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Nexstar SLT Mount Backlash settings not working?

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#1 Caldrie


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Posted 31 July 2020 - 05:21 PM

Hey folks! This is my first post here on CN so go easy on me.


I own a Celestron 130SLT and have for 2 years now. It's been good to me so far. But I've always had an issue when it comes to tracking Nobjects. I've never had super accurate alignment, but I've tried getting the alignment stars as center as possible. Once I have the alignment going, I slew to an object using the Go-To feature and it works reasonably well. Using a 25mm or even 9mm eyepiece, the object usually appears in the FOV of the EP.


But then something weird happens. As soon as the mount slews over to the object and stops (even if the object is center or not) it will begin to move out of frame. It never quite gets there though, it usually drifts off a bit but remains in the eyepiece. 


I've done some research and found that backlash could be the issue. Now I've always noticed a delay between the button press and the mount moving. When slewing on the AZ or ALT axes I can hear a click before the normal ticking sound of the gears slewing. Only when changing directions. Sounds like backlash to me! The slower the slewing rate, the longer the delay before the gears start turning.


This wouldn't be so much of a problem if I wasn't trying to image some distant objects with a fairly low FOV camera. Now it's very annoying when attempting to center slewed to objects. Especially those I'm trying to image. Although with some manual correction they stay in the frame and track as normal (normal without guiding that is).


So to fix this problem I decided to fiddle with Anti-backlash settings. I followed several guides including Michael Swanson's guide on the Nexstar resources page. But something is significantly different in my mount.


It's mentioned that the negative and positive values for each axis should be the same. E.g. 50 on AZ positive and negative & 30 on ALT positive and negative. But this doesn't achieve the desired results. 


See when I set the AZ positive to the max (99) I notice that turning left has a significantly longer delay (than 0) before the gears turn. But turning right (changing directions) has the drive engage instantly. That's perfect! Just what I want to see! 


But I don't want turning left to also have that long delay. So I set AZ negative to 99 also. But now turning both ways engages the same delay between movement. This happens with any value added, if they are the same on both negative & positive (or close values) the two directions will exhibit a longer delay than if they were both 0. So basically I'm stuck with backlash compensation for one direction on AZ and ALT axes. One can be high and the other has to be low. But not both. This is okay for tracking (i suppose) but not for aligning. The HC firmware (afaik) tracks motor movement and not gear movement, so it will be off regardless of which direciton has backlash compensation. 


I wish to negate backlash compensation for both directions on both axes. I'm not an expert at mechanical engineering and have a very basic understanding on how gear systems work. Am I correct in assuming that what I want is impossible? Or is there something specifically wrong with my MC firmware? Because all the guides I've seen state that the two direction of each axes should have the same value. But all that does is make the backlash worse than before.


Any help would be very much appreciated! Thanks a bunch!


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#2 mac57


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Posted 31 July 2020 - 06:01 PM

I run my SLT mount at around 35 or 40.  These mounts are not super accurate, and rely on a level mount.  I think you should try backing off a bit on the backlash, and make sure you use your lowest power ep to get your object in sight.  It's gonna take a little time for you to learn your scope.  Alignment is key.  Mark

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