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RA Setting Circle Slipping When Forks Are Moved Manually

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#1 Orion68

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Posted 01 August 2020 - 11:11 PM

In trying a new technique for polar alignment I've noticed an issue with the RA setting circle. Either this is new or I missed it previously but not sure how I could have overlooked this. Anyway, here is the issue.

 

If I grab the fork arms and manually move the scope in right ascension the RA setting circle immediately moves a very small amount (1/4 degree or so) then makes a small, distinct clicking noise and stops turning as the fork arms continue to turn.

 

However, if I move the fork arms by using the RA adjustment knob only there is no movement in the RA setting circle.at all - which is the way it should be.

 

Also, I can rotate the setting circle to set it without any issues, it seems fine in that respect. And, as far as I can tell the setting circle is being driven correctly by the motor drive - I'll try to check that tonight.

 

Hoping someone can let me know what might be causing this. Thanks in advance.

Clear skies, Tim

 

 



#2 Optics Patent

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Posted Yesterday, 09:24 AM

The RA ring is frictionally engaged to the synthetic platter (not visible inside), and that is frictionally engaged to the drive discs above (which slips for slewing) and the main drive gear below (usually but not always pinned). 

 

The 1/4 degree motion is most probably some slop in the gear train internal to the motor.  It's normal but some might be better or worse.  a 1/4 degree movement in a 1-degree marked system seems fine.  YOu can easily check this by removing the base plate, and removing the motor, and see how much the pinion wiggles.



#3 Orion68

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Posted Yesterday, 12:46 PM

I looked again and the slip is closer to a full degree, maybe a little less. I'll remove the bottom plate and the motor, as you suggested and see if there is any play in the pinion gear.

 

Many thanks Ben.



#4 Optics Patent

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Posted Yesterday, 07:40 PM

It may also be interesting to observe whether the drive takes that same portion of a degree to start driving depending on which direction you skewed from (the slow motion adjustment shouldn’t be a factor. ).


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