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Losmandy RA loosness

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#1 stargazerken

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Posted 08 June 2009 - 06:48 PM

For a lack of a better word, I used loosness. Just relubed the mount and I am readjusting the RA worm gear. How much play should the RA axis have and still obtain good tracking? Right now the scope has about 1 minute +/- (according to the RA vernier) play in it. I am trying to adjust the mount to permit decent tracking with an SBIG ST-7. Thanks!

#2 blueman

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Posted 08 June 2009 - 09:32 PM

You should have about 1-2mm movement at the end of the counter weight shaft.
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#3 rsbfoto

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Posted 09 June 2009 - 11:35 AM

For a lack of a better word, I used loosness. Just relubed the mount and I am readjusting the RA worm gear. How much play should the RA axis have and still obtain good tracking? Right now the scope has about 1 minute +/- (according to the RA vernier) play in it. I am trying to adjust the mount to permit decent tracking with an SBIG ST-7. Thanks!


Hi,

Is it a Gemini GoTo equipped mount ? Does it have Level4 Eprom ?

If yes, then this helps you in a simple way to adjust your RA backlash. Adjust the backlash consecutevily to a lower value until the Gemini tells you " RA Motor lags "

Do this for your full RA motion eg. 180 or wahtever amount of degress your mount is capable to slew from East to West or West to East. It is known that in some point of the full travel you will have the message " RA Motor lags ", maybe 2 points (one on the east limit and one on the west limit. If you got that feel yourself lucky as you will have a nice run from East to West.

You will find an adjustment where The gemini will not tell you " RA Motor lags " this is the best backlash adjustment you can achieve.

If you are a bit risky you can even leave the mount in some RA position to tell you " RA Motor lags " if afterwards you do not get the Blue scrren of death :grin: eg. Gemini tells you " RA motor stalled ". If you get oj a point " RA Motor stalled " then backoff a bit with your adjustment.

Also watch the temperature of your motor while doing this.

Anyhow if you do it during Summer, then in Winter your backlash will be a bit bigger, not smaller. This sounds like impossible but the Losmandy mounts have the property to have more backlash, with the same adjustment, in Winter rather then in Summer.

#4 stargazerken

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Posted 28 June 2009 - 09:05 PM

Soory for the delay, the mount is a non Gemini G-11. I adjusted the mount to obtain 1-2mm motion that Blueman noted. Unfortunaly the sky clouded over just before sunset and the bottom fell out. Now another problem cropped up-the dec motor has some issues so it will be replaced. The motor is 10 years old so I guess it is time.

#5 Kaizu

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Posted 29 June 2009 - 12:06 AM

I have a G11 with Gemini. I have adjusted it as Blueman said, couple millimeter gap at the end of counter weight shaft. There is two reasons for that. First, for imaging, the mount should be balanced little heavy to east and it is impossible to feel that if there is no gap. Second, if there is no gap on warm weather, the mount stals when it's cold. Also I'm afraid that the gear may get worn faster if there is no running clearance.
I have also tried to image when the mount has balanced equal or west heavy but the results have been poor, ca. 1/3
of exposures have failed because of sudden jumps.

Kaizu

#6 gnowellsct

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Posted 29 June 2009 - 12:29 AM

If you have a ten year old mount you are overdue for new motors. This is not because they are worn out. This is because the new steppers are so much better. So far as wearing out goes, your Hursts will probably last longer than you will (or me). Around 02 there was a change (in the stepper models) from Hurst to SAIA. The new SAIA motors are much smoother: quieter, and no vibration at high magnifications. The swap is easy.

Most people here are giving you the proper advise for a Gemini system. But you don't have that. You have tough as nails steppers. And they can't burn out. The power gets cut off if they stall. So the answer to your question is that you can adjust to zero backlash. Not tight, but get to zero backlash.

Now the fine point here is that at zero backlash your steppers may or may not be able to to power their way along in most conditions. The reason being that the worm wheel (the teeth on the mount) has very slight irregularities. If you are feeling really compulsive, the thing to do is turn your way through the full 360 degree circle and note where it is hardest and easiest to turn. When you got to a "hard spot" put some tape on the mount to indicate its location and keep turning till you get to an "easy spot," etc. What you want to do after you know where the hard spot or spots is/are, is go to a hard spot and adjust to zero backlash. If you do that you'll be OK for getting through the hard spots and you'll have slight backlash that will appear from time to time as the worm wheel turns into the "easy spots." That's the obsessive way to do it.

Or you can just set the thing up, adjust to zero backlash, and run it for a couple of days and make sure it doesn't stall. If you can crank at 15 degrees an hour you're golden, and you can check it before you go to bed and wehn you wake up, etc. You can even put a bungee cord on the counterweight shaft and set it up so that the steppers are chugging but the mount is slipping against the clutch.

Your steppers actually make for a fairly easy adjustment because you don't have to worry about burning them out. All this business about winter and summer settings etc., that's all servo speak. Servo motors die horrible deaths. I run my stepper G11 from -15F to 100F and never adjust the worm at all. That's the beauty of your steppers. If you want to get PE down get an Ovision worm. It will cost $500 but you'll have PE in the 5 arc second or better range (that would be peak to peak).

Anyhow the SAIA steppers are great. I suppose the frugal thing is to get only one for RA but heck I decided to treat the ol' gal right and giver her two brand new steppers.

Here she is:

Greg N G11 with Tak in winter

#7 stargazerken

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Posted 29 June 2009 - 10:34 PM

Greg, thanks for the advice! I will replace the steppers with the new ones shortly. I may have to do that one at a time starting with the RA motor. I noticed that Losmandy has a high precision worm on their parts list. How much better in that worm compared to the original? The Ovision worm may have to wait a little bit; but it has been added to my upgrade list. I don't have any plans to upgrade to the Gemini, my 1973 vintage goto system still works well (thats me!).

#8 Kaizu

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Posted 30 June 2009 - 05:04 AM

In my case, changing the worm gear wouldn't help because the PE was originally less than 5".
(My G11 is almost two years old.)

Kaizu

#9 hydrogia

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Posted 30 June 2009 - 02:12 PM

Great Article Greg!

I don't have a G11, but it's promising the difference in quality on can achieve with such a simple (albeit slightly expensive) upgrade. Better $500 than $4-5000

#10 gnowellsct

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Posted 30 June 2009 - 06:59 PM

There have been several reports on Yahoo! losmandy_users of out-of-the-box PE in the 5 arc second peak to peak range which is excellent. The general consensus is that the Losmandy "precision worms" which cost $50 are good but "ain't necessarily gonna get you there." The main design fault that has been identified in the mount is that that cube-shaped bearing blocks on either side of the worm are not maintained perpendicular to one another. The Ovision system solves this problem with a single block, and adds to the mix an extremely precisely machined worm gear which is apparently close to the limits of what contemporary machining can do.

Most of the mounts these days come in around 10 to 12 arc seconds peak to peak; the autoguiding crowd has been extremely focused on a 76 second spike which is difficult to impossible for an autoguider to compensate. This error is not PE "related to the worm." It is related to where the worm is in relation to the bearings in the bearing blocks. So it doesn't "match" with the worm four minute cycle and thus is not really something PE can compensate.

I had my stepper G11 out last night, I was using it with a C8 (of all things). It's a very nice setup and I wouldn't be surprised if people are still using them twenty years from now. They have much to recommend them: the electronics are almost invulnerable on the stepper version, and pretty durn good on the Gemini version.

If you do get the stepper version another "must" upgrade is the aluminum motor covers. These are essential to keeping the mount from bashing one of its motors. I've always thought that Losmandy should just STOP selling the stepper mounts without hard motor covers because you really are cruisin' for a bruisin' if you don't have them. That said, it is true, I just USE this mount, I sling it around, sometimes I wrap it, sometimes I don't, it is probably my most durable piece of equipment.

My view is that these mounts are among the best quality options out there "out of the box" and the results that people are getting by swapping in the Ovision makes it a very competitive option. There are lots of USED G11s, around $1700 stepper-ized, around $2600 with Gemini; if you put in the $500 Ovision worm you will get a smooth PE that is easy for the autoguider to correct. Ovision, however, only guarantees its worm to ten arc seconds peak to peak (results are usually better and often much better). The results for G8s are half as good, because there are half as many teeth in the worm wheel: a worm certified to 6 arc seconds peak to peak on a G11 will get around 12 arc seconds p to p on a G8, but it could be "worth it" because you get a very smooth curve.
regards
Greg N






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