I'm not sure the donut analogy makes sense to me. I think the mask has to be well out of focus to work. It's well out of focus on the front of the telescope and if anything, it's closer to focus on the filter but still well out of focus.

I was hoping you'd have an idea if it would work. I wonder if we could get an idea by putting a fine line in an X pattern on a filter to mimic a spider and seeing if it causes spider type diffraction spikes.

David