Problem doing automatic focus with FocusMax

[jimao22]

Hi,

I have a problem regarding my Moonlite focuser drived by a Lunatico Astronomia steper motor via FocusMax. Is been a while since this unwanted behavior started, but I did not found a cause for that.

If you look at the V-curve, you can see it is very unbalanced. Using this curve further on,  I can find the focus, but each time I need to refocus, my focus point decrease with few dozens steps (I have a very small step size and around 90,000 steps for a whole ride of my focuser). For example, first time when I re-calibrate the focuser, everything is in focus at 8000 steps, let's say. But if I focus again and again and again, several times, this focus point go down to 7000, then 6000 and so on...

This is not good, because I do not have a consistency of the procedure of focusing and it give me some head ache. 

I suspect I don't have a good mechanical connection between stepper and focuser, but when I checked, everything seems to be alright. Have anyone have a clue for this strange behavior of my  focusing system? I should add it wasn't like that from the beginning...

[alan4908]

Hi Jimao22 

I also have a Moonlite focuser which is driven by a Moonlite stepper motor and use FocusMax for automated focusing. To achieve consistent focus results with FocusMax you need to have a good set of V-Curves, which you don't appear to have at the moment. I've found that you need about ten good V-curves to achieve very good focusing. There is no point attempting to use the V-Curve (above) to achieve focus since it simply will not work.  So, I would suggest you concentrate on trying to get one good V-curve, if you cannot do that then I'd suggest you check your optical configuration, perhaps something has become misaligned.

Alan

[jimao22]

Yeah, but no matter I'm doing, I have the same V-curve (unbalanced), so I don't know how to fix this....

[perfrej]

That absolutely is because you have motor backlash. Set backlash compensation in FocusMax to the opposite direction you are focusing and start testing how big it is.

One way to test backlash is to drive the motor a few hundred steps in one direction, then do 1-step clicks in the other direction with your hand on the focuser's axis. Keep clicking until you feel the axis move and note the number of steps it took, then enter that number in FocusMax's settings for backlash. Your vcurve will then be more symmetrical and focusing more reliable and precise.

 

All the best,

Per

[jimao22]

Thank you, Per! I'll do as you said.