Backlash Compensation?


#1

I’m really confused here, so please help!

I have auto focus motors on my two refractors. A Microtouch motor on a 120mm with R&P focuser. And a Moonlight motor on the 80mm with a Moonlight Crayford.

I used a dial indicator on both of them to measure and record the microns per step.
And that is when I noticed both focusers/motors have some backlash.
So, I set about trying to adjust it in SGP by putting some numbers in the provided box.
However, I just can’t seem to understand whether to put the compensation “IN” or “OUT”.
In other words, I’m not sure what direction is correct for each focuser.

I can see by the indicator movement (step-step-step) that the compensation is being applied by SGP.
But, trying it each way does not seem to remove the backlash, it just shifts it to the other direction.
Under actual imaging conditions, I think i have the backlash wrong no matter how I set it.
It’s making the autofocus miss the best results, although sometimes I do get good curves, just not consistently.

Please, what am I misunderstanding about how this system works?
A step-by-step review of the backlash compensation setup procedure would really help me.


#2

Doug,

The first thing you have to do is establish in which direction most of your focus changes will take place. In my case as the session goes on the temperature drops and I have to move the focus in to keep in focus.

Having decided on the focus moving in, the backlash compensation is set to the opposite, i.e. OUT.

Next you need to determine how much backlash you have when moving the focus out, measure this with your dial gauge and convert that into steps. You may find that 150 microns movement is needed before the focus starts to move, find out how many steps that is, i.e. for me 150 microns is 30 steps of 5 microns. Add a margin for safety, say 20 and you end up with 50 steps which is what you put in the backlash compensation box in the OUT direction.

So what this means is, when the focus is moved in by 10 steps, it moves IN 10 steps. If the focus needs to move OUT 10 steps, it actually moves 10+50=60 steps then moves back in 50 to end up 10 steps further out.

Also make sure that you have not set any backlash compensation in the focus driver or hardware or it will get very confused.

I hope that clarifies this for you.

Mike


#3

Thank you, Mike!
Your explanation helped me understand the adjustment much better.


#4

Your very welcome.
Mike


www.mainsequencesoftware.com