Eliminating Focuser Backlash

My focuser is an HSM20 controlled by a Pegasus Astro Ultimate Power Book v.2. I use 9 data point and 200 steps. When the autofocus routine is invoked it moves out appropriately but the first two measurements have almost the same HFR before the desired “U” pattern emerges. (see images below). I attribute the lack of a significant reduction between the first two data points to backlash. In SGP I have tried check Backlash selecting IN and values of 200 to 500 but I always get the same pattern. This is troublesome because the last curve fit widens out to take account of the first data point. I believe that this incorrectly moves focus on account of a datapoint before backlash removal. Although it is difficult to see in these consecutive screenshots, the 77% and 88% curves indicate a focus of 7205 whereas the 100% moves it out to 7275 (incorrectly in my view). Am i doing something wrong in setting up the backlash removal? Any suggestions appreciated.D886D083-A2B4-43D0-9EC6-E52E66DDD72D_4_5005_c B3DF3ECB-9309-49E6-9AE1-7D18087AED67_4_5005_c 3C1262F5-DE8A-488B-9148-FFA06DBC6402_4_5005_c

Do you have the SGP log from this run? Or maybe one with multiple auto focus runs?

Thank you,
Jared

Jared

Found a link to logs under Help. I have four files from last night. (I am not sure which is which).

Unfortunately I cannot figure out how to attach files (as v. images) through the Support portal.

Regards, George

George,
You can zip them up and use dropbox or similar to share them. Or you can send them to support@mainsequencesoftware.com. Just be sure to include the forum post URL if sending them via email.

Thank you,
Jared

As I am sure you know, you really cannot set backlash too high unless it causes your focuser to exceed it’s travel limit. I would try a much larger number for backlash, maybe 1000 or 2000 and see what happens.

Thanks CCDMan. I set it to 1000 and am now getting a more traditional “U”. My Focuser travel is 2.5” (94,000 steps) with good focus around 71,000 so there’s plenty of room on either side. I may even increase it to 1500. Appreciate the help.

Regards, George

I have made the same observations as you.