Hi Lester
On my Celestron C9.25 I use the JMI NGF-CM motorised focuser. The JMI website is here:
http://www.jimsmobile.com/index.htm
Do a Product search for NGF and look for the SCT models.
I first focus using the Celestron standard focus knob and this does shift the image, although only within the central 2/3 of my field of view, with x2.5 barlow and ToUcam.
Next, I lock the Hutech SCT focus lock which can be seen here:
http://www.sciencecenter.net/hutech/...uslk/index.htm
Then, I use the crayford focuser function of the JMI to get manually better focus. Then, I lock the JMI crayford focuser movement so control is passed from the manual knobs over to the built-in motor. Finally, using the battery operated motor control keys on the hand paddle, I achieve critical focus.
IMO, it is the only was to rapidly, reliably and easily achieve critical focus when imaging at these long focal lengths.
You can purchase quite expensive software controlled focusers that monitor focus through a software program and automatically make adjustments for e.g. temperature fluctuations which can change focus due to key metal parts expanding/contracting.
Cheers
Dennis