I built a home-brew autofocuser for my 600mm f/4 Nikon lens using a stepper motor and a timing belt. It is able to move the focus barrel accurately enough to focus when I operate it manually. However, it doesn’t work with SGP autofocus because the belt requires a fair amount of tension, and hence a barrel position is not the same depending on the direction it is approached from. SGP autofocus typically finds one side of the “V” perfectly, but then uses all of the points for the other side of the “V” tensioning the belt and leaves the lens out of focus.
Would it be possible to add an autofocus mode where SGP walks the focus in one direction - say “in” - until a minimum HFR is found, then backs off in the “out” direction and walks “in” again until within 10% of the previous minimum?
Hi
If I understand you right, I think that is how my feathertouch focuser is working on my edgehd sct.
In sgp on the equipt. profile mgr under focuser there is a button called “Other” that allows you to set a backlash compensation value that rewinds the focuser to a point before the ideal focus point and then walks it back in.
But the stepper motor counts must be repeatable for it to work.
You need to set it up so the focus count goes from high to low as the lens turns towards infinity. Then set the backlash to kick in when going from a small value to a large one. That will be when the lens is on the close focus values.
Then keep the curve narrow so it doesn’t go very far past infinity.
With a home made focuser and driver you have control over how your focuser works. Can you put something in the driver so that the focus position is always approached in the same way? I think that means that all positions will be approached from the same direction and from far enough away that the effect of the way that your hardware works is always the same.
The things I suggested would apply to any focuser and don’t need changes to the focuser itself - or sgp. Many people think that if a system “has backlash both directions” it somehow means it needs to do something special in two directions. But that isn’t true - all you need to do is choose one backlash direction and stick with it. The one thing special about sgp is that it requires the autofocus curve be taken from high values to low values. But sgp also allows you to change the numbering system of your focuser so it can work that way.
The only physical problem with this setup is that it needs to continue focusing enough past infinity to capture the other side of the v-curve. I have done autofocus with a setup like this and there was enough room on the dslr lens for this to work - and it never had any need to go far beyond focus because the backlash unwind is on the other side - and there is plenty of room. But the width of the v may need to be small by decreasing the step size.