🚀 go-pugleaf

It is a well established result that SCTs which have moveable mirrors for
focusing are at their best at only one point among their many possible
positions. Since many things require the mirrors to move in order to
focus, it is unlikely that Joe Amateur is using the mirror to its best
capabilities. Different eyepieces, different diagonals, differences in
distance to the object focused upon, etc. will all change the position
of the mirror required to focus.

It occurred to me that it might??? be possible to use this fact to advantage.
Jim's Mobile now sells a focuser for SCTs that works without moving the
mirror (What an idea!). If positions of the mirror on one side of perfect
lead to undercorrected images and positions on the other lead to over
corrected images you could use this to tune you telescope for no
sperical abberation.

1) Do star test using JMI focuser (or other brand that doesn't shift mirror.)
2) Note whether under or over corrected. The star test is very sensitive at
revealing this if you know what to look for.
3) Using the focuser that moves the mirror (the standard unit) compensate for
under or over correction.
4) Star test again. Iterate above steps till everything is "just right".

Any chance this will actually work in practice? 

BTW, yes I am aware that what you gain by moving the mirror is tremendous
depth of focus and lots of ability to move the distance of the eyepiece,
camera, etc. with regards to the mirror.