Focused Backlash - Accurately measuring. SharpSkyPro.

[rogerg (Roger)]

Does anyone have any tools/tips on accurately determining the backlash present in a motorised focuser?

My RoboFocus has never had backlash issues and I've never specified any backlash compensation.

My SharpSkyPro seems to have significant backlash. This could be as much a fault of the Megrez focuser as the SharpSky Pro, but regardless it's there. I can determine backlash by incrementally racking the focuser in and out step by step, watching to see when the star changes, but this is error prone as atmospheric changes make it impossible to be sure the star has changed focus due to the focused movement as compared to "seeing".

So, I'm thinking surely something can automate this and measure the backlash for me to then configure in the settings of SharpSkyPro drivers and/or FocusMax ? Surely ??

I predominantly use FocusMax but have tried TheSkyX's @focus2. Last night I did manage to get FocusMax to auto-focus successfully once but only just by the skin of it's teeth. Auto focusing is problematic because each time it's slightly different, for example the V-curve shifts around, seemingly due to backlash.

Current backlash compensation for the SharpSkyPRo is set to my best guess of 35 units/ticks.

Roger.

[Shiraz (Ray)]

On my old mk1, I bluetacked a drinking straw to the unused focuser knob. Then varied the step size to find what setting just took up the gear slack and started the end of the straw moving when the motor was reversed after a few steps in one direction. Used that as the backlash setting in SGpro with good results. From memory it was around 20-30 ticks.

If you have the focus point drifting around, you could have trouble with slippage in the Megrez focuser itself. Never used one, but a quick look around the web is not comforting. FWIW, my similar low end Crayford focusers (modified GSO and SW) can reliably hold an imaging train of about 1kg, but not much more.

[rogerg (Roger)]

Originally Posted by Shiraz

On my old mk1, I bluetacked a drinking straw to the unused focuser knob. Then varied the step size to find what setting just took up the gear slack and started the end of the straw moving when the motor was reversed after a few steps in one direction. Used that as the backlash setting in SGpro with good results. From memory it was around 20-30 ticks.

Thanks. About as accurate as my existing measure I would guess.

Originally Posted by Shiraz

If you have the focus point drifting around, you could have trouble with slippage in the Megrez focuser itself. Never used one, but a quick look around the web is not comforting. FWIW, my similar low end Crayford focusers (modified GSO and SW) can reliably hold an imaging train of about 1kg, but not much more.

Look on the web and you'll find anything you want There's a zillion different iterations of megrez focusers so not really a reliable indication. I don't believe slippage is a problem, it feels rock solid and impossible to shift the tube. That said, it does have an issue, with flop not slippage. Getting it to hold the imaging train straight without any sag is an issue, but that doesn't seem to vary, it's fixed. Hmm. Worth investigating physical setup/influences some more

[peter_4059 (Peter)]

I attached a dial gauge to measure mine but I recently saw a post on the sgp forum describing how to use a bahtinov mask to measure it. Focus with mask then step focuser a large number of steps in one direction then same number in other direction. Then check with mask and adjust with extra.steps if necessary to return to perfect focus. This tells you how many extra steps of backlash are present. Repeat process in opposite direction.

[Shiraz (Ray)]

another thought Roger, do you have the SharpSky attached to the 10:1 or directly to the main shaft? The 10:1 might have a bit of absolute position variability.

[rogerg (Roger)]

Originally Posted by peter_4059

I attached a dial gauge to measure mine but I recently saw a post on the sgp forum describing how to use a bahtinov mask to measure it. Focus with mask then step focuser a large number of steps in one direction then same number in other direction. Then check with mask and adjust with extra.steps if necessary to return to perfect focus. This tells you how many extra steps of backlash are present. Repeat process in opposite direction.

Hmm, interesting. I think that method is just using the mask to make the changes more evident?

Originally Posted by Shiraz

another thought Roger, do you have the SharpSky attached to the 10:1 or directly to the main shaft? The 10:1 might have a bit of absolute position variability.

It is attached to the main shaft, good to check

[peter_4059 (Peter)]

Originally Posted by rogerg

Hmm, interesting. I think that method is just using the mask to make the changes more evident?

I don't think so - in theory the position of the focuser is the same when you first focus with the mask and after you step it in one direction then back the same distance (number of steps) in the other. Backlash causes the drawtube position to be different and hence causes a shift in focus as indicated by the mask. If you then re-focus using the mask and record how many steps this took you have effectively measured the backlash.