Celestron CGEM Mount Hypertune
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This article covers my experience performing a Hypertune on my Celestron CGEM mount, and what lead me to do so in the first place. I bought my CGEM second hand, but it’s a relatively new (less than a year old when I bought it) unit. I’d heard about the cogging issues (not fixed through firmware) and general poor tracking some units experienced, but decided to go with a CGEM regardless. My first mount was a CG-5 that performed well for me, and I’d certainly seen other astrophotographers using CGEMs with success. When a lightly used one came up, I decided to go for it.
Initially the mount seemed to work a bit poorly. The motors seemed to struggled at parts of the rotation in both axes. Doing some research, I found it’s possible to adjust the tension of the worm gear against the ring gear (more on how to do that below). These initial adjustments helped, and I had a fair amount of success imaging with the mount for a few months. My last time out with it though, the tracking was so bad, I was throwing out as many exposures as I did with the CG-5. The mount just wasn’t working smoothly. Neither axis moved very freely with the clutches loose, so balance was never as exact as it probably should be, either.
When I first got the mount, I looked into sending it to Celestron. After all, it was less than a year old. What I found out was that the Celestron warranty only applies to the original purchaser, so that was a non-starter.
I looked at selling the CGEM and just getting another mount. Something like a Skywatcher Atlas or iOptron EQ45 Pro. Researching these, it seemed there really weren’t any guarantees I’d wind up with a substantially better mount. Consistency between units there was not. Unless I was willing to step up to the $3,000+ price bracket, the variability between individual mounts appeared to be very high, and there were basically no manufacturers that advertised their mount to be within a peak-to-peak periodic error range. (The exception here seems to now be the SkyWatcher AZ-EQ5, which advertises ±5 sec. The next-cheapest mount with an advertised minimum performance would be the iOptrion CEM60.
So, if a clearly better mount was going to cost me 2-3x as much as a CGEM, Hypertuning started to sound like a reasonable option to invest in. I was already familiar with the concept of Hypertuning – certainly it gets discussed frequently on Cloudy Nights. I didn’t particularly want to go through the exercise, but with a mount running so poorly, it seemed like it would be worth a go. I bought a Hypertune kit from DeepSpaceProducts for $195. I’ll say I think this is a little on the expensive side. The kit itself includes new worm wheel bearings and PTFE spacers, along with some of the tools you need, sandpapers, and new lube and metal polish. The main thing you’re buying though is the instructions for how to do this without screwing up your mount even worse. In that respect, the two DVDs are definitely valuable for someone who has never taken apart a mount before. Ed is also there to provide support if you get in over your head. There are some tools you’ll need that aren’t included in the kit, and some like round tip pliers you probably don’t have in your garage or toolbox. It’s worth investing in so you don’t do what I did and wind up struggling with some of the pieces and adding a few score marks here and there. Shipping for the kit was very quick. Two days after ordering, it was at my door.
I’m not going to offer step by step directions here. There are other resources for that, like the kit from DeepSpaceProducts, which this article is no substitute for. What I will call out is some of the little problems I ran into along with what I observed about the mount that appears to have contributed to the impaired performance. If you do a hypertune yourself using the DeepSpaceProducts kit, I do recommend watching the videos all the way through before you start, as I did.
Disassembly of the DEC axis was relatively easy following the instructions. It was clear however that two of the four hex bolts holding the worm gear housing on had the threads stripped out from being overtightened (with steel bolts going into aluminum, this can be easy to do). Pulling the DEC axis apart really didn’t present any other surprises, but did require some banging on with a rubber mallet to free the ring gear and rotational axis from the housing.
The Right Accession axis offered up an unexpected and unpleasant surprise. It seems that when the mount was assembled at the factory, there was some sandblasting residue left in the housing, due in part to a flaw in the casting that left a gap where the residue built up. Well, that sandblasting residue had migrated to the ball bearings and taper bearing on either side of the housing. These had worked their way into the rotational axis surfaces, which no doubt contributed substantially to the apparent stiction and tracking error. I suspect this also caused the RA worm to be a little off-center with one or more of these little balls working it’s way up to the outside of the ring gear (the gear and housing both showed rough spots that required sanding and polishing).
Both ring gears proved to be very tightly seated in their housings – neither rotated easily, which would also account for stiction, particularly when combined with a lot of backlash between the worm and ring gears. Sliding them out initially, they would move a little, then get stuck. It required a rubber mallet to actually remove both axes, because the ring gears were so tightly packed into the housings.
Worm Gear Housing Disassembly
The worm gear housings are a pain to disassemble. You absolutely need to have the right tools. Even then, I needed to actually apply heat to part of the casing to loosen up some thread lock on one of them. I never actually was able to completely remove the RA worm wheel from the housing, due to one of the set screws holding the spur gear to the worm wheel having the head stripped. Either someone previously opened up this mount (which seems unlikely) or it was over-tightened and stripped at the factory. As the worm seemed to spin just find, I didn’t bother fighting with it, and just left the factory worm wheel bearings. The DEC worm wheel came apart (no stripped set screw heads) and i was able to replace the bearings for that worm.
With (mostly) everything disassembled, cleaning off the old grease and the casting residue didn’t take that long, but did go through quite a few shop towels. The process was made much easier with citrus degreaser, and for the gear teeth, a toothbrush (you’ll want to throw that out when you’re done).
Sanding the ring gear collars and the RA housing took the most time. The kit included 400, 600, and 1000 grit sandpaper. For the DEC ring gear though, I broke out the 220 to start with, as the fit in the housing was way too tight. Even now, I’m pretty sure I could have sanded it a bit more. One of the main goals of the HyperTune is to get the axes rotating freely. Both of the ring gears in my mount were too tight in the housings to do that (with clutches released, they were still ‘sticky’). After the sanding down to 1000 grid paper, I applied some metal polish (included in the kit), then the degreaser again to shine the pieces up. After a lot of work (probably an hour in sanding and polishing for the two ring gears alone) they both spun much more smoothly in their housings.
The two taper bearings got cleaned with degreaser, as did the housings. The larger bearings (3 on each axis) simply got wiped down with shop towels to remove the old grease. As Ed says in the video, don’t bother trying to do anything with these bearings. As long as they spin freely with no strange sounds, they’re good. Cleaning overall took probably three hours and half a role of shop towels, a few toothpicks (great for getting into the gear teeth and other tight corners) and the aforementioned toothbrush and sandpaper.
The RA Housing is a casted piece of aluminum, and was the source of the tiny steel balls. Examining it closely, there is a flaw in the casting that left a gap where what I assume is sandblast residue got trapped in the housing during the manufacturing process, then over time worked it’s way out into the RA axis bearings and ring gear.
When I passed a magnet over this area after otherwise throughly cleaning it, the magnet picked up more of the contaminate, confirming my suspicion.
Since I couldn’t be sure I could get all of the contaminant out, I mixed up some 2-part epoxy and covered the gap inside the housing. This is a rough cast part of the housing, so there are no parts in contact with the area. I also went over the whole thing pretty thoroughly with the toothbrush to kick loose any other contaminate and cleaned, sanded, and re-polished the RA housing.
Reassembling and re-greasing the mount goes easier than the disassembly, now that the parts fit together with better tolerances. It still took a couple taps with the rubber mallet in a few cases, but overall, it went smoothly. RA axis goes back together first, then the DEC assembly.
One element to be aware of with the re-assembly is to keep the bolts for the worm housings loose to do the final tuning of the mesh between the worm and ring gears. This is best done with the mount powered and doing minor adjustments with the set screws on either end of the worm housings. Ed go through this in detail in the videos with the kit, but this is where you are able to eliminate much of the backlash in the mount.
Conclusions… coming soon!
In all, the process took me the better part of a weekend to complete, including watching the two DVDs all the way through before starting. I took my time with it though, so it’s something that could be completed in a day.
I haven’t yet been able to get the mount out under the stars, due to rain clouds and thunderstorms, to test out whether or not I’ve actually improved the real world guiding performance. At this point though, I can’t imagine it won’t be improved given the debris I removed from the mount. For what it’s worth, I emailed Ed asking if he had seen that issue before, and he said he had not. I just seem to have good luck with refractors (see the AT65EDQ review), and apparently mounts too.
I can say with confidence that the axes turn much more smoothly (can be easily spun around) and slewing is completely smooth in both axes with significantly less backlash than before. Because of the smooth rotation, balancing equipment should be much easier and more precise. I’ll add a guide graph similar to the pre-hypertune graph as soon as I get the chance.
Is a hypertune worth the price and time? Check back for updates. I hope to be able to answer that question after a clear night!
Is a CGEM a good mount to buy? I think I got a bit unlucky with my particular unit, and getting it second-hand with no warranty support. If I hadn’t come across a second-hand CGEM for a good price, I was actually looking at the Orion Atlas Pro AZ/EQ-G, which also happen to be on sale for $200 off right now.
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