CGE Pro Declination "Play" Fixed
Posted 20 January 2011 - 05:23 PM
Here is an image of the motor, gears, their support frame, and the support frame that attaches the motor drive to the mount.
Mounted to the declination axis is an aluminum plate where the gear box housing is mounted (it was removed prior to the photo). On this plate, a support frame is mounted (red arrows). The motor and gear assembly (black arrows) is mounted on the mount support frame by the two nuts on the mount support frame. These nuts attach to bolts on the gear and motor assembly frame.
The tension on these nuts controls the distance between the worm gear (under and parallel to the motor) and the declination gear. However, if these nuts are loose, the gear and motor assembly frame can move laterally relative to the mount support frame. This is what causes the "play" in the declination axis.
Also, this is a tuning issue with the mount. Setting the correct pressure to fully engage the motor's worm gear and declination axis gear is important to ensure minimum backlash in the gear without binding the gears against each other. So, by using the combination of nuts, the pressure and "tilt" of the gear can be accurately set.
At this point, play could result from 1) the nuts holding the motor and gear assembly frame to the mount support frame being too loose; or 2) the worm gear being too far from the declination gear. I found that when I wiggled the declination axis, the entire motor/gear assembly moved slightly.
Shown below is another view of the nuts holding the motor frame to the support frame. I turned each of the nuts clockwise 1/4th turn and eliminated the play. The worm gear seems to be firmly engaged with the declination gear and when I mounted a scope on it, the play is gone and everything appears to be working normally.
Those who are not able to get the motor fully secured without disengaging the worm gear from the declination gear could get some teflon washers and place them under the steel washer. It's possible that when they configured the mount at the factory, they didn't use enough spacers to keep the motor secure under heavy loads (I run my rig with about 60 lbs on it, so that may have help loosen things up).
Just be careful when fully removing the gear box covers that you are careful to remove the blue connector from the circuit board on the lower cover.
Posted 20 January 2011 - 06:25 PM
Seems like an easy tweak, but for $5k you would think this would be part of QC.
Posted 20 January 2011 - 06:47 PM
Posted 20 January 2011 - 07:17 PM
Seems like an easy tweak, but for $5k you would think this would be part of QC.
I think it probably passed QC because you really don't "see" any movement of the gearbox relative to the support frame until you look at the end of a long lever arm (the end of the optical tube assembly) showing that the slop exists. The QC probably checked to make sure the block was secure and that the worm gear was properly meshing with the declination gear.
Posted 20 January 2011 - 07:43 PM
Great post! This will help a lot of people.
Posted 21 January 2011 - 08:27 AM
Posted 21 January 2011 - 05:11 PM
Posted 21 January 2011 - 07:44 PM
Posted 22 January 2011 - 12:15 AM
Jim -- It could. I used to repair Criterion mounts back in the late 70's and early 80's. The worm gear would get askew with respect to the RA gear and have a problem similar to what Lleege is experiencing. It took some trial and error to get it re-seated properly.
Posted 22 January 2011 - 03:05 AM
Posted 22 January 2011 - 10:47 AM
No matter what, the best thing is to look in the gearbox and watch what happens when the dec axis "slips". Just remember that there is a blue power connector attached to the bottom cover (it comes off in two parts -- the top has the 4 philips head screws and the bottom has the 8 5.5mm hex nuts -- slowly remove the bottom cover and unplug the blue connector). You'll also need a small philips head to remove the encoder sensor (it's the little roller that presses against the declination axis). It remains connected to the lower cover.
Celestron's gearbox design, by the way, is basically the same design that's been in use since the 1960's. On the upside, it's a well proven design that's very reliable and adjustable. The downside is that it's really no better than what we started using 40 years ago and uses some "kludges" to improve performance... which means that getting it to perform "right" can be a huge pain in the neck.... but it can be done.
By the way, I had mine out last night and was able to check the tracking. Clouds were coming and going, so I only ran a few exposures and watch the tracking graph (I use metaguide) and the stars were respectably small and well shaped for the seeing in exposures as long as 6 minutes -- which is where moonlit sky saturated the sensor).
I had the lowest tracking numbers I've ever had... and the wind was blowing a steady 5-10 with some gusts to around 20... just what I'd hoped for when putting my 30'ish pound dslr rig on this mount
p.s. If you can't figure out what nuts/screws to turn after looking at the gear and seeing what's happening, put it in an email to Celestron support. I'm sure that if you can give them a clear indication of where the slippage is occurring, they'll know how to fix it. It's a very hard problem to diagnose without a scope on the rig to help visualize the problem.
Posted 22 January 2011 - 11:28 AM
Posted 22 January 2011 - 04:10 PM
Although it sounds like LLEEGE has tried all this...
On my mount the DEC has no play around the entire gear. The only fault I've ever had with the mount was the grease wasn't applied terribly carefully between the gears. 5 or 6 q-tips later everything was cleaned up nicely.
Posted 22 January 2011 - 05:55 PM
Posted 23 January 2011 - 05:03 AM
Posted 23 January 2011 - 01:16 PM
If you removes the gear housing covers and mount a scope, you can watch the worm gear and see where the play is coming from. In your case, it sounds like the teeth are not fully meshed with the gear. If you look and see that they are, then watch the worm gear itself -- is the entire thing moving? Is it the motor itself or the metal it's attached to also? Keep looking... these systems are far too simple to hide it.
Basically, you have to see exactly what's able to move freely (that's the play). Once you know what's slipping, it's easy enough to fix it, although sometimes it's counterintuitive to know how. For example, tightening the two nuts on the bottom of the support frame will increase play if you pull the worm gear away from the declination gear.
Posted 24 January 2011 - 08:33 AM
Posted 24 January 2011 - 06:17 PM
Posted 24 January 2011 - 09:29 PM
Posted 25 January 2011 - 04:49 AM
Posted 25 January 2011 - 10:39 AM
Btw.. here's a "first light" test of my modded T1i (astrodon filter), Astro-Tech AT111APO, and recently fixed-up CGE Pro... Of 34 seven-minute subs, I only threw out one due to bad tracking when my cat tried to play with the cables. The only defect I could find was a slight misalignment of the sensor in my camera, so I opened it up last night (after this image data was capture) and found that one of the three spring-loaded supports for the CMOS sensors was slightly out of position, causing the stars to have a very slight "triangular" appearance. Obviously, a field flattener is needed with this scope (as I was told before I bought it).. and that is already on a Fedex truck scheduled for delivery tomorrow.
It's not the prettiest picture of M33, but I'm happy with the performance of the gear. When I said "first light", I meant it -- the camera was modded last week, the scope was new two weeks ago, and I repaired the slop in the mount whenever I started this thread, so this data was captured during the very first imaging session.
Fyi, the total weight of the imaging rig was 34 lbs (scope, guide scope, cameras, plates, adapters).... 54 if you count the big, fat cat that slept curled up on the accessory tray during most of the evening). Perhaps the most impressive part is that said cat failed to ruin more frames while getting on and off the accessory tray (he has to make frequent patrols of the neighborhood for stray, tasty critters... not unlike many of the denizens of these fora, I'd guess.)
Posted 25 January 2011 - 12:59 PM
Your posts give me some confidence in the CGE Pro. I'd all but given up the idea of using a Pro mount for my long refractor (133 mm, f/12). It isn't very heavy, perhaps 40 lbs for the OTA, but it is over 6 ft long!
Most of the people I've talked to have told me that a CGE-Pro wouldn't hold my long scope without a lot of play and poor tracking - what would you think?
BTW, I currently use it on a CGEM, but it is way undermounted! The play and tracking is terrible, barely acceptable for visual work, completely unacceptable for photographic work.
Posted 25 January 2011 - 02:47 PM
Below is center crop of a single 10 minute exposure of M74 through a C14 at its native focal length of F/11 (f=3910mm) with an unmodded DSLR (reduce to 25% of original for posting). The total weight of the rig on the mount at the time was 59 lbs. This was done prior to when I fixed the "play" in the declination axis and was guided via a 50mm guidescope.
At this focal length, the differential flexure of the guide scope and collimation of the imaging scope become critical. I think the C14 was pretty well collimated. This picture was taken during the same session at f=8850mm (using a planetary camera), so I think the collimation was pretty good (so was the seeing).
Given that the weight of your refractor will be at the ends of the long tube and not centered over the mounting plate like with the C14, it's hard to predict, but the weight of the tube is probably fairly substantial relative to the weight of the objective, so the 70-100lb class of mounts should work pretty well.
My only advice is to try the scopes out on a few mounts and see what meets your needs. I used to own a Losmandy titan and (flame me if you must, purists) it doesn't hold a candle to the CGE Pro. It's more expensive, has the same weight capacity, and was far FAR more mechanically unreliable. I'll take the CGE Pro over that anytime (especially thanks to the Celestron software and alignment routine).
On the upside for the Titan, I used to run a 16" f/5 newt on it with no problem. That tube was 7 ft+ long, but most of the weight was in the rear cells, so it's another vague comparison. But things have changed since the day when the G8's and G11's were "king" -- back then, there was no internet that allowed that people with problems to shout-over everyone else... and to the contrary, there were no real mechanism for people to learn just how many problems those mounts had because they only really spoke to others at the occasional star parties. The dealers (gasp!) were rarely reliable sources of information and (to their credit) really didn't have many options to recommend.
Last thing... with a 130mm F/12 scope, what is your objective? It's going to have a very narrow, dim field for DSO imaging. If it's for planetary visual use, it will be great.. but for planetary imaging it's a pretty small scope. I'd hate to recommend a mount that won't meet your needs or suggest you spend a fortune for features you don't need.
I'm not sure how much that helps, but I hope there's something in all that that sends you in a more clear direction
Posted 25 January 2011 - 04:39 PM
Posted 25 January 2011 - 08:25 PM
The problem I was having with the declination play resulted in erratic tracking -- it worked well if I slightly overbalanced the mount and aggressively tracked in declination. It was very, very sensitive, though. Considering it was guiding at anywhere from 2500 to 4000 mm (sometimes I used the F/6.3 reducer).
Given the types of targets that work well with a C14, long exposures weren't really a problem for me since the skyfog would saturate the chip after 8-10 minutes (with an LP filter in an orange/red zone). You can't really go after extended objects with a C14 anyway.. and planetaries, small galaxies and globs work well with lots of short of exposures, rather than a few very long ones. Also, with the air and satellite traffic here, you lose a lot of frames to streaks... keeping them short helps
Here's a link to 60 x 3 minute exposures of M27 -- a better example of how well it worked at times and on targets more appropriate to the scope...
So, to answer your question, I don't think that fixing the play made the mount track "better"... but it certainly feels more "solid".