Stepper motor RPM for electric focuser

[martindale]

Hi

I'm trying to put together an electric focuser using the original stepper motor and control left over from when I upgraded my old EQ6 mount to GOTO.

These stepper motors have a RPM of approximately 2.

Will this be slow enough, or should I reduce it with drive belt and pulleys,

Also, what's the preferred way of connecting the motor to the focuser, direct drive with flexible coupler or belt and pulleys.

Don't know if it's relevant but, the scope is a 12" f4 Newt with OO crayford focuser with 10:1 fine focus reducer.

Cheers.

Paul.

 

[Tinker1947]

Stepper Motors don't turn unless there switched on so when its connected there need to be a clutch system or a way to unattached it from the focus if its to be used for normal focusing...

[martindale]

Hi Tinker1947,

I hadn't intended using normal focusing, just the electric.

[hughgilhespie]

Hi Paul,

The speed of a stepper motor depends on several things. It takes a fixed number of pulses sent to the motor to make it turn a full revolution, this number can be anywhere from 64 to 400 pulses per revolution. Then, to complicate things, each pulse can be sub-divided into micro-steps. So you can have 1, 2, 4 etc. micro-steps per pulse. 

I guess you want to use the motor and it's control electronics without making any changes which is fine but it does limit how much control you have. For your f/4 telescope, the Critical Focus Zone is about 10 microns. I don't know the details of your focusser, but if it uses a 3 mm shaft which is typical, then one full turn of the main focus knob will move the focus by about 10,000 microns. A full turn of the 10:1 knob will give 1000 microns focus change. Thus you need to move the fine focus knob by less than 1/100 of a turn to acheive good focus. With a 2 rpm motor, this means that you will have less than a third of a second to stop the motor when you acheive focus. Not impossible, but .................... So, I think you will need at least a 3:1 reduction using belts and pulleys and probably 5 or 6 :1 would be better.

If you could control the motor by varying either the pulse rate or the microstepping you could probably get away without any belts, pulleys, etc. and just couple the motor shaft directly to the fine focus shaft.

 

Hope this helps,

Regards, Hugh

[Gina]

That's what I do but using the little 28BYJ-48 geared stepper motors run from an Arduino via ULN2003AN stepper driver chips.  These stepper motors and also driver modules are readily available for just a few pounds.  I use either timing belts and pulleys or 3D printed spur gears between stepper motor and focus knob or shaft.

[ChrisLX200]

Focusers generally use geared stepper motors, to get both speed and resolution within a useful range.

ChrisH

[martindale]

Cheers Guys,

Very helpful, lots to think about,