Celestron focus motor

Hi Mohamed, I got the thread pitch info from these sources (not authoritative, but the info seems trustworthy):
agenaastro.com/articles/guides/miscellan...reads-explained.html
agenaastro.com/articles/guides/focusers/...ser-on-your-sct.html
Cheers, Doug

dmsummers wrote: Hi Mohamed, I got the thread pitch info from these sources (not authoritative, but the info seems trustworthy):
agenaastro.com/articles/guides/miscellan...reads-explained.html
agenaastro.com/articles/guides/focusers/...ser-on-your-sct.html
Cheers, Doug

Hi Doug,

I read those articles before, and the 2" 24TPI standard mentioned in this article referring to the mounting thread on the OAT backside. I don't think they meant the thread size of the focuser shaft connected to the back main mirror. I still think it is much coarser than ~1mm because of the long-distance it can travel.

Yea....you're probably right. I had a similar question for Celestron Tech support when I fit my ASI EAF motorized focuser to my RASA 11. They told me it was 0.75mm. It's entirely possible that they've used the same pitch on most of their scope sizes, but I wouldn't bet my life on it. I suspect if you use 0.75mm for your calcs, you're not going to be very far off....

The Celestron and Crayford focusers work in totally different ways. The Crayford works by having a small shaft against the side of the focuser tube so one rotation moves the focus by the circumference of the focuser shaft. Fo4 a 4mm diameter shaft about 12.5mm.
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.

The Agena Astro article is talking about mounting thread pitches, nothing to do with the focuser internally.

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.

Thank you Chris for confirming the thread size of celestron OAT focus shafts. You are correct, with this pitch, even 200 steps motor will give 3.75 um/step which is still sufficient in C8 with CFZ 97


Thank you
Mohamed