This is the first post of a sort of blog to show what happens to a G11 mount when the Ovision (www.ovision.com) worm is installed.
My G11 that I bought secondhand has the 76 second error introduced by the skewing of the worm in the mounting blocks, causing drag on the bearing at one point. The only way to eliminate it completly is to replace the worm with one of these. Many have tried to shim the mounting blocks and adjust them, but the 76 second error is still there. So I bit the bullet and bought one of these worms.
I will be installing it tonight and hopefully I'll have a PEC run. I have a few already from PemPro to compare.
The Worm itself is a nice looking piece of kit, baeutifully machined and rock solid.
Watching with interest. Let us know how you go. I have been working the past few months on sourcing bearings and bits and pieces to make my own worm block. I'm 99% there. My version is spring loaded and comprises thrust bearings in the assembly. I'll be interested to see your results.
yep, count me in as watching with interest as well!
I'm unsure why Scott Losmandy hasn't put out his own version. From what I read it's not really the worm as they are already using the newer "high precision worms" but the housing that is causing the problem.
yep, count me in as watching with interest as well!
I'm unsure why Scott Losmandy hasn't put out his own version. From what I read it's not really the worm as they are already using the newer "high precision worms" but the housing that is causing the problem.
He does but it's not commercialised yet. The trick is to have everything inline in one solid block.
To work best the gearbox also needs to be decoupled from the base plate and mounted directly on the worm block. The oldham coupler can then be replaced by a bellow coupling. Then what I want to do is spring load the whole worm block so the meshing is more flexible and you get rid of any backlash.
Many have tried to shim the mounting blocks and adjust them, but the 76 second error is still there.
I think the error comes from the fact that if the end blocks are not square in relation to the worm axis the radial bearing will pop every now and then. The one block addresses this issue. Any chance you can post some pictures of the bits and pieces of the ovision block?
Installation is as per the instructions and pretty easy. Simply place the scope in a position where you can release the RA clutch and not have the scope crash into something. I had mine resting against the mount. Then unbolt the old worm, remove the part of the Oldham coupling and refit the new worm. Adjust the Backlash in the RA axis to a couple of mm movement of the end of the CW shaft. Mine's a bit tighter than this, so I may back it off a bit. The instructions say that the worm is pregreased, I couldn't see anything on it, so I whacked a bit of bearing grease on it.
My clutches are really hard to get tight enough to hold everything when there's no camera attached, so I had to use a lot of force to undo the RA, in doing so I managed to stuff up my polar alignment.
So after I had installed the Ovision I had to drift align again.
tried out the drift align wizard in PemPro, it has some problems, can't seem to lock onto a star.
used Startarg instead, which I use my webcam for, so the SBig had to come out, refocus etc.
Drift aligned the azimuth OK, but not perfect.
Started PemPro (again screenshots to come tomorrow), ran the first acquisition without PEC on to get the native PE of the worm, looked OK except for some drift from the imperfect polar alignment. Curves overlayed pretty well. Analysis showed the 76 second error has gone.
Programmed the mount with the new PEC and acquired another four worms worth of data. Tried to use the refine part of Pempro, things seemed to get worse, tried subtracting one curve from the other, still rubbish. Did another couple of manipulations of the PE curves, eventually I have it down to +/- about 3.5", P-P 7".
Tried to run the align wizard again, better, but still having difficulty locking onto the stars.
First impressions of the worm, worth every cent. If I can work out how to get Pempro to work properly (probably operator errors), then I think I should be able to refine the PE down to <5" P-P, which should be good enough for unguided exposures at moderate focal length.
So, is the 76 sec PE something I can expect to occurr in my Titan when it arrives from Losmandy?
Matt
Hi Matt,
I don't think so, I believe that it's a G11 specific problem. It has to do with the mounting blocks for the worm.
As the blocks are in two separate pieces, the worm can, and almost always does, go in ever so slightly off centre. This causes it to drag on the bearings evry 76 seconds. As 76 seconds is not a divisor or multiple of the worm period PEC cannot correct for it.
reading this thread with interest too
Have you thought of posting it in the ATM section too.
It might not come under the gaze of dabblers possibly?
Anyway, my 2c:
You need to make the worm stay centred and with zero
end thrust before you can even hope to quantify a meaningful
PE in arc sec. (error attributed to the worm anyway)
Also, having let this run with the (random)? periodicity of 75 sec
for months/ years, this could have also ground a slight random
error into the tooth facets of your worm wheel?
I hope you get to the bottom of it Stuart. Interesting stuff.
You need to make the worm stay centred and with zero
end thrust before you can even hope to quantify a meaningful
PE in arc sec.
Correct - on closer inspection the ovision block doesn't seem to provide for thrust bearings (unless they have changed their design recently).
I've followed Rainer's advice and have two thrust bearings on each end of the worm assembly behind the radial bearings so there is zero axial play in the block I'm working on.
Finaly the refined PEC, i.e. a PEC run made after the PEC was uploaded and turned on. The derived curve has a residual PE of 4" P-P. The raw data was about 7", which probably mean that the seeing was about 3" last night, typical for Melbourne after a warm day.
You need to make the worm stay centred and with zero
end thrust before you can even hope to quantify a meaningful
PE in arc sec. (error attributed to the worm anyway)
I hope you get to the bottom of it Stuart. Interesting stuff.
Steve
Hi Steve,
The worm stays centered, or at least in the same place because the weight imbalance of the mount (slightly East heavy) keeps it there. There is a brass cap on the end of the Ovision worm, which may have a thrust bearing under it, you can just see it in the pics, I don't want to undo it. The worm will move longitudinally in the mount, but only if I lean on the counterweight bar.
I didn't need to get to the bottom of the problem, it's a common fault with the G11 mount, been solved long ago.
