Amongst other uses, the adapter allows the direct connection of the 2.5" PHOTOLINE corrector for APO refractors to focusers with female M63x1 thread, like the 2.5" Starlight Feather Touch focusers or the modern 2.5" rack and pinion focusers from TS.
Whether youre doing planetary visual observing, planetary imaging, or deep sky imaging, the Bahtinov Mask is a great accessory to get the best focus ...