Here is a picture of the focuser. Note the tension spring to eliminate any slack in the belt.
Here is an image taken with a Canon 85mm F1.8 at F2.8 focussed with the stepper motor.
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http://members.optusnet.com.au/~cheekyfish/SAG.jpg
and with the Canon 300mm F2.8L at F2.8 focussed with the stepper M11 Wild Duck
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http://members.optusnet.com.au/~trlee8/WDS2.jpg
I think you will agree that very accurate focus is noticeable in any final astrophotograph no matter what focal length.
It certainly surprised me.
Bert