Celestron CGEM tuning questions

[raf.serrano (Raf)]

Hi guys, I was so frustrated with my CGEM that almost led me to buying a seconds AVX but I'm glad I did not.

I was getting "C" shaped stars so I wondered what the culprit was. Found out the tappered roller bearing on the RA axis was a bit warped so I tore down the mount.

Given I need to replace the RA axis bearing, I also have some questions in mind:

would it be better if I replace all bearings or is it not necessary? Planning to leave the old bearings inside the worm wheels and only relace the besrings in the RA and Dec steel casing.

Also the dec tappered bearing seems heavy duty should I replace this too?

The dec axis tapered bearing has red grease in it, is it good or should I replace it with superlube?

How much grease should I apply on the tapered bearing? The dec axis is overflowing with red grease. Should I do the same with superlube?

Cheers guys, clear skies

[toc (Tim)]

That red grease has been there for a while - probably a good idea to just remove it all and replace with new stuff.

[raf.serrano (Raf)]

Originally Posted by toc

That red grease has been there for a while - probably a good idea to just remove it all and replace with new stuff.

Thanks for the response Tim I'm enjoying the tear down and tuning. I'm also planning to hide the usb and 12v cables inside to the dovetail clamp too. Anyway would you know what the red grease is? Is this the one available at supercheap auto with ptfe and would you recommend that for the unsealed bearings better than superlube?

[toc (Tim)]

Originally Posted by raf.serrano

Thanks for the response Tim I'm enjoying the tear down and tuning. I'm also planning to hide the usb and 12v cables inside to the dovetail clamp too. Anyway would you know what the red grease is? Is this the one available at supercheap auto with ptfe and would you recommend that for the unsealed bearings better than superlube?

Yes pretty sure the red stuff is what you describe - not sure I got it from Supercheap, but its definitely possible.

It seems fine, but I would probably use superlube these days - can't really say with any authority that it would actually work any better, but the astro herd all seems to go with superlube The red stuff is pretty damn messy though.

[raf.serrano (Raf)]

Originally Posted by toc

Yes pretty sure the red stuff is what you describe - not sure I got it from Supercheap, but its definitely possible.

It seems fine, but I would probably use superlube these days - can't really say with any authority that it would actually work any better, but the astro herd all seems to go with superlube The red stuff is pretty damn messy though.

Haha fair enough, thanks Tim! just bought the smaller bearings and re-using the larger tapered one with the superlube. Feels like restoring an old Camaro or Mustang though this is much less complicated

[ChrisV (Chris)]

First time I did mine I thought the problem was the bearings. I replaced a couple of the large bearings ( but not the tapered one). Superlubed everything and it was much better.

But later I cleaned out the old bearings, superlubed them and tried them again. All was good. So really, it just needed a good clean and regrease.

I still got egg shaped stars at longer focal lengths. So eventually trashed the coffee grinders motors, and replaced with steppers/pulleys/belts using the onstep system. You wouldn't believe it was a cgem anymore - quiet and beautiful tracking - well good enough for me

[raf.serrano (Raf)]

Originally Posted by ChrisV

First time I did mine I thought the problem was the bearings. I replaced a couple of the large bearings ( but not the tapered one). Superlubed everything and it was much better.

But later I cleaned out the old bearings, superlubed them and tried them again. All was good. So really, it just needed a good clean and regrease.

I still got egg shaped stars at longer focal lengths. So eventually trashed the coffee grinders motors, and replaced with steppers/pulleys/belts using the onstep system. You wouldn't believe it was a cgem anymore - quiet and beautiful tracking - well good enough for me

Oh wow this looks promising Chris! Future project for me when I get the initial tuning done do you have any links you can share for this particular mod?

Edit: coffee grinder motors

[ChrisV (Chris)]

I put my cgem mod on the onstep website.
https://onstep.groups.io/g/main/wiki/4414

Scroll down the page to the cgem section. A few people have done it. I've found it a great improvement, but don't do it unless you are unhappy with the performance of yours after tuning it.

[raf.serrano (Raf)]

Originally Posted by ChrisV

I put my cgem mod on the onstep website.
https://onstep.groups.io/g/main/wiki/4414

Scroll down the page to the cgem section. A few people have done it. I've found it a great improvement, but don't do it unless you are unhappy with the performance of yours after tuning it.

Noted, thanks Chris, would probably be a project in the future

[raf.serrano (Raf)]

So I have removed the motors from the housing and found some play in the gear on both the RA and dec motors, is this normal or does this imply that something is wrong with the motor? Compared to my Skywatcher AZ-GTI the motor on that mount doesn't have any descernable play if I try to turn the motor gear.

[ChrisV (Chris)]

Just pulled my CGEM motor out of its storage box. It has about half a tooth's width of backlash/play.

Chris

[raf.serrano (Raf)]

Originally Posted by ChrisV

Just pulled my CGEM motor out of its storage box. It has about half a tooth's width of backlash/play.

Chris

Hey Chris, thanks so much for going through the trouble. I opened the motor gearbox and looks like not much can be done to remove the backlash unless the gears are made a bit bigger and fot snugly to each other

Raf

[ChrisV (Chris)]

Same with mine Raf!

After modding mine I've come to the conclusion that the cgem is mechanically great - good enough for me anyway.

It's just let down by the crap gearbox/motor

[raf.serrano (Raf)]

Originally Posted by ChrisV

Same with mine Raf!

After modding mine I've come to the conclusion that the cgem is mechanically great - good enough for me anyway.

It's just let down by the crap gearbox/motor

Looking for a 1:50 or 1:25 reduction gear that would spin in reverse so I can try belt driving this first before I mod with onstep

By the way Chris, I should have checked your google drive page first from the link you provided, it has a LOT of amazing info for CGEM mounts like a literal Bible for it awesome work you put in there mate.

Also interested, are you able to control the mount via mobile device / wifi? Or is it through a PC?

[ChrisV (Chris)]

With onstep I control the mount via WiFi and USB. I don't use a hand controller. On a mobile phone or tablet you can align the mount using the onstep app.

I then stick with WiFi and use SkySafari for GOTOing if I'm just doing live imaging.
If I'm imaging I use the USB port to control with SGPro, Nina or APT via ascom.

I only needed a belt reduction of 3.2:1 with a good stepper (400steps/rev) to get enough power and accuracy. This gives 0.14 arcsec/step with microstepping.

[Syd_Gra]

Hi, I've just replaced the motor control board on my CGEM after the RA port drive appeared to go faulty.
Is this a common problem and does anyone know what might trigger the fault? I'd like to not have to replace it again. Thanks