The focal point of an SCT mirror is not constant. If you move the
mirror forward, it would not move the focal point out.

If you turn the focus screw to pull the mirror out, the focal length
increases. If you turn the screw to push the mirror forward the focal
length decreases. Also, the focal length of an SCT is given generally
for the length when it focuses at the back end of the tube. If you
have a diagonal in your f/10 SCT, it is not operating at f/10, it's
longer.

edz