Friday, July 14, 2017

The Saga of the Celestron CGE Mount

It is with great sadness that I announce the passing of my Celestron CGE mount.  Adopted from my Uncle Chris, it lived a good life, and helped me get some awesome images and take my fledgling astro-imaging hobby to the next level.  You will be missed!

So basically, here's the rundown.  I'm sharing because I have seen other CGE users run into similar issues - although, I'm the first case of a complete failure that I've yet come across.

In the beginning...

Looking back through my log, it's clear that the real issue started early on, I just didn't recognize it for what it was yet.  I got the mount in February 2016, and it wasn't long after that I started complaining of it "not being able to handle the cold."  I mention having very bad initial guesses for alignment stars as early as late February, and definitely in April.  Also in April was the first time I recorded that it "moved on its own," probably an instance of it "jumping" when I either touched a cable or the cold shrank the metal in the shielding contact in the port, causing bad ground contact and the motor to jump.

Long story short, here is what was happening: So the Celestron CGE is two parts, the electronics pier, and the mount head itself.  The two are connected by...wait for it...ethernet cables.  Worse, they used shielded ethernet cables - and worse yet - they use the shielding as a ninth pin to conduct the ground signal to the RA axis.  (Whenever I tell my electrical engineering friends about this, they are as flabbergasted as I am.  If you are a Celestron engineer and reading this...please please please never do something so dumb again.)  To top it all off, if the ground signal to the motor is lost, the motor starts slewing out of control, at a speed higher than the highest normal slewing speed, and that motion is not encoded (meaning, if the mount slews this way for just a short period, it doesn't know that it moved, and I have to restart the entire mount and re-align all over again).  I hit the internet over this issue, and eventually it came down to those blasted ethernet cables.

The first thing I tried, back in July 2016, at the recommendation of many other CGE users, was to replace the shielded ethernet cables with flat, more-flexible ones.  This appeared to work, and since the weather was warm, they were pliable enough and the metal expanded enough that I had several nights of success without any issues.  I also got cable guides that I attached to the mount to let me create some slack on the cable where it goes into the mount housing so that it wouldn't get pulled.  This worked for a while, but then as the cold weather of fall came again, it became clear that a more drastic solution was needed.

The Bennett Mod

Many CGE users online recommended the Bennett Mod.  There were so many CGE users who had this exact same issue that a Canadian, Gary Bennett, finally created a mod kit to fix it.  Basically, it replaces the ethernet ports, both in the electronics pier and the mount housing, with an 8-pin screw-on connector that was much more robust.  The chassis was still used as the RA ninth pin, but a good connection was ensured because the cable screws on at both ends.  After reading about all the success that many other CGE users on forums had, and saw how many people swore by it and said it solved all of their CGE issues, in December 2016 I bit the bullet and paid the $400 for the "easy" version of the kit, which doesn't require you to drill new holes into the mount housing.  I received the kit, and it was indeed rather easy to install - no soldering required, just unplugging headers and plugging in the new ones, which even came labeled.  The instructions were quite clear and had helpful pictures.  It took me about two hours (I meticulously checked the tightness of the connections, triple-checked that I was plugging the right cable into the right header, etc), but then it was done.

So I plugged int back in, turned it on, and after getting some connection errors and tightening up some connections, got both axes talking to the hand controller.  I hit Enter to find switch position...and while dec worked on finding the switch position no problem, RA started slewing out of control!  Pressing keys on the HC yielded error codes about no response, and finally I just had to cut the power before the mount tipped itself over (it's rather top-heavy).  

I got in contact with Gary Bennett, and he was very helpful at providing me with ideas of things to check to try and figure out what went wrong.  He even sent me the schematic for the mount, and I dug out my multimeter and meticulously went through every connection, testing for proper voltage and connectivity (or grounding).  Everything looked by the books, at least as far as I can tell, being a physicist and not an electrical engineer.  The dec axis worked just fine, and it continued to work even when I swapped the RA and dec cables to test.  I've got a whole bunch of notes on all of the strange behavior that was going on, but I won't exhaust you by putting it all here.

Calling In the Pros

After struggling with it for a few weeks, I finally got in touch with Dr. P. Clay Sherrod, who does a "supercharge" service for several kinds of mounts, hoping that his familiarity with the inner workings of my kind of mount would be able to help me figure out what was wrong, and how to fix it (his website is here).  He gave me a great deal of helpful advice, and I re-did a number of things I'd already tried, and also tried some new things to test it.  I looked for burned out resistors, checked the function of the limit switches, swapped out hand controllers, used different power sources (AC wall power vs DC battery), checked resistance across the optical encoder, plugged the ethernet components back in (essentially "un-doing" the mod), all sorts of stuff.  But, two months later, I was still getting nowhere.  So I had a choice: throw in the towel and get a new mount, or take a several-hundred-dollar bet and ship it to Dr. Clay for diagnosis and, hopefully, repair.  I just had a feeling (and my hunches are usually right) and there was just a single bad connection somewhere, some ground wire that needed to be re-soldered or replaced, something cheap and easy that would just unravel the whole mystery.

Dr. Clay spent several weeks on it, even taking it into his electronics lab at the University of Arkansas to test all the circuitry.  He was just as baffled as I was.  After much effort, I finally had him send it back to me.  I started shopping for an alternative mount, although I was unable to find anything that matched the capability and payload of the CGE.  Everything was either too light (55 lb payload or less) or too big (75 lb payload or bigger).  The CGE had a 65-lb payload capacity, and only weighed about 40 lbs.  In astrophotography, you want your mount to be able to carry twice your actual payload in order to have good tracking; between my 11-inch telescope, the guide scope, and the camera, I am at about 32 lbs, so I really do need about a 60 lb capacity.  But anything with that capacity was beyond my pricepoint, so I was going to have to settle for something like the Celestron CGX, which has a 55-lb payload capacity and cost $2200.

Another miracle, of a sort

So far in my astrophotography journey, I've had a great number of small miracles.  Nearly all of my equipment has been given to me, or at least sold at a steep discount.  Literally the only things I have purchased on my own at full price are my DSLR and some miscellaneous parts and pieces, like dovetail bars from ADM or cables.  My C8, C11, and Borg refractor were all gifts; my Orion ST-80 was sold by a friend on the cheap; my QHY5 was also sold to me on the cheap.

So when Dr. Clay emailed me and told me that he had a Celestron CGE Pro, just back from Celestron refurbishment (and Hypertuned!) that he would sell me for $2k, I about fell out of my chair.  The Pro is the successor to the CGE, with a payload capacity of 90 lbs, and - wouldn't you know it - screw-on 8-pin cables (and even a screw-on DC power port).  It sells new for $4,500.  My only reservation that I had to sleep on for a few nights about it was that it weighs a lot more than the CGE did - the EQ mount head alone weighs in at a hefty 75 lbs, and then there's the counterweight bar and electronics pier in addition to that.  Luckily for me, the mount head comes apart into two sections, but they are not very evenly distributed - the upper portion still weighs some 50 or 60 lbs.  So, of course, I bought it!  And, bonus points, it arrived just in time for the 2017 Texas Star Party.  Blessings all around.

So...now what?

As far as Dr. Clay and I can tell, something got fried when I installed the mod kit.  Dr. Clay thinks it's a juxtaposed cable in the mod kit, but Gary Bennett says he tests each kit on his own CGE before shipping.  I followed his instructions to a T, and I am no foreigner to circuitry - I am an experimental physicist, after all, and have soldered my fair share of both low- and high-voltage circuits and have thus far not blown anything up.  And this didn't even require soldering, just plugging stuff in, it's very nearly fool-proof.  So, who knows.  It may have even been completely unrelated to the Bennett mod.  The last night I used it, in November, it did the slew-like-crazy thing, and then it under-slewed in dec several times in a row, even with power cycling.  Maybe it broke that very night, before I even bought the Bennett Mod.  We will never know.  

As far as the CGE itself goes, I would like to sell the parts to other CGE owners, since they are hard to come by now that Celestron has discontinued it.  But I'll have to sell them with some kind of money-back guarantee, since I don't know which part is the fried one.  I haven't gotten around to it yet, so we'll see.

As far as the new CGE Pro goes, it was a champ at Texas!  I guided as long as 7 minutes, and the mount wasn't even level (my tripod has a gimpy leg, and of course that always ends up on the lower side).  It is, however, prohibitively difficult for a single person to set up and use, so it is going to become my long weekend and star party scope, situations where I can set it up and leave it up for several days (and have some help getting the heavy mount head on top of the rather tall electronics pier and tripod combo).  

10 comments:

  1. Lol, I had the exact same problem with my CGE mount - the RA would slew out of control and the only way to stop it was to cut the power.
    I looked for information on this problem but there wasn't really anything too helpful out there but info on the cable issue, which in turn also led me to the Bennet mod.
    I installed the Bennett mod and initially it worked just fine but after a couple of sessions with my CGE 1400 the RA started to slew out of control again - I was fuming mad lol.
    Anyway I ordered the ORION HDX 110 and lived happily with that for a long time... in the meantime I took the CGE apart to see if I could find the problem and I did - it was an intermittent bad ground on the RA - the RA motor needs the be grounded properly to function (which really was the original issue a lot of CGE owners experienced). So after finally grounding the RA motor properly (in my case I drilled and tapped a hole in the RA housing and fitted a small screw that I attached to the RA motor ground and voila!) I have a fully functional CGE mount once again.
    Interestingly though my HDX 110 now has developed a similar issue where the RA briefly stops at the requested object but then immediately starts slewing again until I cut the power *facepalm*.

    R.

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    1. I thought about just soldering the ground wire of the motor directly to the RA housing, lol! Maybe I will 😂 A screw would probably be better though, yeah, and I need to get a tap kit anyway..

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    2. I realize this is an older post but would you have images of your mod that you could link to? I’m in the process of pulling my CGE apart to clean the gears and relube everything. I’d love to do the ground wire mod. Did you just solder a lead on to the ground wire post and attach it to the metal housing? My CGE is going on 20 years old and I don’t want to replace it. It’s perfect for what I need and want just do some DIY tuning. Thanks for the help.

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    3. Hi Scott, I didn't end up doing the ground wire -- I came to the conclusion that the motor was dead, and I found someone selling one on AstroBin earlier this year. I bought it, swapped it in, and it worked! But then my other axis was behaving oddly (dec axis), and it looked like the grub screw might have come loose -- the one that holds the brass drive gear to the motor shaft. In trying to tighten it, I stripped it, and the spent months trying to remove the grub screw so I could replace it. I've given up for now...

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  2. Greetings to all owners of CGE frames. The mount acquired in 2005 has had some episodes. Undoubtedly good load capacity. It was necessary to change the original control and de RA and dec cables. It currently does not work. I understand that it is necessary to change the control and cables RA and DEC again. I bought some for Amazon and they sent me wrong ones. Star the return process.

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  3. Interesting. I may have to pick your brain and ask for help - if you are willing of course. I picked up a used CGE with the Bennett mod, but it was sold as a problem child at a price I could not refuse. I'm trying to sort through its problems and parts may be an issue eventually. I do, oddly, have another CGE through work since on was purchased for the university back in 2006 and although I can't cannibalize it (inventory would have a fit) it does allow me to swap out modules to see what is going on with mine.

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    1. Sure! You can email me at astronomolly.images (at) Gmail (dot) com. I've really been through the ringer with my CGE and CGE Pro mounts, so I can probably help you out. (and maybe sell you parts...)

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  4. I got into the mount today and the limit switches seem to be giving the wrong logic state to the drive. When both are off "0,0" the mount will rotate cw but not ccw. However "0,0" is when the mount is between the hard limits and should move either way. I did get it to cut off when it hit the meridian for cw "0,1" but it will only go ccw if I manual hold one switch closed when the other is open - logical "1,0" to go ccw! Weird.

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  5. I think one of the limit switches is bad since it gave intermittent correct behavior, cw and ccw enabled when both switches off, after clicking the switch manually. It then aborted mid slew and went back to not running ccw when it was in the 0,0 state. Do you know a source/part number for the limit switches?

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    1. Yes! They're super cheap. I bought a couple to see if they were causing my issue (they turned out to be fine). All you have to do is desolder the old ones and solder on the new ones. Very quick. https://www.jameco.com/z/SS0505A-Highly-Electric-Company-Snap-Action-Switch-Short-Hinge-Roller-Lever-Solder-5-Amp-250-Volt-AC-0-44n-Screw-Mount_159599.html

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