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CGEM DX - binding, now slop in DEC axis

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#1 DaemonGPF

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Posted 26 June 2013 - 10:32 AM

I finally got around to setting up my CGEM DX. I was playing with the mount inside before taking it out and found that I had a serious binding issue in the DEC axis. The mount would slew just fine until it was about 15-20 degrees off center to either side then it would begin to slow rapidly and stutter. The motor sounded as if it was struggling to push some insane weight. With and without a payload. So I tinkered with the backlash adjusters in front and back of the DEC housing and problem gone. However, now I've noticed there's a bit of play in there. It's very slight, but after taking a first light test drive last night I can see how it was throwing off the go-to and alignment crossing over the meridian. The axis and consequently, OTA would shift. Through the EP the motion was significant enough to take a centered object and move it just out of FOV on my C8 with a 25mm EP.

So, did I simply not notice the slop when I bought it? Or did I possibly introduce this messing with the backlash screws? I only made slight adjustments, maybe 1/2 turn to loosen one and tighten the other.

At this point I'm wondering if maybe it's not meshed fully and I need to open it and make an adjustment or two? The slewing last night was fantastic and smooth, until you hop over the merdian, then the OTA shifts, and it's back to running smoothly.

Since I'm likely to crack this bad boy open anyway, should I just proceed with a Hypertune as well? It's a used mount.

Suggestions? Something I'm overlooking? I've owned a regular CGEM before, and a CG-5, and I didn't have DEC axis play in either.

#2 jrcrilly

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Posted 26 June 2013 - 10:40 AM

Rotate the dec axis, checking the slop as you go. When you fimd the spot with the least slop, adjust the gears for no backlash there. That will result in some slop at the looser locations, but that's what it is. If it is adjusted for no backlash anywhere else, it will bind at that tightest spot.

Half a turn is a big adjustment. You'll want to use finer increments.

#3 orlyandico

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Posted 26 June 2013 - 10:49 AM

DEC slop is inevitable.

If you tighten down the DEC worm too much, you will get binding, noisier slews, and the risk of burning out the motor.

#4 DaemonGPF

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Posted 26 June 2013 - 12:03 PM

Thanks guys for the input.

John, I suspect you're right that I may have overcompensated in the adjustments.

It seems like an abnormal amount of slop. There was a tiny tiny amount in my other GEMs, but nothing quite like this.

Just so that I am clear, it is "push-pull" with the two adjustment screws, correct?

#5 orlyandico

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Posted 26 June 2013 - 09:59 PM

it's push with the center screw. The outer screws are pull and tighten the worm against the wheel. What I normally do is tighten the outer screws until I can't turn the worm spur gear by finger (this requires opening the mount and removing the motor). then i tighten the center screw to relieve the pressure until the spur gear can be turned comfortably by finger.

if you can't turn it comfortably with your finger tip, the motor won't be able to either.

#6 EFT

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Posted 27 June 2013 - 07:55 PM

The best mesh of the worm and ring gear come when the gears are properly centered on each other which is not the case from the factory. The only way to properly center them is to disassemble the mount and take measurements to determine the proper thickness that needs to be used for the shims.

However, in most cases with these mounts, eliminating all backlash is not possible. The gears are simply not of high enough precision to allow for a tighter mesh. The best you can generally do is minimize the backlash and that is easiest to do by making the adjustment with the axis slewing. That way you can hear the gears start to bind and back off from that point slightly.

#7 DaemonGPF

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Posted 29 June 2013 - 09:01 PM

I had first light with it the other night. Spent most of my time tweaking settings, messing with autoguiding parameters. After dialing in the software and backlash on the hand controller it was guiding perfectly in RA - very smooth in RA actually. Every so often I'd see a dramatic "skip" in DEC, however. Fairly certain I'm still seeing a mechanical issue here, perhaps over adjusted the mechanical backlash screws. Unfortunately the weather is bad for the next several days so I can't go back at it to make further field adjustments.

Ed,

I was thinking of tearing it down anyway, but if I'm going to break down the mount and make adjustments and clean it up, I might as well Hypertune it also and get everything done in one shot. I just spent a ton on new gear though, gotta see if I can squeeze a few more bucks past the "CFO".

#8 DaemonGPF

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Posted 29 June 2013 - 10:39 PM

Hmmmm... check this out:

https://www.youtube....h?v=JI4BEli149o

Something extra to keep an eye out for I suppose while I'm inside there. I saw several other videos out there from various people showing the EXACT same issue with play in the gear.

#9 cherokawa

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Posted 30 June 2013 - 12:53 AM

Josh,
At least for CGEM (not sure about DX), I found that Dec balance was very critical for proper guiding performance. I had to balance on all 3 axes and what made this difficult was that the standard saddle was heavier on one side than the other - and I'm not talking about just the weight of the knobs. Achieving good Dec balance turned out to be much easier with an ADM saddle than the stock saddle. Perhaps as a previous owner of a CGEM you already know all this, but just thought I'd throw this in as one of the things to check. :)

#10 DaemonGPF

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Posted 30 June 2013 - 09:31 AM

I noticed on my prior CGEM, the DEC liked to have a bias to the rear of the plate, both East and West. I don't know why specifically, but that was how I managed accurate tracking with it. With the DX having regulated power and a few other features, it is supposed to be much more tolerant to imbalanced payloads, but I suppose I better test that theory more closely.

Ironically, Anthony from ADM is where I got the OTA on this particular setup. I planned to follow up with him shortly on a few hardware bits, but have to let the dust settle some after my recent spending spree building up two separate rigs.

#11 DaemonGPF

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Posted 30 June 2013 - 04:00 PM

Further digging - there is no play in the spur gear on the DEC motor itself. I took a look under the side access while inducing the play. No motion in the spur gear while I was moving the saddle. The play seems to be isolated to the worm gear assembly. Perhaps the spur gear on the end of the worm may have play in it? Perhaps isn't meshed fully? I don't have the tools with me to remove the retainer rings to really get inside and validate. Looks like a trip to the hardware store.

However, I did find something else interesting... I removed the saddle plate temporarily to get it out of the way while I was trying to diagnose. The retainer ring on the top of the DEC shaft had quite a bit of marring above one of the ring holes.. This mount was bought used, and had a trip to Celestron before me. It was to address the cogging issue. I don't know if they swapped in a new mount all together, or went in and did some work, but whoever handled the assembly/disassembly was seemingly careless. It's just boggling how near perfect this mount is in RA to have this much slop in DEC.

#12 DaemonGPF

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Posted 01 July 2013 - 04:19 PM

Taking the plunge. Going to rebuild/overhaul the mount. Talked to Ed a bit on the phone today and a few other people. I'm thinking I can rectify a number of issues I see in here. When I get the Hypertune bits, I'll try to document and photograph/video the before and after.






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