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[Chicago Astronomer - Astro Joe]

Repairing a Nexstar "Evolution" 8" Single fork wobble

05 September 2016

Chicago Astronomer crew member Brian informed me of a problem he developed on his Nexstar mount which carries a 8" SCT.  It had developed a wobble in the Declination axis...and despite tightening down the clutch, the axis still wobbled, making any observations aggravating and accurate slewing to targets difficult.  Brian is a great Chicago Astronomer crew member and I will do whatever I can to get him back and attending to our fans.


(NOTE: I do not recommend Single-arm mounts, as I feel too much stress is put on the Dec. axis assembly and things go wrong - such as here.)

Brian and I meet by the Adler Planetarium and he shows me the wobble...

Yep...a lot of wobble and not cool.

I accept the task and take the SCT, mount and legs to the C.A. workshop...

I should have it back to Brian and ready for our next Chicago Astronomer event in less than a week.

Over the three day Labor Day weekend, I had some free time to investigate on what is is causing this unacceptable wobble...

There is an issue with the OTA as well, but I'll address that after the mechanics have been dealt with and corrected....I hope...  

 I remove the tube and we start the investigation....

There is about a 1/4 deviance in a side-to-side motion.  Tightening the clutch did nothing to reduce the play and we got to get inside and see what has betrayed us.

Starting out OTA side, I attempted to loosen the hold down nut...




But it won't budge...in either direction.  I fear if I force it, that I will snap something.  No bueno....    

We attack it from the other side, hoping for better luck.

I need to remove the clutch grab rings, clutch knob and bottom housing to get to the hidden hex screws and innards...

Let's take off the saddle and see whats on the other side, but nothing is there to assist in the disassembly, so it's back to the other side.

Popping off the cover via several Hex screws reveals the innards, but I need to remove the bottom cover of the mount to access screws, revealing the R.A. assembly...

 

Wiggling the axis, the issue is someplace behind that large gear wheel...

I inspect the workings and do not like the cheap Chinese lubricant that is sloppily applied to the gears.  We will take care of this in a bit...

Rechargeable batteries look good...




I remove the clutch and grip rings...





I check the wobble and it's behind that large gear somewhere...








Removing the clutch friction plate wheel...

And bingo....I have found the problem...!

This nut, deep inside the mount, has backed off and come loose...allowing the 1/4" wobble and causing the headache.   We will snug it after some more investigation and we go deeper...

The OTA side assembly pops off with the removal of the offending nut, displaying needle bearings on both sides.  I clean them off and apply quality lubricant...






Nuts, washers and bearings...








Dow Corning High Vacuum Grease is recommended for telescope machinery.  It does not out gas - making it suitable for baffle tubes inside SCTs and is not affected by high/low temps.  It also does not thin out and I use this on my own gear and apply it to the gears here...




Hmmmm.....




Lot of parts to reassemble....

I snug the hold down nut....eliminating the wobble completely...

Not too tight, as not to place strain on the motors.  I test it several times and satisfied on the "grab" on the the torqued nut.

Thank goodness these needle bearings are set and do not fall out....!  I clamp the assembly as I return the clutch friction plate home and tight via two inset allen wrench sets...

The large drive gear has more of that lubricant goop and it is removed...

Denatured Alcohol does a good job in cleaning.  I apply the Dow Corning lubricant before reinstalling.  

(There is a serial number of #3116P on the gear - good for reference.)

Checking the bottom housing of it's gears since it's all open now...

....and more of that awful Chinese goop....

Before popping the covers back on, I place my business card inside, as I always do on repair jobs...

Everything comes back together nicely and without complications....and no left over parts...!  





All back together.  

I power the mount up, lights turn on and all axis slew nicely, whirring and moving the R.A. and Dec as it should....without wobble.

With the mechanics of the mount successfully attended to, we turn toward the C8...

At a recent Chicago Astronomer Star Party, Brian brought to my attention this metallic powder resting on the secondary housing.  I had no idea what it was from, but let's find out while we are at it.  The Corrector Plate is somewhat dirty, but not too bad...

I suspect that it is a Corrector plate hold down ring screw that is causing this, as I observe a thread filament on the glass....

To keep the filings and gunk from dropping unto the primary, I keep the OTA tilted horizontal...

It sure looks like a thread fragment....

And there it is...

The mis-drilled screw hole has nicked the corrector plate. I have seen this before and something at the manufacturing of the OTA.   Quality control was sleeping that day.

The screw has nicked the edge of the plate.  Doesn't look bad, but it may create a stress point.  It's important not to tighten these screw screws tiight, but just snug.

The secondary mirror has some haze and dust on it...

I blow off the dust, but also lightly drag several cotton balls to dislodge the sticky ones that won't blow off..





The primary also has some haze, but I will not correct that in this job...

I clean and reclean and....reclean the corrector plate...






After blowing out the OTA, I reinstall the corrector plate in it's proper alignment position and give it another cleaning...





Everything looks good, motors purring and glass clean.  I take it out and give it a preliminary test run...

All seems to be working fine.  


No wobble and mount is solid and the keypad slews the mount in both directions nicely.  It is a bit out of collimation with slight coma in the in/out focus -  and with a couple of slight turns with a screw driver, I get concentric circles and airy disks in the eyepiece - viewing Polaris and Polaris B nicely and sharply.  I recommend "Bob's Knobs" for easier collimation on this Fastar secondary for future tweaks. 

I'll leave it to Brian to give it a test run and see how he likes it and if any other adjustments are required.  I do think that the offending nut will loosen again as nothing is there to keep it from backing off, other than the friction plate and clutch knob - tightened down by hex screws.   If so, then we can add some "Thread Lock"...but that would be for a spring project. 


End of project.


Astro Joe

[Chicago Astronomer - Astro Joe]

Reunification of Telescope and Astronomer

08 September 2016

My only free evening to return Brian's Nexstar Telescope was Thursday, the 8th of September and right before our 606 Bloomingdale trail Public Observation session the very next evening.  We met by the Adler and set-up the repaired mount and OTA for Brian's test post-repair run...

I check the clutch and any play on the Dec axis...and it is solid....

...and the little lights come on too.





For a quick test run,  I manually slew the OTA toward the 1/4 Moon for a peek..

With the cleaning of the corrector plate and nudge in collimation, the lunar surface was tack sharp.  Brian slewed the scope here and there and was satisfied in the performance.  With the removal of the old crappy Chinese grease and the application of the superior Dow Corning lubricant, the gears purr nicely.

We discuss our next Chicago Astronomer Star Parties this weekend, (We got three) and happy on the new life on Brian's Nexstar Scope.   He will test out the gear this evening on a clear summer September evening and report back to me on his evaluation of the repair.

Addendum:


Brian gave his Nexstar a shake out session and gives the repair a thumbs up and very satisfied with it's performance.  He did some Lunar viewing and the the delivered flawlessly...

May you and your scope Live long and...and prosper Brian...

[enthusiast]

How did you remove clutch friction plate. Did it just lift off. Mine is not coming off.