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Astrotech 90EDT focuser backlash

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#1 PeteS_MA

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Posted 07 March 2020 - 10:38 PM

Recently I had a difficult night in terms of focus stability and so I'm taking a closer look at the focuser performance on my AT90EDT. I've noticed two issues: first, I've got some backlash due to wiggle in the pinion housing. I've linked a short video demonstrating the problem. It's really easy to see: https://www.youtube....?v=iqNhrgIzcFM I've tried cranking down the four screws on the bottom of the pinion housing but that binds the entire shaft.

 

The other issue is with the fine-tune focuser knob. It barely works at all and I have the grub screw cranked in as hard as I dare. It will spin all day long but neither the tube or the course-focus knob move at all. I've seen the instructions here: https://www.cloudyni...74259440284.jpg  but you can see from the video that I don't have a '#2' screw so I don't know where to go next.
 

 



#2 orlyandico

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Posted 07 March 2020 - 10:44 PM

I used to have an AT90EDT, and my recollection is that there really is some backlash that you can't get rid of. However that pinion moving back and forth doesn't look normal. Maybe you could take the entire pinion off and see if the screw holes have gotten out of round.

 

As for the fine focus knob.. if it spins freely and does nothing, I suspect the tuning documented here might help:

 

https://agenaastro.c...rd-focuser.html



#3 Jon Isaacs

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Posted 07 March 2020 - 11:12 PM

My two cents, what I see:

 

Clearly the pinion block is loss, that needs to be fixed, not sure what is going on there.  

 

When you say the grub screw is cranked down as hard as it will go, which screw are you referring to?  Your focuser seems to be somewhat different than the one in the photo.  

 

One thing that can happen:  The small set screw that is visible below the knob you are turning in the video, that can come loose.  That allows the silver piece between the knob and the black body to turn.  If that happens, the fine focus knob quits working.  Check to see if the silver piece is turning when you use the fine focus knob.

 

AT-90 focuser.jpg

 

Otherwise, it's probably something inside.

 

Jon



#4 PeteS_MA

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Posted 08 March 2020 - 05:49 PM

OK, I think I've nailed down the pinion housing at least. There was a lot of finesse involved with backing out the four screws at the corners and very slowly screwing them back in 1/4 turn at a time in a star pattern until the backlash fell off to almost nothing while still letting the tube rack back and forth with just the right amount of resistance.

OTOH, the little set screw that Jon points to for adjusting the fine tune knob doesn't seem to do anything.



#5 Jon Isaacs

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Posted 08 March 2020 - 08:29 PM

OK, I think I've nailed down the pinion housing at least. There was a lot of finesse involved with backing out the four screws at the corners and very slowly screwing them back in 1/4 turn at a time in a star pattern until the backlash fell off to almost nothing while still letting the tube rack back and forth with just the right amount of resistance.

OTOH, the little set screw that Jon points to for adjusting the fine tune knob doesn't seem to do anything.

 

It was worth a try, if its loose, it's an easy fix.

 

Otherwise, its probably internal, there's a nut that adjusts the force on the two speed planetary "gear" train. For that, the focuser needs to be disassembled.

 

If you're anywhere near Lowell, Ma, I'll probably be back there sometime this year and could take a look at it.

 

Jon



#6 PeteS_MA

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Posted 08 March 2020 - 08:53 PM


If you're anywhere near Lowell, Ma, I'll probably be back there sometime this year and could take a look at it.

 

Jon

If you consider Wakefield near Lowell, then yeah, I'm pretty close



#7 PeteS_MA

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Posted 09 March 2020 - 09:10 PM

I took the pinion housing off and right away I saw that there's a missing teflon bearing circled in red under the axle:
 

pinion-housing.jpg
I'm guessing that teflon is 10 mil thick. Is there a way to get the axel out so I can get the new bearing under it easily?


Edited by PeteS_MA, 10 March 2020 - 08:28 AM.


#8 moxican

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Posted 11 March 2020 - 08:48 PM

I have the same scope. Did you find the solution for this issue?



#9 Jon Isaacs

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Posted 11 March 2020 - 08:55 PM

There are should be some setscrews holding the knob in your hand to the shaft and one at the other end holding the two speed to the shaft. Sometimes they are hidden.  

 

Does the focuser knob move up and down?  There maybe a bearing further out so the inner bearing is unnecessary. 

 

Your fine focuser issue is at the other end and once you remove the nobs and assembly, (that set screw that I circled will have to come out), you can maybe adjust it.

 

Jon



#10 PeteS_MA

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Posted 12 March 2020 - 07:04 AM

I have the same scope. Did you find the solution for this issue?

I think so. The missing bushing is resulting in too much play in the focuser axle. I've ordered some nylon tape that I think I can slide into place if I remove the course focusing knob (the one I'm holding in the second photo). Removing that knob gives the axle a little play and I think it's enough to slide some tape into the gap. The tape will arrive next week so I'll update after that.



#11 moxican

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Posted 12 March 2020 - 07:42 AM

And there is a way you can remove the course focusing knob? Is there a little allan screw for it?



#12 PeteS_MA

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Posted 12 March 2020 - 09:34 AM

Yes. Just next to the taps for the set screws that hold the silver collar in place there's a hole that exposes the two allen screws that hold the knobs in place on the axle. Turn the knob and you'll see them go by. Just take them out and the course focus knob will come right off. They're tiny so be careful. Once you take the course knob off you should get enough play in the axle to slide a new nylon or teflon strip into the gap on the course-focus side. The old one that had come loose was still in the pinion housing when I took mine off.

Be careful taking the pinion housing off too. There are four tiny o-rings you'll need to keep track of that will probably fall out when you back out the four screws that hold it in place against the focuser tube. You can see them in the second photo.



#13 moxican

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Posted 12 March 2020 - 01:16 PM

Thanks Pete. Do you happen to know the diameter of the axle after taking off the knob?



#14 PeteS_MA

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Posted 19 March 2020 - 11:03 AM

The shaft looks to be 8 mm.

 

I cut a small strip of nylon tape and got it under the shaft to try to reduce the friction but it hasn't helped at all. I even added a bit of lithium grease to the axle. I noticed that it really takes very little lateral force on the axle to make the fine focus knob slip: even putting a moderate amount of pressure on the axle with my finger on the course focus side will cause slipping. If there's a flat spot on the fine focus axle where the grub screw is supposed to seat I haven't been able to find it.



#15 moxican

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Posted 19 March 2020 - 03:59 PM

If you are talking about the fine focus (smaller) knob slipping, I have read it her on CN that it is normal. Mine is like that also. I don't know why.

If you mean what is on that original Youtube video, the only thing I can see causing this is those four screws at the bottom of the focusing mechanism. They look to be loose. Other than that....



#16 PeteS_MA

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Posted 11 April 2020 - 03:04 PM

Yeah, it's the fine focus knob. I just played around with it a little more and I've confirmed that the knob is definitely turning the fine tune axle. I marked the axle with a red dot and I can see the dot moving when I turn the knob, but that's all. It's not engaging with the course adjustment axle, even though when I turn the course knob the fine tube spins exactly as you would expect.

At this point I'm starting to wonder if replacing the entire focuser wouldn't make sense


Edited by PeteS_MA, 11 April 2020 - 03:15 PM.



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