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Because the camera has a different focus point from the eyepiece. TURN the focus knob until the camera comes into focus. OR you can buy a "focus ring" for the eyepiece which attaches to the barrel of the eyepiece and is adjustable-- then focus with the camera FIRST and then insert the eyepiece with the ring on it and lift it UP until the image comes to focus-- then tighten the ring. NOW you can use that eyepiece to roughly focus the camera.
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.
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.