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Old 05-06-2018, 10:51 PM
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OuterObsession (Jameson)
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Join Date: May 2018
Location: NORMAN PARK
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Hi Astrofriend,

I find this all very interesting as I have modded the crayford focuser on my ed100 to use a nema17 stepper motor.

I got a comment from IanL over at Stargazers Lounge about calculating the CFZ (critical focus zone) for a telescope - a similar calcultation you have shown on your website.

"Yes but typically each filter would start from a known position where it is near focus. Over time that position would be invalidated. Whether you can use only full steps depends on:

With regard to whether to use full steps or not:

Critical Focus Zone in microns = focal ratio x focal ratio x 2.2

So my scope is 6.38, making a CFZ of 89.41 microns (i.e. the range of focuser travel within which the image is in good focus).

Distance moved per motor step in microns = Distance moved per focus shaft turn in mm x 1,000 / steps per motor turn x motor gear teeth / focus shaft teeth

So my motor has a step angle of 1.8 degrees, making 200 steps per turn, I have an 18 tooth pulley on the motor and a 36 tooth pulley on the focuser shaft and the distance moved for one turn of the focuser shaft is 1.43mm.

Thus I get a distance per step of 3.575 microns, and 89.41 / 3.575 equates to 25 steps in the CFZ. The distance moved per step needs to be a reasonable fraction of the scope's critical focus zone (say 10-20 steps), so mine is a bit too high but not unacceptable.

If there were too few steps, then one would try half, quarter or eighth microstepping to increase the number of steps by 2, 4 or 8 respectively. If too big then there are a number of strategies - change the motor, change the gearing or just increase the number of motor steps per reported step number in the Arduino firmware."

Some thoughts I had were with your calculations you don't really need to know the total length of the draw tube as you'd never practically need to use the full length of the draw tube to achieve perfect focus.

Also as was pointed out to me, when you use less then one step on a stepper motor the motor will 'instinctively' get pulled to the nearest step after power is removed causing the software to lose track of which step the motor is on.

IanL says to have around 10-20 steps in CFZ does this sound correct? Also I'm assuming if you're using a stepper motor with a gearbox you would deduct the backlash from the total amount of steps in the CFZ?

Cheers,
Jameson
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