Autofocus problems

[AlistairW]

Hello,

I am trying to get a feel for an issue I am having getting repeatable V curves. Can someone give me some advice on whether the following is correct.

On my refractor I have a draw tube that has a distance of fully IN to fully OUT of 80mm. My focuser is calibrated so that IN to OUT is covered by 13000 steps, therefore :

13000 steps --- 80mm

162.5 steps --- 1mm

or 1 step --- (1/162.5)mm

I was using SGP last night with a Clear filter and had 10000 as a midpoint with a step size of 50 and 9 points. So in other words .....

The Clear filter had a focusing run between something like +4 and -4 steps around the midpoint, and this equates to + 4x(1/162.5)x50 to - 4x(1/162.5)x50 or -1.23mm to +1.23mm about the mid point.

SUMMARY.

Given the initial position of a perfectly focused filer, is a total travel of 2.5mm too much or too little for the auto focuser to do it's job. (Reason I ask is initially I had a great V curve but later in the night (as the temp dropped) the focuser struggled).

[ Another separate question, - I physically measured the backlash in the system as 348 steps. Therefore I set the backlash to a bigger value of 500 and to applied this to the IN movement. I think this is correct. So the operation would be that the focuser would crank out to its maximum value (+500), - then return to the maximum value. This should then have 'soaked up' the backlash. The subsequent steps are all IN, and therefore there will be no backlash to uptake on this inward movement. Hope that is correct ?) 

Thanks

Alistair.

[Filroden]

SGPro recommended noting your HFR at focus then unfocus until you get a HFR score about 3-5 times bigger. Your full travel would then be twice the distance in steps between these two points and your step size would be the full travel divided by your number of points less one (so 8 in your case).

And your backlash settings look good. I have mine set to IN. (And IN physically moves my focuser inwards)

[alan4908]

Hi Alistair

I also struggled a little with getting reliable V curves and focus until I read through some very good tutorials on how to set up Focus Max (attached). Since I'm not sure what version of FocusMax you are using, I've attached two version of the FocusMax Tutorials - V3 relates to the freeware version.  Personally, I found the Precision Focusing presentation very informative. 

Precision Focusing Using FocusMax.pdf

FocusMax Tutorials v3.pdf

FocusMax Tutorials v4.pdf

Alan

FocusMax Tutorials v3.pdf