Bart wrote: I use a motor similar to this one:
there exists also a version with 400 steps per revolution.
Bart wrote: You don't need all that resolution on a Celestron. it is already 'geared' by the focuser thread inside. 200 steps/rev is plenty. 400 would also be fine.
I use 400 steps/rev on my Maksutov Newton telescope with a Crayford style focuser. This is a much more sensitive focuser, because the translation per rotation of the focus shaft is more. Diameter of shaft: 4 mm, and so: 4 * Pi / 400 = 0.0314 mm/step (full step!) This is a faster system ( f / 4.8 ) which is more critical to focusing than the Celestron ( f / 6.3 ) and even then it -just works- with no problem.
dmsummers wrote: @mhammady: The 8SE appears to have a thread pitch of 24 threads/inch or 0.94488 threads/mm (1.0583mm/thread(or 1 rev)). You're at F10, so your CFZ is on the order of 2.44*10^2 (244um). If you were to use the Celestron (1k cnts/rev), the math works out to 1.058um / count or 230 counts "length" for the CFZ. The ASI has 5760 cnts/rev, which works out to 0.1837um/count (1328 counts CFZ length). That's overkill unless you know you'll be adding a focal reducer. As Bart indirectly points out, at F10 and 200 counts/rev, you'd have a CFZ length of 45 counts. That seems "skinny" to me. 400 counts/rev would give you 90 counts CFZ length. 1000 would give you room for that focal reducer you're going to want . My 2 cents. Cheers, Doug.
dmsummers wrote: Hi Mohamed, I got the thread pitch info from these sources (not authoritative, but the info seems trustworthy):
ChrisRowland wrote: The Celestron focuser shaft is a fine pitch screw thread and one rotation moves the primary mirror by the thread pitch. This www.cloudynights.com/topic/645522-celest...ct-and-edgehd/page-8 says the pitch is 0.75 mm so 0.75 mm travel per rotation. At 1000 steps per rotation 0.75 microns per step. Even running the SCT in hyperstar mode at F/2 the CFZ is 10 microns so no problem. Maybe someone at Celestron thought about this.