I have a late-1980s AstroPhysics refractor with great optics, but a not so great focuser. Joe LaCour of KineOptics came up with an accessory that provides a very smooth fine focus capability without requiring any modifications to the scope itself. The accessory uses a KineOptics HC-1A focuser with a 3D-printed “snout” that plugs into a 2” diagonal. Now I use the old focuser for rough focus and the accessory for rapid fine focus adjustment. For those who may not be familiar with the HC (Helical Crayford) focusers, they are extremely smooth and backlash free. Unlike the more typical threaded helical focusers, the Crayford adjusts very quickly. I added the extra long nylon bolt to give even more leverage and feel.
There are a couple important considerations. First, the focuser adds about 1 3/8” of length to the existing focuser plus diagonal. Therefore it is essential to have at least this amount of available inward travel on the main focuser to compensate for this extra length. You can see in my photo that there is not very much inward range left for this particular eyepiece. Newer scopes commonly are designed to accommodate binoviewers and imaging trains, and so are likely to have plenty of inward travel range. For older scopes, first find the focal position for each eyepiece without the accessory and note whether there is sufficient remaining inward focusing range.
A second consideration is that very tall heavy eyepieces are not well suited for the HC focuser as the extra weight will require the friction adjustment screw to be tightened excessively, loosing some of the adjustment feel that is the whole point of the exercise. Because my Lunar and planetary eyepieces are all quite compact, there are no issues for my application. Of course, one can also go back to relying on the original focuser as needed.
KineOptics charges a little over $100 for the HC-1A. I don’t know the price for the custom “snout”, but it will certainly be quite affordable.