What is the precision required for Stepper Motor

ocuser is precise tool – it must allow to adjust imaging train position with high accuracy. But what is the exact accuracy? Well, it depends – mainly on focal ratio of instrument. There is a quality called Critical Focus Zone that, simply saying, describes the distance where the image is sharp. You can see in the table below how CFZ varies depending on telescope focal ratio:

Focal ratio	Critical focus zone in microns
f/3	22
f/4	40
f/5	52
f/6	90
f/7.5	140
f/10	249
f/15	560

Focuser must provide at least 3-4 steps for the distance of CFZ. So if you have for example f/4 instrument with CFZ 40um, your focuser step should be smaller than 10-15 um. This is not so hard to achieve. Most Crayford type focusers have shaft with 6mm diameter, so about 18mm circumference. That gives 18,000 / 10 = 1800 steps required per shaft revolution. It can be achieved with simple 200 cheap step motor and 1:10 gearbox. If you have 1/10 instrument, then you can use for example 400 steps motor coupled with main focuser shaft without any additional reduction.

What is the power required to run focuser?

Power is not the most precise term – to move focuser shaft we need quantity called torque. Torque is expressed in N*m or N*cm units. Imagine 10cm long lever attached to your focuser knob. The force you need to press the end of this lever to move focuser is the torque it requires. If you need 0.5N (it is more less the weight of 50g), then torque is 0.5 * 10 [N * cm] = 5 N*cm. 

See more:

http://www.blogeri.hr/blog-postovi/post/operation-of-stepper-motor-35780/

http://www.loopdesk.com/article/25893/which-nema-size-step-motor-is-best-for-high-torque