CEM25/CEM26 word of warning

[david_taurus83]

I'm not quite sure about the new CEM26 but owners of the CEM25 will be aware of the adjustable spring loading on both axis. Too tight and the worm mesh will bind and won't budge, too loose and it will slip over the ring gear. If you set it just right though, with a bit of force, unbalanced load or an accidental knock it's possible to still slip the worm over the ring gear. So far, I've not managed to accidentally knock the counterweight bar into the doorframe for example but I have had the RA axis slip the worm over the ring gear a few times due to the load being incorrect on scope side or counterweight side. Nothing too serious and it was usually because the spring load was too weak. The mount generally performs very well with an average guiding error of below 1" each session and regular spells of 0.5" RMS. Recently though, I've been getting some huge spikes in RA. Sometimes it looked periodic, sometimes it was random. I decided to strip the mount down last night and have a look.

There was some very fine swarf like material on the worm.

 

 

 

I cleaned up both the worm and ring gear and applied some new grease. You can see on the ring gear from side on, the marring caused by the worm sliding over the ring. I don't think this is the cause of the recent blips in guiding as it would only be in that one area of the ring. More likely to be the swarf on the worm. A marred ring gear can't be good either though. The good news is that with the CEM mounts you only ever use 180° of the RA ring gear. By installing it back on the mount turned 180° I now have the bottom half of the ring that has never been used facing the worm. So like new again.

 

 

 

Ouch!

 

"Bottom" side of the ring never been used

 

Heres the last hour of guiding from a few nights ago. This is the worst I've seen it. The first time I noticed it it done 3 large deviations but nothing for the remainder of the session.

 

 

So the lesson to be learned is don't let the worm slip over the ring gear, no matter how mild you think it is!

 

It's clear tonight hopefully so will try and get a session in. I will probably strip down the Dec axis today and have a look. Shouldn't be any issues there but a clean and regrease can only be good. I can also evaluate the condition of the ring gear should I ever need to swap out the RA one!

PHD2_GuideLog_2021-04-03_210939.txt