removing gradients in astrophotography images using Photoshop as described in a recent post by Trevor Jones of AstroBackyard.com. In his post, Remove Gradients in Your Astrophotos with Photoshop, Trevor describes a fairly common method of removing gradients using Gradient Exterminator (which I use), but also another method which may give better results in certain cases (Figure 3).
The method involves: 1) Removing the DSO from your image, 2) Creating a synthetic flat frame, and 3) Subtracting the flat frame from your image (Figutre 4). I am not going to go over the process, Trevor does an excellent job of that and he has a video to go a long with it. Since I just got done processing M94 as I normally do with Gradient Exterminator and heavy additional processing, I gave the other method a try. Since I don't use flats (yet), my unprocessed images typically have plenty gradients and not only that, the night I captured M94 the sky looked clear but there must have been some additional moisture or something because many of the subframes were redder than normal.
The results are as follows:
Figure 2 - Minimal Processing (Levels) - Lots of Gradients
Figure 3 - Crop Processed With Gradient Exterminator
Figure 4 - Crop Processed By Subtracting a Synthetic Frame
As you can see from Figure 2, plenty of gradients and vignetting to start with. The results of processing were good either way at least to me. The gradients were successfully eliminated in both images and it is arguable which image is better. So which method involved less processing thus was easier? I took a while doing the synthetic frame method but that was because I am not familiar with this method not because it is inherently time consuming. In fact I think this method may ultimately be quicker. I can see how this method works well for small objects and as Trevor pointed out, not be good for objects which fill up the field of view.
If you have never visited AstroBackyard.com before, I would highly recommend you do. The site contains easy to follow tutorials and well thought explanations.