Refractors and Newtonian reflector telescopes come equipped with externally-mounted focusers that precisely position an eyepiece or camera near the focal plane of the fixed primary lens or mirror to achieve a sharp image.

Schmidt-Cassegrain telescopes, however, are a little different. They use a single external focusing knob attached to an internal mechanism that moves the primary mirror itself, at the back of the tube, to focus the image. This clever scheme has the advantage of keeping most of the focusing mechanism inside the telescope itself, out of harm's way. It results in the modest convenience of a stationary eyepiece position during focusing. It also allows a very large range of focus that accommodates the extra path length of binoviewers and other accessories as well as the ability to focus on terrestrial objects relatively easily.

But standard focusers for SCTs have two main disadvantages. For some observers, the standard focusing mechanism for SCTs is somewhat coarse, exhibits backlash (play in the focus gears), and only operates at a single 'speed' without any fine control of the focus position. A more serious drawback for SCT focusers, however, especially for astrophotographers, is image shift. This is a consequence of the geometry of the focusing mechanism. The focusing knob of an SCT is located off to the side of the primary mirror, and as it's turned a rod pushes the mirror near its edge to displace it. However, the mirror itself is held near its center on a long baffle tube. So when the knob is turned, the mirror tilts slightly. This results in a movement of the image in the eyepiece or on the camera sensor. It's easy to see visually and it can be annoying when observing at high magnification. And when trying to focus using small sensors at long focal ratios, the image can shift all the way off the sensor!

Read the full Guide here: https://agenaastro.com/upgrading-a-focuser-on-your-sct.html