That's very nice. Thors Helmet is a tough and dim target. The stars aren't necessarily out of focus that could just be seeing at that focal length.
I use a white teeshirt over the end of the scope (tie the arms together to form a bit of tension and smooth it over the end of the scope) and take dusk flats aiming for about 1.3rd histogram. I usually only take 3 and 3 darks. Light box would be nice as you could take darks anytime but not needed.
I've also used a piece of opaque white plastic packing material fixed with a rubber band over the end of the scope. Geez I'm giving you all my high tech secrets here!
Greg.