G11 with gemini need help

[Striker]

Guys I need someone with experience with both the mount and gemini.

Everything is working well and accuracy is very good but PEC is really giving me the *****.

Not long ago I was getting up to 5 minute unguided exposure...now I am lucky to get 20seconds before I get severe star trails...I have done my drift align and thats fine....all my problem's are pec related...the stars are just jumping up and down.

So easy fix you say....just train the PEC..yep so I do the PEC training.....its says pec is active....nothing changes.....my pec is terrible.....for example a bright star is moving from side to side about the width of 5 stars in less then 1 minute....so you could imaging what the stars look like in just 30seconds.

Am I doing something wrong...this is frustrating me ATM but everything else is working perfectly.

Any help is very much appreciated.

[acropolite]

I don't know anything about the G11, only seen one in the flesh once before. I seem to remember [1ponders] saying something about a similar problem with the Mapleton G11. Maybe something's become loose inside, that would account for your initial good performance.

[rogerg]

Hmm, not sure if this will be of any help, but...

My GM8, I have had some problems with it's DEC. For all of the problems, I've asked the Yahoo! Losmandy group about it. I always get the same answer: Loosen off the adjustment screws enought o move the worm a little, re-seat it and tighten it up again, ie, get it back in to a snug fit.

I've found this fixes my problems sometimes and not others. Right now I have a problem where I can't remove backlash. For some reason no matter what TVC value I enter, it appears to have no effect. Strange. Again I've been given the same advice, but haven't tried it yet.

Roger.

[[1ponders]]

Tony I agree with Roger, however there may be another thing to look at. How did you go about training your PPEC? If you used a camera to autoguide then it might be worthwhile checking the end play in your worm blocks.

Oh and is the problem just in one axis or in both and if only one - which one? Bet it's Dec or RA only.

Also if you're autoguiding to train your PEC what guiderate are you using what camera and other equipment/programs. What star were you using/how near to the zenith, what were your seeing condition like when training? How's the balance on your scope when training and do you rebalance when putting on the camera etc?

Try this. Grab your OTA and try to rock it in RA and then in Dec. Not actually move it in Dec and RA, but just rock to see if there is any play or free movement in either axis.

[JohnG]

Hi Striker

A couple of things, how old is your G-11, if it is new you should have the High Precision Worm which was introduced a couple of years ago, how is your balance, it is critical with Losmandy mounts, as mentioned in a previous post, is your worm adjusted properly, I can send you articles on how to adjust it properly using feeler guages. If you feel it is definately PEC then it will either be balance or worm adjustment. You have to get the worm tight enough so it dosn't bind, you check this by moving it by hand for a full rotation, you will feel the tight spots, you have to balance the tightening with the backlash.

With your Gemini, what version are you running, there is a bug in the current Level 3, V1.13 concerning training PEC, it is well documented on the Gemini Users Group. Most users have given up trying to train PEC and are now waiting for the release of Level 4 which is now in advanced beta, the problem of PEC has, apparently been addresseed together with a number of other bugs.

If you aren't a member of the Yahoo Losmandy and Gemini Users Groups, I would join, the archives of those groups have a lot of info.

Hope this helps a little

JohnG

[JohnG]

Stiker

One thing I forgot to mention, if your mount is an older one, there was a problem with the grease Losmandy was using, it apparently reacted with the grease in the bearing and became a gluey mess, I pulled my GM-8 apart last year and found the bearings were sticky, cleaned them up and relubed using a Silicon based grease and they are now smooth.

Something else to tink about.

JohnG

[Striker]

I do have backlash that needs adjusting on the Dec but the RA has no slop movement or backlash it's perfect....and thats where the PEC needs adjusting...I am not getting much PEC on the dec axis.....thats fine....all this has to do with the RA axis.

I did pec training manualy but with the use of the toucam so I was visualy looking at it on the monitor instead of constinently looking through a iluminated reticle eyepiece.

Pec training was easy...I could see I needed to make large corrections on just the 1 axis but this was no problem..I had the star centered very well for the entire 4 mins.....I even did the PEC training 3 times in a row again.

The star I was using was around 10-20 degreez of the meridian.

I have balanced the mount.

Here is diagram I just made up on what is happening regarding star movement on the RA axis...this is approx 2800mm focal length with a toucam.

[Striker]

Thanks John...the G11 is new....around 5 months old..purchased new.

Any info on how to adjust the worms is much appreciated...I will try to read up on it aswell.

Just joined the group..will have a browse before I start asking questions.

Thanks.

[[1ponders]]

So what sort of movement is that Tony in arcseconds?

[JohnG]

Tony

I get the impression by looking at the diagram on your post that the worm is moving from side to side, you say you have backlash, it looks to me like your worm is moving in the worm blocks ie side to side, this very easily adjusted, that would definately account for you star moving only in RA, also have you let the autoguider do it's own thing and see what happens, gentle reminder, make sure the Gemini is in Photo mode.

JohnG

[Striker]

hahaha arcseconds.....speak my language....ask me in mm's.

Sorry Paul....I'm not that educated in those terms.

At the focal length of 2800mm I am getting star trails 3 wide on the RA axis within 30 seconds...if this means anything.

[Striker]

John I dont have any backlash in the RA..only Dec which is not causing me any problems at the moment.

[JohnG]

Ummm interesting, are you sure it was not seeing, you are photographing at a long focal length, just thinking out loud.

JohnG

[Striker]

3 months ago I took a 7 minute unguided image at this focal length with no star trails....I have done something wrong but just trying to work it out.

[JohnG]

Tony

There is one other thing I can think of that might be causing the problem, take off the worm cover and have a look at the Oldham Couplers, there is a known problem of them becoming loose on the shaft and causing similar to what you are seeing, use an allen key and tighten the small grub screws, 4 of them from memory. While the cover is off, have a look at your worm and see if it is moving in the worm blocks.

JohnG

[Striker]

Thanks John....I will have a look.

I am just very cautious when asked to open something up that is worth so much money that I have no idea how it works.

What info have you got regarding back lash or worm gear adjustment anyway.

[JohnG]

OK, taking the cover off is no problem, just undo the 2 small grub screws and lift the cover off, inside you will see the worm, the Oldham Coupling is to the left ( I am running off memory here, I don't have my mount in front of me), you will see the coupler which is a white nylon thingy, either side of the coupler you will see the shaft with the metal part of the coupler, there are grub screws that hold those parts to the shaft, check that they are tight. With the worm, just rock the mount from side to side and see if it moves within the blocks you will see on either end.

PM with your email address and I will send you what I have, including articles with photographs. I have found that you have got to be prepared to play with Losmandy mounts to get the best out of them.

JohnG

[JohnG]

Tony

This might help you identify the parts

JohnG

[Striker]

Thanks John...I downloaded a PDF of the Yahoo group on how to adjust...I am trying to work out what those Oldham couplings do....their purpose.

[JohnG]

They are meant to take up any mis-aliagnment between the motors and the worm shaft ie if the motor is slightly off centre, it would try the bend the shaft if it was a straight through fitting, the Oldham Couple allows a slight sliipage to counter the bending effect, I think I got that correct, someone else might be able to explain it better.

JohnG

[JohnG]

Ahhhh, my spelling went out the door.

JohnG

[Striker]

I just played around with the gears.....I did adjust the Dec a little so maybe I have fixed the backlash but I didn't see too much wrong with the Ra axis...anyway I loosened them off and retightened everything...so I will wait till I test next clear night.

It's a simple but affective gearing method on the G11....I can see that being a much improved system over the conventional geared mounts.

Thanks for your help John...I will let you know how I go.

[[1ponders]]

Tony after you retightend the gears were you still able to turn the worm gear by hand?

[JohnG]

Tony

A couple of important things, did you do a complete rotation by hand as mentioned in Paul's post, it is critical that you do that so you know you have the worm adjusted correctly, you will feel the highs and lows when you do that. Were the Oldham Coupler grub screws loose?.

When you run the system, make sure you keep an eye on your motors, the Gemini servo's will heat up quickly if the worm is too tight, if they are getting warm to touch there are 2 possibilities, 1. the worm is too tight, you will get a Motor Stalled or Tracking Stopped messages on the Gemini HC, turn the unit off immediately or you will fry the motors. 2. Slow your slew speed down, I think the default is 1200, drop it down to 800 and see what happens. This is where the balance between gearmesh and backlash comes into play, you have to balance each out, it takes a little time and practice but it is worth it in the end.

JohnG

[Striker]

No I didn't turn it by hand.....to do that I imagine you would have to pull the motors off and the holders.

I have already dropped the max slew speed down to 600....I dont need any quicker then that.....I usualy only work on 1 target a night so fast slew is not needed.

[[1ponders]]

Probably a good idea to take the servo motors off and try turning it by hand. Without doing that you can't know whether you have meshed the gears tight enough or too tight. Nor can you find where the "tight" spots on the gears are. I'ts not a big job and can make a big difference.

[JohnG]

Paul
Unfortunately taking the motors (servo's) off a Gemini equiped mount is a little more complicated than the standard digital drive, there are a number of small parts that are fragile and could quite easily break.

Tony
Whilst what I am going to suggest is not ideal it should give you an idea if the worm is binding or not. Turn the Gemini on (you can do this indoors or whenever), counterweight down, do a warm restart, disregard the aliagnment procedure, press the directional buttons and let the mount slew to it's east and west limits, you should hear a warning squeal when you are near the limits, run it back and forth a few times and listen to the servo pitch, if the worm is binding you will hear the servo start to labour and the tone change, do the same in DEC as well, you should be able to go nearly all the way round in DEC, in RA this is not ideal but it will cover 180 degrees of the main gear, you are limited there by the built in limiters. The best way of course is to remove the servo's, that is something you have got to decide.

JohnG

[[1ponders]]

Whoops, sorry Tony :P

John, for the other 180 deg of the RA would it work if Tony were to do as you suggested for a few swings and finishing at the western extreme end of the RA movement (assuming he's pointing at the SCP), then loosen the RA clutch and push the scope in RA to the opposite end and redo the swings. Would that enable him to cover all the worm and gears?

[JohnG]

Yes, it should, your not doing any sort of aliagnment anyway, you can do this indoors, the only thing is that Tony is going to have to do is a Cold Start, any aliagnment model that has been built will be lost, it would also be a good time to check the east and west limits as the default limits are quite conservative.

Good thinking

JohnG

[[1ponders]]

:lol: Don't worry John. Tony has had plenty of practice of remodelling after cold starts, hey Tony :face:

[JohnG]

Same here, I do them in my sleep, fact of life with a Gemini.
At least I got my A and E numbers down to 2 and 0 now. I don't want to tear down the GM-8 but will have to soon for the G-11, ah well back to cold starts again.

JohnG

[RAJAH235]

FWIW, speaking from a strictly mechanical pov, the so called "Oldham Coupler" is a 'self aligning universal joint'... ( in other words, it moves in both axes).
We used a similar coupling in our radio controlled IC power boats, between the motor & prop shaft. Bit more substantial of course.. HTH. :D L.

[JohnG]

There you go, I knew someone would describe it a bit better.
Thanks for that.

JohnG

[Striker]

Thanks both John and Paul.....Yes I have no trouble cold starting...I have done it approx 20 times now.
I would rather get all this sorted out now and then I would have to do the cold start and remodeling anyway....that does not take long

The idea is to have everything set up and working properly then do all your modelling.....sounds logical.

I will do what you said and slew over 180 degrees then loosen the clutch and cover the other 180degrees..that sounds simple....

I have a fealing that the Dec axis which I tightened may be a bit tight as it has a higher pitch to that of the RA axis....will test and adjust today.

Thanks guys.

[JohnG]

Hey Tony

Don't forget to let us know how it went, might take a couple of goes but it will be worth it.

JohnG

[Striker]

John.

I did take the motors out and adjusting so I could easily turn the worm by hand with no obvious sticky points on both axis's.

Had an opertunity early last night before the showers come over just to test the mount after the adjustment.

I am pleased to say that I now do not have any backlash what so ever on either axis.....but I still have reasonably bad Pec on the RA axis...not as bad but still considerable....I gave the mount a good going slewing at high speeds with no motor lock ups and had no motor over heating issues..all looks good except for the pec......is it possible the small worm has a ever so slight bend in it.

[JohnG]

Hi Tony

OK, if you can turn the worm easily with your hand it is still probably not in tight enough, I don't know if you have tried this but get a set of feeler gauges and try the spacing the right hand worm block with various thickness gauges, you have to place the feeler gauge between the right hand block and the mount itself, most opinions I have read suggest you leave the left hand block in place and use your fingers to hold the right hand block in against the worm and mount face, have a look at that diagram I posted. Another way mentioned on the Losmandy Group is to use a small G cramp to hold the block. It is a matter of adjusting, correcting, you should be able to feel the highs and lows of the main gear, I doubt very much that the worm itself is bent, you should have the new high precission worm anyway. Unfortunately itis trial and error, you have to get balance right between backlash and gearmesh, once you do that your PEC should improve dramatically.
Something I use to turn the worm is a bit of 1/2 inch dowell with a groove cut in the end, makes it a whole lot easier on the fingers. Not such a hard thing to dismantle is it.

JohnG

[Striker]

I will keep playing with it....keep you posted.

Atleast I only need to play with the RA now as the Dec looks good with no backlash.

[JohnG]

Bout the only other thing I can think of, and I asume that you are doing this anyway, is to have the mount weighted so it is pushing against the worm ie slightly unbalanced.

JohnG

[[1ponders]]

Tony is there much endplay in the worm? Is there much movement left a right of the worm gear in the block?

Also could it just be a bad spot on the gear? Have you tried guiding on different sections of the gear (not the worm but the main gear itself)

Btw Tony what do you mean by "reasonably bad pec" How do you measure it?

[Striker]

Reasonably bad pec means taking in consideration that I am measuring Pec at 2800mm focal length with a toucam so i will expect some Pec plus seeing will have to added to the equation....but still not good enoug....lol

Yeah I tried on Orion and the tarantula...both very ordinery.

Should there be any movement regarding End play paul....I think their both tight.

[[1ponders]]

No you don't want to have any movement,within reason. It doesn't want to be so tight as to restrict worm rotation.

This is sometimes part of the problem when trying to use autoguiding to correct pe. When you're autoguider tries to make an adjustment or the error in pe "reverses" then instead of making an adjustment by the worm moving the gear, the gear acts as a pivot point and the worm moves backwards/forwards in its blocks. From the autoguiders frame of reference it has made an adjustment and nothing happened, so it has to make another adjustment. However depending on what is happening with your pe at that particular moment it might infact double the adjustment needed or still not be enough so the mount continues to drift or jumps too far, which requires another adjustment. Which again might not be the adjustment needed because the worm may move b/f again. But it might be too late by that time anyway, you have trails starting to appear in your image from the first mis-autoguidement :P. Either way over correcting or undercorrecting you still end up with trails and your autoguider spends most of the night fighting the endplay and not actually guiding. Does that make sense???:P:)

[[1ponders]]

It's pretty important you know how many arcseconds per pixel you are imaging at. Looking at the K3 graph isn't much use if you don't know that. If you look at the graph and it is swinging 5 pixels above and below the line if your resolution is 0.25 arcsec then the total swing is 5+5 = 10 pixels = 2.5 arcsec. Now while not perfect as far as pec is concerned it something your autoguider should have no problems dealing with. I've heard that many people are very happy if they only have a 5 arcsec pe. Me I'd be ecstatic.

But if your resolution is 2 arcsec then your pe will be 20 arcsec. So its important to know your resolution so you can work out your pe in arcsec to make a value judgement on the ability of your mount. Looking at the graph on its own really doesn't give you much info except that your mount isn't tracking perfectly. How imperfectly you don't know.

BTW have you set your telescope type/ F/ratio, ToUcam pixel size and chip size in K3?

[Striker]

Um...I dont use K3....lol

I use Guidedog.

Tested again tonight...no different....I even tried tightning the worms as much as physicaly possibly...besides the odd motor stall the pec was identical to when the worm was physicaly loose not even pushed up against the main gear.

I give up...all I know is the EQ6 has less pec then this mount.....yes the problem may be me...I accept that but it's all fairly basic to adjust once you get in there.

The amount of hours I have spent trying to get this mount just reasonable for imaging is rediculaous.....It's like having a dragcar where you spend most of your time under the bonnet instead of driving...

I think it's time to call a pro.. EDDIE

[JohnG]

Well Tony, you got me, everything I have told you to do worked fine on my mount and I have never had an ounce of trouble. I would not have tightened the worms that tight, you have to have a slight amount of clearance, the only thing left is the known bug in Gemini Level 3. I have pulled my mount apart probably 4 times now over the past 4 years, relubed, worm adjustments etc, no drama, very bizzare.

JohnG

[Striker]

When did you say your coming up to Brisbane to give my mount a service John....lol

I am going to test it tonight if weight is an issue.....taking the guidescope off.

Thanks again for your help John & Paul.

[[1ponders]]

Tony when you check the pe of your EQ6 what scope are you using and when doing the Losmandy what scope are you using?

[Striker]

both with the C11......I have not checked the PE on the EQ6 since I had it in my Observatory.....checking the PE on the EQ6 is not needed now considering I am imaging at 600mm...you wont even see it.

I just remember the star stayed in the same spot most of the time instead of jumping from side to side as it is doing with the G11 atm.

[JohnG]

Tony
With the C-11, the mirror hasn't started to flop a bit more has it.

JohnG

[Striker]

No John...I dont think any mirror flop would cause the image to go both ways like PE.

I think my C11 has very little mirror flop....I always use the SCT focus knob to focus and the image still stays central....and thats with a JMI electric focuser installed....I rarely use the JMI.

I have a few more option to play with tonight...let you know how I go.

[Merlin66]

With or without the guidescope, set the counterweight to give an out of balance condition so that the telescope would tend to "fall" to the East of the axis if the locks were not tightened, check the PE in this condition. Sounds like the gears/worm are OK but still may have a slight mis-match which could cause the perceived error. I'd also push the scope east then west and see if the PE reflects the "back pressure" Could be you just need some "pre-load" on the worms.
My 2cents worth.
NB haven't found this problem on a GM8 clone.

[Striker]

Thanks Merlin,

Thats the plan tonight....I have always made sure the weights is heavier on the east side but when I say heavier it is a very minimal difference...not enough I am hoping...I have always being so wrapped in getting perfect balance with the G11...maybe it was too good and thats working against me.

[Striker]

Ok...we are now heading in the right direction.

All I did was apply more weight to the eastern side as some have already mentioned including the Yahoo group.....I think I had the mount to well balanced from the start....The RA motor did stall on 2 occasions so I slowed down the slew speed and slightly adjusted the weight again...I may have a gone a little to far on the first attempt as the motor was getting a little warm.

I will still getting some PE but less then before...so I tried guiding through the C11 at FL2800 and all went well with some adjustments of figures in Guidedog but had no problem guiding for 10 minutes.

So I am happier then I was last night so thats a good thing.

Atleast this has given me an opportunity to get to know my mount.

Thanks guys...I am still not out of the woods until I do a test image but heading the right way.

Thanks.

[JohnG]

Great news Tony, sounds like you just need to loosen the worm a tad and you may have it nailed, good work, your an expert now. lol

JohnG