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ZWO EAF backlash setting

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#1 Barns

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Posted 12 October 2019 - 03:35 PM

Hi all, I've got a ZWO EAF connected up to my Skywatcher Esprit 100 and after some mucking about with the bracket etc think I have it all rigid now. Since it's been (of course) cloudy for ages here I've used SharpCap's contrast-edge-detection autofocus routines to test it out in the daytime pointed at distant stuff.

 

I noticed that if I scanned for focus in the OUT direction vs IN direction, the "Best Position" value found was different by large amounts (~1000 steps) between the directions even though the actual position on the focus tube was the same. So obviously some backlash there and quite a lot of it! I've since re-tightened everything, changed how the bracket is fixed, and remeasured the backlash and now have a value of around 65-70 (still needs some fine tuning measurement). When this is entered into the EAF ASCOM settings, the focus point found is more or less the same in the IN and OUT directions (within ~20 steps), which implies to me that the backlash is now being pretty much compensated for.

 

My question is, is this a reasonable expected value for the backlash to end up at? What kinds of backlash values have other people ended up needing in the EAF ASCOM settings in order to get repeatable focus positions in both directions? Do I need to revisit again the fixings or the possibility that the EAF itself has too much "wobble" internally and needs exchanging?



#2 wrnchhead

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Posted 12 October 2019 - 04:00 PM

That is unusual in my experience. My EAF seems to have none or very little on the Moonlite. Has been repeatable to the last reported digit.
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#3 OldManSky

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Posted 12 October 2019 - 06:50 PM

Mine is at about 25, but it was 40 on my previous scope. Because much of the measured backlash is from the focuser, not the EAF.

#4 Barns

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Posted 14 October 2019 - 06:09 AM

Thanks for the replies. I've spent this morning trying to pinpoint where the backlash is coming from and am coming to the conclusion that it's internal to the EAF unit itself and not with the scope focuser.

 

With the EAF attached (unpowered), I can turn the microfocuser about 1/8ish of a turn each direction until it's stopped by the EAF. With this turning I can see the focus tube immediately go back and forth a little - so I diagnose no backlash between the focus mechanism and the tube (i.e. not on the rack/pinion). However I can see the EAF motor shaft being turned as well whilst I do this little back and forth turn on the microfocuser, meaning the backlash isn't somewhere in the coupling between the EAF and the focus mechanism. Hence I think it has to be internal to the EAF itself; it's allowing a small amount of back-and-forth play on the motor shaft.

 

Does this sound like a "bad" EAF unit? I've read elsewhere that the EAF itself shouldn't have any "play" on the shaft like this.



#5 ThatsMyCoffee

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Posted Today, 08:58 AM

In the EAF unit, it's highly unlikely that there is any play.  I don't believe there are any gears, so where would it come from.  But....  If you think there is, take it off and feel by hand.

 

I'd guess that there is a tiny backlash in the rack & pinion, but a super tiny amount by hand is a HUGE number to a motor like that.

 

In the end...  if it works, just leave it alone.  Adjust your software to compensate and worry about other things.


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#6 OldManSky

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Posted Today, 10:31 AM

The EAF does indeed have gears. There is backlash in any geared train. On my EAF it’s very small, but it’s there.

I have yet to see a focuser that doesn’t itself have some backlash...so the total backlash of the system is a combination of the two sources.

Measure the backlash (ZWO gives one method in the manual, I used NINA’s automated routine), set the amount in the software, and go have fun :)




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