Hi Raymond,
I'm not a huge fan of the focus knobs on the back of SCTs as they are *very* prone to image shift... which isn't an issue at low power/wide field, but becomes a huge, annoying problem at higher power when you go to adjust the focus and your target "image shifts" right out of the field of view! And for astrophotography, image shift will quickly have you pulling your hair out.
On my Celestron CPC1100, I have an aftermarket focuser (pictured) that includes knobs for both course-focus and fine or "micro" focus. This type of focuser (and there are a variety of them out there at many price points), not only makes it easier to tweak to a fine focus, it also eliminates the image shift issue because it no longer uses the movement of the primary mirror to achieve focus.
That said, I did not remove the original focus knob (or lock it), since I still occasionally use it for a very course focus at the start of an observing session.
Good luck!
Jonathan
