Here is a mask I made for the 18". I tested this last night and seems to work. Stereo glasses needed for this image. Joe.
-------------------- JMI 18" f4.5 Newtonian, split ring mount
10" f4.5 Newtonian on GEM (was a DS-10 once upon a time)
50 and 110 mm Mamiya RZ lenses with homemade adapter
ST-2000XM camera