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Celestron Advanced GT (CG-5) mount tuning tips

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#1 cattom1985



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Posted 25 June 2021 - 03:33 AM

I got my CG-5 second hand a few years ago. It has been working well except for the stiffness in its shafts. I wasn't able to do any reasonable balancing on this mount, the tracking was okay but not ideal. It works fine for visual usage, however for imaging, the mount struggles a lot pushing the telescope and camera, and the tracking on PhD was a mess.


Recently I got quite intrigued by the puzzle solving videos on YouTube (such as this one https://www.youtube....h?v=9eKbIvgIdtU) and wanted to solve one myself. However, most of those puzzles cost hundreds of dollars. Therefore I thought why not use this opportunity to tune my CG-5? It's just like a puzzle, two birds, one stone.


I would not cover all details in my tuning, as most of them are covered on the following webpages:


I found this video of AVX tuning quite useful also:


Instead, I would like to share the lessons and tips I learned during the process, and hopefully it can be helpful to people who're planning to tune their own mounts. I am not an expert in machines so please feel free to correct me if I am incorrect or inaccurate.


1. RA/DEC gear stuck on shaft
This is the first problem I faced. The RA/DEC gear and the shafts are tightly fit together. A very tiny misalignment, or a dry fit without enough lubricant, or a change in temperature can cause them to bind with each other and stick as one piece.


If you run into this situation, do not panic, do not use too much force, and do not try to hit the gear with a hammer! Here are two remedies that may quickly get you out of trouble:


Remedy 1
Try heating the gear under hot tap water, while placing ice in the shaft. This action will expand the gear while shrinking the shaft. This method enabled me to take off my DEC gear with ease.


Remedy 2
When fitting my RA gear to the shaft, a misalignment caused the two pieces to bind with each other. I applied more force trying to unbind them and soon regretted: they bound even tighter. To fix this, I took off all plastic parts, and placed both of them in the oven. I cannot remember exactly which temperature I set but probably around 200C or 400F, because there was a PTFE ring inside and I remember I had to control the temperature lower than 500C to make sure PTFE wouldn't deteriorate.


After baking the gear and the shaft for a while, I don't remember what the exact time was but probably 5-10 minutes, I took the parts out with a baking glove (good thing to be single because I don't have a wife to irritate), held the upper section and gently tapped it on a piece of carpet. The RA gear came off easily.


2. Cleaning the glu-ease
The manufacturer used a lot of black grease on this mount. It was sticky and dark as graphite. Wearing a pair of rubber gloves would help a lot. The gloves can be reused, if you always wipe the grease off and wash your hands with the gloves on with detergent and hot tap water.

I used a toothbrush, kitchen towel, degreaser and kitchen detergent to clean the grease. Warm water helps a lot.


3. Whether or not to polish the parts
Several tutorials mentioned polishing the contacting surfaces. My mount was 10 years old and its parts had a lot of time to run-in. I didn't notice any unevenness in it. Still I attempted polishing some surfaces, but didn't see much improvement. I guess the experience learned here is if the mount is new, then polishing the parts may help, otherwise it may be better to just leave it as is.


4. Is it worth it to replace the original nylon washers with PTFE ones?
I think it depends. I did purchase PTFE bearings but I feel my nylon washers are still quite usable. If I happen to work on a second mount one day, I may choose to stay with the nylon washers or cut my own PTFE washer. However if you are a perfectionist on a budget, then upgrading to PTFE may be an option for you.


To cut PTFE washers, you will need to purchase some PTFE sheets. The washers in CG-5 come in two thicknesses. The large washers are 0.5mm thick, while the small ones are 0.3mm thick. The PTFE sheets can be easily found on eBay. To cut PTFE sheets, you will need a compass circle cutter, which is available on Amazon.


5. Is it worth it to replace the worm gear bearings?
Probably. If you decide to replace them, here is the size: 6x13x5 (Internal Diameter X External Diameter x Thickness in mm). The model number of these bearings is 686.


Do you need to upgrade them to ceramic ones? I did so but I'm not sure. I found these ceramic bearings quite affordable: https://trbrc.com/tr...hybrid-seals-2/


6. Do/Can I upgrade the RA bearing to a tapered roller bearing?
If you do not know what a tapered roller bearing is, it's basically a bearing with rollers installed in an angle. It allows the mount to place the load of the telescope horizontally.


The stock RA bearing is a generic 6007RS ball bearing, its size is 35x62x14 (Internal Diameter X External Diameter x Thickness in mm). It is rubber sealed and I am able to feel some friction on it.


The closest tapered bearing is 35x62x18, which is 4mm thicker. It will push the RA shaft 4mm outwards and expose the RA gear. Therefore I wouldn't upgrade the ball bearing to a tapered one.


An example of the tapered bearing: https://www.vxb.com/...-p/kit12451.htm


However, one can still upgrade the RA bearing to a Japan made ball bearing with metal shields. The metal shield reduces the friction a lot. I purchased the Nachi 6007ZZE.


Nachi 6007ZZE: https://www.vxb.com/...n-p/kit9537.htm


Some people may argue ball bearings are not the best for the use case of GEM mounts. I did check the specs of Nachi 6007ZZE and its dynamic load rating is 16,000N. Although the force from the telescope is not spread horizontally with a tapered design, I assume this bearing is more than enough to handle a telescope.


7. Which grease shall I use?
My original grease was a tube of 4 years-old SuperLube. I applied it to the mount and felt it was more sticky than what other people are describing on Cloudy Nights. Therefore I purchased another tube. The new tube did feel smoother than the old one. So if your grease is old, get a new tube.


8. How much grease shall I apply?
Here I would like to quote a reply from wrvond: "To quantify it a bit better, a two inch strip of the grease is more than enough for the entire mount." I followed this instruction and it helped a lot in loosening my RA and DEC shafts. I applied only a soybean size of grease on the RA/DEC gears and it was just enough.


9. Do I need to upgrade the flat bearing on the rear end of the RA shaft to a thrust needle roller bearing?
I don't think so, the original ball bearing does it's job pretty well. I spent some time doing research on this but eventually decided to keep the original one.


10. Is it worth it to tune the mount at all?
If it's just for the performance of the mount, it may be better off to upgrade it into something more classy, such as CEM-25. CG-5 is a budget mount and it has some intrinsic issues in its design, such as metal on metal grinding on certain surfaces, and slightly decentered shaft gear in locked position. Such issues can never be improved through tuning.


After the tuning, my CG-5 got much smoother than before. I am now able to do balancing on it, and remove most of the backlashes. However, it is still not as smooth as my CEM-60. I guess if one is really on a budget (like me) and/or would like to enjoy the fun of solving a delicate puzzle, it is worth it to tune this mount themselves.

Edited by cattom1985, 25 June 2021 - 03:44 AM.

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#2 cattom1985



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Posted 25 June 2021 - 03:55 AM

Here I would like to thank the following friends who helped during my journey of mount tuning:

  • Special thanks to c2m2t - He got out of his way and sent me many messages to help me out with the gear-shaft binding issue;
  • wrvond - He helped me with quantifying how much grease to use and shared several images of his tuning work;
  • Geo. - He shared the idea of using the oven to take out the shaft "while wife is out".

#3 syam


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Posted 08 August 2022 - 11:15 AM

Thanks for the original post - it helped me a lot! I myself just hyper-tuned my CG5-GT mount, and before I forgot the details, here are a few additional tips from my fresh experience.


* RA ring nut removal (also applies to Dec ring nut removal). Don't make my mistake: when you unscrew the three set screws in the ring, you have to unscrew them a lot - to the point when they are almost protruding from the ring. They are set into deep and soft threads, so they need to be raised a lot not to scratch the threads when you unscrew the ring. I didn't raise them enough, and struggled a lot unscrewing the ring nut - even with a DIY removal tool (3mm thick steel plate with two small holes, where I put two drill bits). I ended up breaking 2-3 drill bits (and damaging a bit of threads under the set screws) before I realized my mistake.





* The main (original) RA bearing is stuck. Even with my pretty sturdy DIY bearing puller tool (2mm thick plate placed under the bearing, with a thick bolt going through it to the top, then through a massive 3mm steel plate with a hole, then a wing nut), I couldn't lift it even a tiny bit. What helped was warming up the bearing surroundings with a hair drier for a few minutes, and then using my DIY puller. Even then the bearing got stuck when it was almost out. I then semi-gently tapped on the bottom side of it with the wooden end of a hammer, and then it came off. This is not ideal, but the surface of the mount where the bearing sits can take some scratching with no consequences, as the bearing sits there firmly, with no movement.





* The cleaned/regreased RA gear is stuck when trying to put it together (when setting the cylinder with worm grooves onto the main RA shaft). It stuck a few times not badly (so I could easily remove it), but at the end it seized so badly that nothing worked - even placing it in the oven for 10 minutes at 400F (as described by the OP). What eventually worked was placing it in the oven at 200F for 10 minutes, then loading the back of the black part (on the right in the photo below) with ice. Then the two parts easily came apart.





* The safe-proof way of putting the RA gear together. After the above accident (which did scratch both parts, so I had to sand and polish them), I didn't want taking any more chances, so before trying putting the two parts together, I put the black part in the freezer, and the top part in the oven (200F for 10 minutes). After that, the assembly was easy. With a hindsight, I probably shouldn't have used the freezer  - just keep the black part at the room temperature, but instead heat the silver part to 300F (it would be the same effect). The problem with the freezer approach was that the air was very humid, so it instantly got covered with a layer of frost, so I ended up having some moisture inside the gear after assembling it. Hopefully it won't cause issues, but you were warned!


* Dry the parts you just degreased really fast - especially the bottom RA bearing! Don't just wait until it dries up by itself, use hair fan instead. I made that mistake, and was horrified to see quite a bit of rust on the bearing bottom by the time it dried up. I spent some time then trying to remove it, probably not perfectly. This might cause issues in the future.






* Regarding PTFE replacement washers. I did make them myself (using a compass cutting tool from a 0.5mm thick PTFE sheet), but then I realized they are very soft and easily deform. One sizeable bend, and they stay in a bent position. This is very much different from the original (transparent) washers, which are made from a much harder (and very springy) plastic. According to Wikipedia article, PTFE is subject to creep, so actually not very good for applications like this (washers subject to significant pressure). The original washers were still in a very good shape, so I ended up re-using them, and I used PTFE based lubricant (Super Lube) for the whole project, so hopefully impact of this will be minimal.




I ended up replacing the main RA bearing, the four Dec small bearings, degreasing then regreasing the whole scope with Super Lube, and carefully adjusting the meshing distance for RA and Dec axes, to minimize the backlash while keeping the movements as smooth as possible. I am waiting for a next clear sky to test it out. My hope for hypertuning is that (1) I can go back to the default 3 deg/second slew rate (I had to reduce that to 2 deg/second since I bought my new heavier scope, RC8), and (2) guiding with PhD2 will become more reliable. We'll see.

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