I got my 50 year-old cat out of mothballs, and used the 35 mm. extension tube off my Lunt LS60T solar scope and the scope ring from my Canon "L" 100-400 mm. IS zoom telephoto lens to make a compact spotting scope.
The Spiratone cat is threaded for a T-mount. The Lunt 35 mm. extension tube is also T-mount threaded, and is the 1.25" eyepiece adapter for my solar scope. The cat was always a macro lens, focusing down to 1.7 m. The resultant hybrid spotting scope maintains that focusing range.
The cat performs better as a spotting scope than as a lens, perhaps because my visual standards are lower. With a nominal 62.5 mm. aperture and large central obstruction, the scope still easily surpasses the views through the standard Bushnell Spacemaster of the 1960s.