How to improve worm gear backlash on Celestron CG-5 Equatorial Mount

Posted by XTSee on 4th February , 2009

I’ve not long had my new Celestron C6-S SCT XLT Telescope, but following a minor problem with the Backlash setting in the SynScan Pro V3 Goto Upgrade hand controller (which was fairly easily fixed by a firmware update to correct the excess Backlash adjustment), my interest was piqued and I wanted to check at an early stage that my scope and mount were adjusted to get the best from them.

A couple of things were niggling me:-

  1. I had read in various places (Cloudy Nights forum, and some personal astronomy websites) about the importance of ensuring correct tension adjustment of the worm gear driving the Right Ascension axis of the telescopes equatorial mount to reduce to a minimum the “backlash” inherent in commercial EQ mounts (preventing backlash altogether only occurs in very expensive top-end telescope mounts with extremely high-precision gearings). While the Goto hand-controller uses software control to help overcome Backlash problems (there is a Backlash compensation setting for both RA and Dec axis) by rewinding/forwarding the servo-motor to take up any mechanical backlash slack, it is better to try to minimise backlash mechanically so that the software need perform the least servo-motor movement necessary. [Backlash occurs in the Declination axis too, but it is considered a less important problem than for Right Ascension tracking]
  2. Although the CG-5 EQ mount supplied with my C6-S telescope is built to quite a high quality standard, I think that the person in the factory is probably less interested in ensuring the mount is constructed and fine-tuned to the level of accuracy that I personally would like. Its a matter of preference, enjoyment of the DIY approach to get the most of my astronomy equipment, and I feel confident enough to dismantle, clean, re-assemble, re-grease and finely adjust the mount, at least if only to assure myself that nothing further can be done to improve things! However I had an inkling that there was room for improvement, because when the scope was rotated about the RA axis, there seemed to be some binding, either due to Chinese sticky grease, or possible over-tightening of the RA worm-gear? Being a perfectionist, this bugged me, and I wanted to do something about it.

CG-5 Mount Maintenance

Initially I considered learning what could be done from scratch, with a mind to take plenty of photos of the process so as I could document the whole procedure, like I have done for my XT10i telescope.

But then I thought that since the CG-5 / EQ5 / HEQ5 are all very similar and popular EQ mounts, then there must already be a wealth of information available on the Internet regarding their maintenance.

I was right, and discovered the excellent article (and PDF) written by “Astronomy Boy”:-

Improving the CG-5 Equatorial Mount

After reading the article through a couple of times, some careful preparation, and going through the whole process for dismantling, cleaning out any swarf from the factory build, regreasing and re-assembling everything for both the RA axis, and the Dec axis, I have to say that Astronomy Boy has it covered pretty well.

The task took me two evenings to complete.

I used some fresh new light Lithium-based grease very kindly donated by my local Ford garage, and really took my time when adjusting the worm-gears, using my fingers to “feel” the amount of backlash, and to carefully tighten the worm gear tension to eliminate as much as possible without causing binding of the gears - I performed a couple of full 360° rotations of the RA head, turning the worm gears by hand, carefully feeling for any excessive tightness. That takes a few minutes I can tell you! In the end I was making very small adjustments to the worm gear tension bolts and grub screw before I was happy.

In fact I discovered that the main reason for the original binding of the RA axis, was that the bearing ring-nut had been over-tightened too much during the manufacturing (it was not the worm gear that was too tight as I originally suspected), and this was causing the ball-bearing race to become compressed, and had actually created a very slight indentation to the inside of the bearing race housing, where the ball bearings were pressing hard. I am gald that I investigated the cause so soon after purchasing the scope, as this may have got worse with use. I suppose if this had occurred, it could have been considered a part of “wearing-in”, but I think its better to have corrected it before any flat spot damage  occurred on the ball-bearings.

After putting everything back together, adjusted to my satisfaction, using new grease, I re-assembled the mount and scope, connected the SynScan Goto electronics and turned everything on.

Re-balancing the scope is noticeably easier: The bearings are overall much more free and easy to rotate, and this allows the centre of gravity to be ascertained more readily for each axis, centering both the telescope tube dovetail mount bar better in the mount jaws for the Dec axis, and then the counterweight position to balance the RA axis.

But the main difference was when I fired up the motors….

At maximum slew speed (Rate=9) the Dec axis in particular was previously fairly noisy and graunchy when accelerating/decelerating, whereas now it is much less graunchy-sounding, and has a much nicer smooth noise, less mechanically strained, and more servo-electronic.

Likewise the RA drive seems very much smoother, and definitely quieter at high speed, and almost silent when at sidereal tracking rate. I guess the motors are working more efficiently due to less friction, and reduced binding, and this in turn can only help by increasing the life of the battery power-pack.

So I feel it’s a job that was well worth doing. I’ve learned a bit more about the mechanics of the mount, and although I have not yet had a clear night to test whether the backlash is improved, I feel confident there will be a positive impact. I have taken note of the Goto unit Backlash compensation settings to make a comparison when I get the chance.

We shall see!

Having decent instructions for the task available beforehand inspired confidence, and everything went very smoothly.

Thank you to Jeff (Astronomy Boy)

UPDATE: I have now had an evening to align on stars and carefully watch and adjust the Backlash compensation setting of the SynScan hand controller unit. Before I performed the above maintenance work on the mount I had noted the settings (which I had tried to get to the minimum possible), the values necessary were; RA=0°10′30″ and Dec=0°05′30″. In other words for Right Ascension when I used the left-arrow button on the controller the motors were having to rewind 10 minutes 30 seconds of travel (plus any actual adjustment I wanted to make in that direction), before repositioning forwards to take up the slack in the worm gear so that the drive is making positive forward motion again. After completing the above maintenance work I have carefully reset the Backlash compensation as best I can, and this is now down to RA=0°04′30″ and Dec=0°04′00″, which I think is a very good improvement, more than halving the RA backlash setting.

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7 Responses to “How to improve worm gear backlash on Celestron CG-5 Equatorial Mount”

  1. Maggie’s Blog » Blog Archive » How To Improve Worm Gear Backlash on Celestron Cg-5 Equatorial …

    [...] However I had an inkling that there was room for improvement, because when the scope was rotated about the RA axis, there seemed to be some binding, either due to Chinese sticky grease, or possible over-tightening of the RA worm-gear? …Next Page [...]

  2. Shari @ Celestron Shop (1 comments)

    Hello, just wandered by. I have a Celestron site. Can’t believe the amount of information out there. Wasn’t exactly what I was looking for, but interesting page. Have a nice day.

  3. Alex (2 comments)

    I’m to new to the group and the CG5GT. . . You probably need to adjust the worm gear itself. These are rarely adjusted well from the factory. From there, you can work with the controller settings. Ed..

  4. XTSee (25 comments)

    Alex, did you actually read this post? It’s ALL ABOUT mechanically adjusting the worm gear to get better results!! I think all you wanted was a link to your site.

  5. Worm Gear Oil (2 comments)

    I us this> worm gear oil. I’ve found it to be very good. What do you guys use?

  6. Worm Gear Oil (2 comments)

    I use this worm gear oil. I’ve found it to be very good. What do you guys use?

  7. Scope Blog

    Celestron C925a Xlt Cg5…

    [...] s probably less interested in ensuring the mount is constructed and fine-tuned t [...]…

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