Hypertuning my CGEM (photos added 5/8)
Posted 06 May 2013 - 01:50 PM
Didn't rain once here in the past 8 weeks prior to my equipment upgrade.
On a serious note, I'll post the real results here when skies clear out. I was having so-so luck with autoguiding and spending way too much time fiddling with software setting trying to account for glitches in the mount. Hopefully, some of the spikes I was getting in RA will be past history.
Posted 06 May 2013 - 03:48 PM
Posted 07 May 2013 - 10:54 AM
1) Large, deep defect inside the aluminum housing for the Dec. ring gear! There is no way this could have happened after it left the factory. I bought the mount new, and I don't believe anyone else opened it up. I sanded out the rough edges of the gouge per Ed Thomas's instructions and it's smooth now. I have to wonder if it was the cause of my sticky Dec. axis.
2) Gobs of dirty, black grease all over everything. That is a common observation from people who do the hypertune DIY procedure.
3) Some dirt/grit inside the Dec. axis taper bearing, which migrated all the way up the Dec. axis and was even under the saddle. This one was my fault for transporting the mount with the counterweight bar removed. Once you take off that bar, the door is open for dirt and foreign matter to get into the taper bearing and stick to the gobs of grease in there. Dirt can also travel up the long, hollow tube that goes up the Dec. axis and get inside the mount. I would also not recommend removing the polar scope cover, which is another source for dirt and grit to get inside the mount.
Some difficulties I had:
* Could not get off the retaining ring for the RA axis taper bearing for a long time. I struggled with this for over an hour using a vice. It was far worse than the problematic one Ed had in his video. In the end, I stripped the paint off the retaining ring, but it opened. Any damage was purely cosmetic, but if this sort of thing bothers you, then think twice before getting the DIY kit.
* Had similar trouble with the retaining ring for the Dec. axis taper bearing. Someone glued the set screws in with excess thread lock. These screws are hard to access, and it took me 90 minutes to get them out.
I am now at the stage of adjusting the re-assembled mount. One thing I can say is that both RA and Dec. axes glide incredibly smoothly! It does feel like a totally different mount, as many people have said.
I will post another update after I adjust all the gears and get rid of the backlash.
Posted 07 May 2013 - 12:43 PM
So, is this mainly a cleaning and re-lubricating procedure? Did you also do the ceramic bearings replacement?
How long have you had your CGEM before you did this procedure?
Posted 07 May 2013 - 06:25 PM
One critical step involved is removing the brass ring (drum) gears from the RA and Dec. axes and (gasp!) sanding them down by hand using sandpaper and metal polishing compound. I have to admit that when I saw Ed doing it in the instructional video, I almost put the mount back together and sent it to him for service. I was scared about sanding down a critical moving part by hand with ordinary sandpaper. However, I mustered up some courage and did the procedure by myself, and I ended up with gears that fit much better into their housings, rotating super-smoothly, and have surfaces that are polished almost to a shine. The difference was dramatic compared to the mount as-received from the factory. The polishing of the ring gears is one major reason why the axes glide so smoothly after hypertuning. The other major reason is replacing the sticky grease with smaller amounts of the fluoropolymer based "Super Lube" included in the kit.
After doing the hypertune, I can definitely see how "individual results may vary." If someone did an inadequate job of polishing the ring gears, I could see that they might not get great results. Only time will tell if I did a good job or a poor one, but I feel really good about the way the axes glide after reassembly. My main worry at this point is whether I can adjust the gears well enough to get rid of backlash without binding up the motors. I have no prior experience in that area, being a chemical engineer rather than mechanical. We'll find out soon enough whether I know how to adjust gears properly.
By the way, we did get a huge rainstorm last night, right on time, just as I finished re-assembling the mount! Hasn't rained significantly in Lubbock since February. One additional benefit of hypertuning: get a green lawn after you're done. I'm sure we'll get a clear night soon enough so I can test the mount on actual stars.
Posted 08 May 2013 - 09:14 AM
Last night, I almost finished adjusting the gears, but the RA and Dec spur gears are still a little too tight with the motor gears. Thus, the motors are working too hard when the scope is oriented in certain directions. I did get rid of the backlash in both axes, though. Some fine tuning of the motor positions is in order this evening.
The balancing of the scope is now vastly improved compared to the original state of the mount. My Dec. axis used to be so stiff that I could barely determine a proper balance point. I sincerely hope I see an improvement in guiding accuracy for astrophotography after I'm done. My autoguider was sending way too many corrections to the mount to account for spikes in RA.
Anyway, here are a couple of pictures from the disassembly:
Here are some blurry photos of the huge gouge I found in the Dec. housing. This is a critical surface against which the ring gear slides. The gouge is possibly a casting defect of some sort. Obviously, I could not sand out something so deep, but I did sand the edges of it to be smooth.
Below is the retaining ring I could not get off with a vice for about an hour. It's now missing significant paint. I'm not proud of what I did to it, but it was screwed on there with such grotesque force that it had to be adversely affecting the tightness of the RA axis. Glad it came off, as I would have been really disappointed if one sticky joint killed my hypertuning effort.
Below is a taper bearing on one of the axes after removing the retaining ring. These taper bearings are coated with gobs of grease by Celestron, meaning that grit and dust can easily get in there if the cover is removed. Also, note the hollow axis tube, which allows dirt to migrate up into the mount as I mentioned.
Finally, here are some gears and ball bearings after disassembly.
The donut-shaped thing on top is a ball bearing. These fit very tightly around the axes of the mount, and one must exert great care in removing and replacing them, as they get stuck very easily and can bind up your assembly at any given moment. The brass drum is the ring gear, and its surface must be sanded down and polished so it rotates smoothly in the housing. Nothing is done to its teeth besides cleaning. Between the ring gear and the ball bearing is one of the plastic spacers that is replaced with Ed's Teflon spacers.
On the right side, you can see a brass spur gear and part of the worm gear in one of the housings. These are not only carefully cleaned and re-lubricated, but the whole assembly has to be removed to replace the bearings with ceramic hybrid bearings. Getting it apart is tricky, and dropping the worm gear on a concrete floor probably means the death of your mount. I hear Celestron won't sell you most parts; they don't encourage customers to disassemble their mounts. Warranty is void the minute you open it up.
Posted 09 May 2013 - 11:18 AM
Posted 09 May 2013 - 09:30 PM
That gouge will not hurt anything, it will just collect some grease.
Agreed. Once the ring gear drum is properly fit in the housing, the gouge will not cause a problem since you have made sure that it does not have any sharp edges sticking up to scrape on the ring gear drum.
Posted 09 May 2013 - 11:13 PM
Posted 10 May 2013 - 10:06 AM
Posted 10 May 2013 - 10:54 AM
I've finished adjusting the gears, and was able to set up briefly the last two nights. The mount axes rotate more smoothly and motors are quieter despite the fact that I have the spur gears much tighter than they used to be. The mount feels great when swiveling around one of the axes manually.
Due to poorly timed thunderstorms, I could not do any testing except polar alignment the past two nights. I didn't detect any problems with the mount, and the backlash in Dec. is much lower after adjusting the gears to be tighter. However, I really want to get a quantitative idea of whether guiding is improved or not by running PhD in a few areas of the sky for an extended period of time. I don't expect that the RMS value in PhD is going to improve, but I do think that some of the spurious excursions in RA that I would occasionally get will be lessened. I feel like sticky spots on the ring gear could have been the root cause. I'll let you all know in 6 weeks when it stops raining!