Celestron Focus Motor Steps


On my refractor I have a focuser that uses a stepper motor and I know the size of each step of the stepper motor and therefore it is easy to setup the step size in SGP. On my SCT, I use the Celestron focus motor and it uses a servo motor and I do not know how to deal with step size on that one.

Anyone here has experience with setting the step size on the servo motor?

Thanks in advance.


The Celestron focus motor does 1000 steps per turn of the focuser knob. I don’t know what that means as change of focus position because it depends on your scope. SCTs are tricky because moving the primary mirror alters the focal ratio of the scope.
If you really want to know the movement of the focal plane per motor step then this should do it:

  • Focus normally. Note the steps.
  • Add a spacer to your camera to move it out by a small and known amount, about 5 to 10 mm but whatever you can manage.
  • Focus again, note the new position.
  • You can now calculate the step size from the space thickness and the difference in focus position in number of steps.

Personally I wouldn’t bother, I’d start by setting 200 steps and trying it. If the movement is too small double it, if too large halve it.

Sounds like an interesting approach, thanks. I am looking to see what other users of this setup are doing as well.


In the Help file under Focusing - Auto Focus - Understanding auto focus there is a step by step guide on how to determine your step size.

Just follow those steps… Did the trick for me.

Clear skies!

I actually have followed that and it doesn’t work for me with a servo driven focus motor. Are you using the same focus motor as am I?


I also use the Celestron Focus Motor, but connect to it via the USB driver.


You mean as opposed to selecting the Celestron Focuser?


Hi Farzad

That’s correct. I connect to the one you have selected there.



I recently got a Celestron Focus Motor and had a difficult time installing it. I’m now using the Celestron Focus Driver which is located on the Celestron Focus Motor webpage.

When following the instructions to calibrate the motor with SGP I got images in the HFR region around 4.0 to 6.0 which calculated to a vary large Step Size (about 150). So I just used a step size of 20 (as shown in the instructions) and that seems to work well. Still testing it out…

Have you made any progress in getting your motor to work well?

Hi Bob. My experience with it is less than satisfactory so far. The documentation leaves a lot to be desired. We know there is a backlash to be considered, but there is no way to measure it or deal with it. When I get SGP to focus it I get a very out of focus result. I think I am going to look into putting my feather touch back on and motorizing it.

After considering the suggestions here and studying the documentations, I find it to be easy to determine the step size for any motorized focuser supported by SGP. The process simply is this:

  1. Bring the system into the best focus that you can, jot down the focuser position (P1, I call it) and the associated HFR (HFR1, I call it).
  2. Use the motor to move the focus OUT a little at a time, capture an image, note its HFR. Do this until the HFR is about 3 to 5 times HFR1, then stop. Note the focuser position, P2.
  3. Decide on the number of data points (DP) you want SGP to use when SGP is attempting to bring the system into focus. I am using odd quantity simply because it is easier and it has a center (balanced) position. I do not know what happens if an even number of data points is selected.
  4. Step size is then calculated as: (P2 - P1) / [ (DP-1)/2]

Simple as that…


P2-P2, is that correct?

ABS(P1-P2) I expect.

I had an error, good catch. Since we are moving the focuser out from a position P1 therefore P2 is greater than P1.