I have an old 8" dob and it has a R&P focuser all metal but I find myself going back and forth because of slop in the system.
I have priced a crawford clone that could be used as a replacement however I would like to use what I have. This is a 2:" metal focuser with a 1.25 insert.,is there a way to make this system better without replacing ? Also is there a 1.25 insert that would give fine tuning focus ? I am seriously trying not to spend a lot of money on this situation. I am sure someone has covered this before so links will be fine.
These focusers can (with care and patience) be made to work well for lighter eyepeices like Plossls or other lighter weight 1.25" eyepieces, but my experience is that they can be less satisfying when using heavy eyepieces. If you mostly use light eyepieces, then I think it is worth the time to try to tune the focuser. If you see yourself ever obtaining heavier 2" eyepieces, my advice is to just invest in a better focuser.
Ok, so you want to tune it.
First, a common issue with these focusers is that the screws that hold the rack to the tube can come loose. Rack the focuser out so that you can see the end of the rack itself, then rock the focuser knob back and forth and observe the rack to see if it has any slop against the tube itself. If it does, then simply tightening the screws on either end of the rack (because both will be loose). If the rack is loose, this may solve your problem.
Next, typically these focusers have a metal back that holds the pinion gear shaft in place. Some of these use shims to set the gear lash. Others use a spring or a backer. Depending on the design, it may be possible to adjust the pinion engagement and while it can be tedious it can make a big difference.
Next, some of these focusers have a nylon strip that is across from the pinion shaft and this strip uses small screws to adjust the pressure of the focuser against the glides. Sometimes it is possible to slightly tighten these screws to make up for wear.
Last, are the glides that are generally cast into these kinds of focusers that sit across from the pinion shaft. Vixen and many others had some kind of surface treatment on these and this treatment does wear out.. When it does, even when you have the zero gear lash, the focuser tube will be loose in the housing and this will cause the focuser tube to "chatter" as you rack it in and out. I have tried with little success to correct this kind of problem. Teflon Tape is marginally effective (Scopestuff) but in the end, I found it to be temporary at best.
Some of these have a plastic ribbed material with glue on it that acts as the bearing. I am not aware of a source of this material so if yours uses this and they are worn, best you will do will be to fabricate your own replacement. I think very thin strips of Bamboo might work, but would likely require replacement from time to time.
Again, if you use mostly light eyepieces, you can probably tweak the focuser a bit. The pinion gear mesh is usually tedious to work with but you can usually get it right.
The worst case is if the focuser has glides with the impregnated bearing surface where that surface has worn. It will be almost impossible to restore smooth movement to this kind of glide. Teflon tape will be to thick and even heavy grease will not provide support. I have had zero success restoring this kind of focuser to really smooth operation. The kind shown in the picture with the glide strips can be made to work because there is sufficient space to allow for the use of alternate materials.
Hope this is helpful.
(and yes, I realize that it is a refractor focuser in the picture, but most of the older Newt non-Crayfords had at least some of these characteristics.)
Edited by Eddgie, 11 March 2018 - 12:23 PM.