Interesting because sometimes I get an mirror image of that graph with my small refractor. The right side will be great, but the left side is very flat. With a refractor and a Moonlite focuser, that is about as basic as it gets. Tagging this so in case anyone has any suggestions on what could cause it.
The same principles would apply for a refractor and during focus you should be pushing the imaging train (instead of the sct mirror) up against gravity. Otherwise you are relying on gravity to help it "fall" and that may introduce slop and stalls that appear to flatten out the curve.
So - with any system aim to push up against gravity and allow enough backlash compensation. And beware of focus exposures that happen too quickly after the focus change - which would result in poor star shapes and poor focus curves.
Frank