I've had the same problem. Unfortunately, these are knock-off scopes. If the shaft that the focusing knobs attach to is still there, you can replace the knobs themselves by ordering THIS part. If the shaft AND the knobs are both missing, you'll have to replace the entire focusing mechanism (sorry, but I don't know where to order the housing from). Hope it's more helpful than the prior answer.
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The Celestron manual for this scope has nothing to say, but on the underside of the focuser assembly, between the 2 focus knobs, you may see 2 screws. They might be thumbscrews or some other screwhead type. One is the adjustment for the focuser internal clearance, or looseness, and the other is a focus travel lock. If present, try backing both off 1/2 turn from tightness, and then experiment to see which is which. The focus lock will have no effect until tight, when the focuser will cease moving, as is now. The other screw will gradually remove looseness and rattle from the focuser until it just moves smoothly, with no back and forth loose feeling at the knob. If neither of these screws is visible you will have to take it to a binocular and telescope shop, where they should not charge much.
The focus knob is not supposed to feel like that. On my Celestron it had almost become detached from the focus lever which attaches to the main mirror inside the tube. There is a threaded shaft and locknut which had become loose.
If you feel DIY enough, and you understand about corrector plate centering, follow this
Focusers can be replaced and or upgraded-- you disassembled the focuser-- you will need to reassemble it properly -- you might have also messed up the screw threads in the holes and may need to try using a slightly larger size screws. Your local hardware store will have them.
If there is nothing on the objective lenses or eyepiece lenses then it most likely needs collimation. This is where you adjust the secondary mirror while viewing an out of focus star. You need stable sky conditions to do this.
There are most likely three small screws at the front of the telescope and these are what you use to adjust it. As one screw is loosened then another is tightened. Do this in very small increments. If you have your manual it should outline how to perform this. If not go to this web site: http://www.astrosurf.com/legault/collim.html. It will show examples of how objects look both in and out of collimation. There is a section further down the page for SCTs.
A very handy tool for this is to replace your screws with Bob's Knobs (http://www.bobsknobs.com/) . These replace the screws and allow you to adjust it by hand instead of a screwdriver.
The other focusing problem I've experience with my Celestron SCT is to have the focus knob loosen up and then slip as I turned it so that I could never focus with it.
I think that the focus shaft has become un-attached by turning too far. Try placing the telescope facing directly upwards and start turning the focus knob clockwise. A little vibration on the tube will help the mirror to settle downwards. This may get the threaded rod back into the knob.
Still not enough information, but taking a stab at it, I'll assume you mean the pinion rod that goes between the focus knobs, and engages a gear rack on the focuser. There are locations where they can be purchased, and you would need to provide the OD and pitch (teeth per radian) to match.
Try: http://www.pinionrod.com/ as a source.
I'm assuming this is the Maksukov-Cassegrain telescope. Turning the focus knob CW should get you focus at infinity by moving the primary mirror away from the eyepiece. As you turn the knob it should move in and out relative to the real of the scope. If that's not happening the focus screw may have been turned CW too far and no longer be catching the threads in the rear plate of the scope.
You may be able to get it to rethread by holding the front of the scope toward the ceiling and turning the knob CCW. Attached is a photo of what the scope looks like under the rear housing. To prevent the focuer shaft from turning in too far the knob should be firmly seated agains the rear of the shaft. I think on this model this is a friction fit.