Combatting Vignetting

If you are at all familiar with SCTs you probably know getting a flat field is not easy due to Coma when adding a Focal Reducer. I don't doubt that taking a flat field is the best way to offset the problem. Right now, the vignetting is so severe, it is not possible to subtract it via software.

So I'm making an attempt with some equipment to help too. Classic SCTs don't have a 2" opening for the diagonal. I've got a Peterson 2" adapter and now I've purchased a ScopeStuff "True 2" Barrel to Canon EOS Adapter" Here's the blurb from the page:

These adapters attach to the front of SLR cameras and have 2" barrels
that fit in focusers and other accessories. The barrels provide an
unobstructed 1.75" (44.5mm) opening, and are threaded for 2" filters.

I like the idea of being able to use the 2" filters along with this adapter. So I'll experiment without the Focal Reducer in the train as well as with.