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Losmandy GM-8 Tracking Speed and Other

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#1 Ranger Tim

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Posted 27 February 2012 - 09:38 AM

Recently procured a well-used GM-8 w/ Gemini 1 and went out last night after extensive tuning and maintenance. It seems to run too fast (or slow, I can't remember which - left button brings it back to center). Can loading the mount on one side heavier cure this? I had it balanced fairly well. Stars tracked off the field of view in a 9mm eyepiece in less than 5 minutes. There was no difference between King or sidereal.

After cold start my initial 2 bright star alignment go-to's are fairly close (first synch and next additional) but the third one was 15-20 degrees off in RA consistently. I had already drift aligned and I know my bright stars. Examples include Dubhe - Mizar - then Denebola, resulting in gross inaccuracy on the third. I used a reticle eyepiece. Any ideas?

PHD guiding was a mess. Pretty wild swings that are indicative of overcorrection but no settings seemed to work. Calibration was barely successful and it took too long. The inaccurate tracking rate seemed to show itself during this period.

Pretty discouraged. Spent my wad on a used mount - again. Losmandy sure makes a well-made mount but I probably should have bought a new Atlas instead. Oh well, another two years of saving ahead of me - again. Looks like I'll miss galaxy season - again...

Yeah, I'm on the Yahoo forums. They don't have the turnaround time that CN's has. The comparison is like glacial compared to speedy :grin:

#2 jaddbd

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Posted 27 February 2012 - 12:17 PM

I have gemini 2 not gemini 1 - however the same concepts should apply...

Check the gemini set up for mount type. If it is set for G11, I believe it will run too fast (2 times too fast I think ???). Check the gear ratios also if that is an option.

Also, changing the balance will not change the tracking speed.

John D
Maryland

#3 D_talley

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Posted 27 February 2012 - 12:20 PM

I also would say that the mount is not set right in the controller. It must say G-8 during the boot up. Everything else will be off if not correctly setup.

#4 Ranger Tim

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Posted 27 February 2012 - 04:05 PM

It says G-8. Could the motors be wrong? Wouldn't some previous owner notice something like this????

#5 MikeCatfin

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Posted 27 February 2012 - 04:13 PM

Another thing to check in the setup is to make sure the tracking speed is set to sideral and not terrestrial. I noticed this on my mount and it took me a while to figure out why it kept drifting. (I didn't even know it had a terrestrial speed)

#6 Ranger Tim

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Posted 27 February 2012 - 06:10 PM

Yeah, lots of menu items. I've got it set correctly. The RA motor sounds stronger/faster in one direction (going west) than the other. I'm beginning to wonder whether I need a new motor.

I set the CWD position at the cold start and it will not return to the same spot. It is off by about the same amount as the go-to's are. There's something rotten in Denmark. Does the mount count the turns of the worm or the duration of the motor movement? Is there another trick I don't know about getting this thing to operate smoothly?

I'd be imaging right now if I had bought a new Atlas. :flame:

#7 kbastro

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Posted 27 February 2012 - 07:17 PM

Hey Tim

I just purchased a used Gm8 too...

after the intial run I too found that the phd graph was all over the place and the unit would not go back to the cwd (though it was close) position after slewing it around..

Having some good knowledge on Losmandy's mounts I took apart my g8 and traced my problem to a stiff gear box,,,,
When the gearbox was removed I could easily spin the wormgear against the pinion, then I spun the gearbox and found it stiff to turn so I replaced it and after that my G8 performed surperbly!

One other thing you must check,,, is that the needle bearings that the RA and dec shaft move on inside the mount turn freely!!! If they are seized then you need to free them by using some type of thinner, varsol, lock de-icer and once they are free (all 4 of them 2 per shaft) then re-lube them with slick 50 or duralube.

I doubt that the motor is to blame but the gearbox I found out is easier to fall apart... remember that the gearbox MUST spin very easy and then make sure that the wormgear is not to tight against the pinion..

kev

#8 D_talley

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Posted 27 February 2012 - 08:40 PM

Sorry you are having problems. This weekend we can get together and do a side by side compare of my GM-8 and yours to see what the problem is.

#9 blueman

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Posted 27 February 2012 - 10:08 PM

Do not forget cables, they can be making poor contact. Also, do you have RA and DEC plugged into the correct socket?
Then, power supply, what at you using and what voltage are you using?
Balance, and worm lash too, everything needs to be checked
Remember, you bought a used mount, anything could have been done to it, or problems from shipping if it was shipped.
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#10 Tonk

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Posted 28 February 2012 - 08:15 AM

I didn't even know it had a terrestrial speed


Just to be sure that you understand - terrestrial "speed" is the motors OFF (so you can look at the church clock etc)

@ Tim

I'm a long time GM-8/Gemini owner and can say the when working its the best mount of its capacity going. I can routinely get < 1 arc second PA with the PAC tool and unguided tracking can be stable for 8 minutes at FL 480 - but 5 minutes is more routine.

It sounds like you need a strip down of the mount to check/clean the bearings, worms and gear boxes. I've had my mount 9 years now and have gone through the clean up (degrease, re-lubing) twice in that time to keep performance up.

The Gemini can do odd things at times but thats only when aligning using particular stars (posibly the star position in the database is off). I would check what version of the firmware you have. I've had the programmable ROM upgraded once (to Level 4, V1.04) - the most current version is Level 4, V1.05 but is only needed if using the Titan, otherwise Level 4, V1.04 is fine.

Stuff about Gemini I firmware is here - http://www.docgoerlich.de/Gemini.html

Does the mount count the turns of the worm or the duration of the motor movement?


It counts via an optical device within the back of the servo motor (a precision wheel with transparant optical slots).

Thats another check you should make - is the Oldham coupler between the motors gears and the worm sufficiently engaged. You have to take the covers off the worm block to check this. In all my time of owning a GM-8 the most frequent issue is a Oldham "decoupling" which causes the scope to mispoint. You normally get an indicator thats somethings up with the coupler when the motor pitch changes! then its out with the hexnut Allen keys :(

#11 Ranger Tim

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Posted 28 February 2012 - 09:30 AM

Thanks Tonk. I've got the same version software so that's probably not it. This problem seems to be encoder/motor related and here's why I think that.

After playing last night I found that the RA tracking to the west is still too fast. The motor speed in that direction is also at least a third faster during slews. Each successive park at CWD or Home is progressively farther west until it becomes dangerous. Aligning stars in a strict west to east sequence works - east to west does not. Any time the RA needs to perform a go-to to the west (tracking direction) it is grossly overshot by 20 degrees or more.

I am going to switch the Dec and RA motors to see if this solves the problem. I can actually use the mount with only the RA working - having clutches can be a good thing ;)

The real question is this; 1) are there any replacement parts available anywhere for a first generation Gemini rig, 2) can I get these parts for a reasonable price that will beat buying a brand new Gemini II (ain't going to happen, CFO will kill that one), and 3) can I take apart the motor and fix the electronic problem?

I'm pretty good at fixing stuff or finding local experts that can.

Oh, by the way, did I say that I've been through the entire Gemini menu and can now run the electronics without the manual, AND have broken down the ENTIRE mount and put it back together with Super Lube, etc. I have checked the hemisphere operation and run the mount off of a 120v inverter and straight off a battery pumping out lots of good 12+v power. The only thing left is electronic repair/replacement.

#12 D_talley

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Posted 28 February 2012 - 10:12 AM

I have two extra motors and can let you use my GM-8 Gemini controller to see if there is a physical problem with the mount. If the mount works with my stuff, we then have to focus on the electronics.

#13 Ranger Tim

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Posted 28 February 2012 - 02:59 PM

Thanks Dwight! I'm getting ready to go out and try the RA with the other motor. If that fails then I will switch the gearboxes. That should rule out the controller or vice-versa. I'm gonna lick this thing and put the mount into tip-top operating condition. I do have a complete set of the digital controller (non go-to) and motors to try also, so I can sell it as originally configured if necessary.

Would really like for this rig to work though. I need this mount. The Sphinx is so random for Astrophotography that I can't bear to use it for imaging anymore. Love it for visual, hate it for AP. Not enough weight capability either.

#14 Billydee

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Posted 28 February 2012 - 03:35 PM

Tim,

Check with Losmandy on replacement parts for your GM-8. I think they test and repair the encoder pc boards. It would probably be easier to just send them to Losmandy for a test.

Luck, Bill

#15 Ranger Tim

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Posted 28 February 2012 - 07:08 PM

It is the controller (brain). I tried the motor switch and no love. I tried switching worms. The only thing I didn't switch was the gear boxes - don't think that's the problem. The switched motor sounds the same as the one I replaced. Behavior is identical. Nice to have a moon out during the day to test with!

I have the original Digital Drive controller with motors. They are going on next. At least I'll have access to the mount for imaging stuff I can find. I'll miss the SkySafari Pro go-to functionality. It will also rule out any mechanical weirdness in the mount. I have a feeling I'll be begging at the altar of the Losmandy gods before this is over.

Thanks to those who chimed in with encouragement and help. This forum is an integral part of my life now. Part of my astronomy habit/disease.

#16 Ranger Tim

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Posted 28 February 2012 - 10:00 PM

Tonight's update:

Installed the digital drive 492 or whatever number. No wonder everyone loves this set-up. Ultra responsive and becomes second nature in minutes. Drift aligning is a dream with it.

Initial tests showed EXACT SAME drift in RA. WHAAAA? Noticed for the first time that the hand paddle was giving me some fits trying to counter the drift. BINGO! One of the buttons must be sticking. And yes, I do have another paddle to try, an older Celestron model made by Losmandy. IT WORKS!!! Dead on tracking and finally can check out the PE on this thing.

Well, over the last few days I've had it apart only, say fifteen times, so the RA worm is tuned pretty good. Not much wobble at all! I'm going to put it through auto guiding with PHD so I can get a look at the graph. Then I'm going to tear it down and re-install the Gemini to see if this was the problem all along... Keep your fingers crossed!

#17 Ant78

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Posted 29 February 2012 - 02:02 PM

Glad you managed to weasel the gremlins out of it! a sticky button...oh man...ss there should be a PEC button on the digital drive board where you can correct for eight minutes allowing the drive to learn any error in the worm/gear.

#18 Billydee

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Posted 29 February 2012 - 02:46 PM

Tim,

Hand paddles can be fixed and/or replaced from Losmandy stock if necessary. There are only a few items that Losmandy can't fix or replace on your early Gemini I (I think only one or two chips are no longer available and out of production and supply). Hang in there.

Luck, Bill

#19 blueman

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Posted 29 February 2012 - 05:35 PM

There are still chips in stock and some people have found another supply of some chips thought to be hard to find, check the Yahoo Gemini Group.

The only thing that is for sure not available is the PC Board.
Blueman

Tim,

Hand paddles can be fixed and/or replaced from Losmandy stock if necessary. There are only a few items that Losmandy can't fix or replace on your early Gemini I (I think only one or two chips are no longer available and out of production and supply). Hang in there.

Luck, Bill



#20 JoseBorrero

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Posted 08 August 2012 - 12:07 AM

I know may be it too late here, but I will tell that the exact thing happened to me tonight. the RA motors was way fast. I found the problem, it result that the gemini I was reset to default, the parameters was for the titan. that was the reason that the RA was running fast. All is fine now :)

#21 gnowellsct

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Posted 08 August 2012 - 11:40 AM

And swap the motors dec to ra and vice versa to see if the problem persists. If it doesn't that indicates the motor is at fault. GN

#22 RAKing

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Posted 09 August 2012 - 07:24 AM

Hey Tim

I just purchased a used Gm8 too...

after the intial run I too found that the phd graph was all over the place and the unit would not go back to the cwd (though it was close) position after slewing it around..

Having some good knowledge on Losmandy's mounts I took apart my g8 and traced my problem to a stiff gear box,,,,
When the gearbox was removed I could easily spin the wormgear against the pinion, then I spun the gearbox and found it stiff to turn so I replaced it and after that my G8 performed surperbly!

One other thing you must check,,, is that the needle bearings that the RA and dec shaft move on inside the mount turn freely!!! If they are seized then you need to free them by using some type of thinner, varsol, lock de-icer and once they are free (all 4 of them 2 per shaft) then re-lube them with slick 50 or duralube.

I doubt that the motor is to blame but the gearbox I found out is easier to fall apart... remember that the gearbox MUST spin very easy and then make sure that the wormgear is not to tight against the pinion..

kev


This is great advice. Thanks!

I also have a nice, but older, GM-8 and I had to change the servo motors on mine. The RJ45 plugs seem to lose connectivity after a while and I got a lot of "RA motor lag" messages, so I have changed to servo motors with DIN plugs and that fixed my problem.

After following this thread, I tore my unit down and checked the needle bearings, worm gears, and gearboxes, too. All of these check out fine and I hope I can keep my little mount running for a long time.

Cheers,

Ron

#23 JoseBorrero

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Posted 12 August 2012 - 07:54 PM

the most important detail about Speedy RA was a setting called advance gemini settings----> mount parameters, visible only thru ASCOM Gemini drivers. First retrieve data from gemini, all parameters there has to be 0. All this settings can disrupt the tracking as this settings are added when additional align has been done several time. In order to proper function all this settings has to be reset to 0.






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