Ovision worm for Losmandy G11

[rat156]

Hi All,

This is the first post of a sort of blog to show what happens to a G11 mount when the Ovision (www.ovision.com (http://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.

Cheers
Stuart

[multiweb]

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.:thumbsup:

[wasyoungonce]

yep, count me in as watching with interest as well!:thumbsup:

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.:question:

[sadia]

Watching with interest too. I am waiting for my G11 to be delivered next week and depending on what kind of PE i get i might order one myself as well.

[multiweb]

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.

[wasyoungonce]

Thanks.

As for the oldham coupler...aren't they a work of modern technology!:lol:

[multiweb]

Well, they'e very well designed to compensate for axial offset but if you have everything inline in the first place you can go for a rigid coupling.

[multiweb]

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?

[rat156]

Pictures tomorrow.

Part 1 Installation.

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.

Test shots tomorrow night hopefully.

Cheers
Stuart

[mtodman]

So, is the 76 sec PE something I can expect to occurr in my Titan when it arrives from Losmandy?

Matt

[wasyoungonce]

±3.5..wow!


That is sooo juicy!:thumbsup:

[rat156]

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.

Cheers
Stuart

[kinetic]

Stuart, all,

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.

Steve

[multiweb]

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.

[rat156]

Pictures as promised.

The worm in its place.

Cheers
Stuart

[rat156]

Here's the original PE analysis from the Losmandy HP worm.

As you can see the PE doesn't line up from worm cycle to worm cycle.

The analysis shows a huge 76 second peak, which is the cause of all the trouble.

[rat156]

Here's the first data from the Ovision worm.

As you can see the Ovision worm makes a huge difference. Both of their claims, i.e. removal of the 76 second error and lower raw PE.

I forgot to change the scale on the graphs to arcseconds, but the image scale is 0.87 "/pixel.

Cheers
Stuart

[rat156]

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.

Cheers
Stuart

[multiweb]

Very cool. Big difference. Looks like the block is now made in a cast.

[rat156]

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.

Cheers
Stuart

[kinetic]

Stuart,

what was the declination of your test star out of interest?

Steve

[rat156]

Zero(ish)

[Bassnut]

4 arcsecs P/P and 1.11 RMS is excellent (with 3arcsecs seeing?, I dont quite get that). Anyway, thats near to PME type results, couldnt get better than that, nice work :thumbsup:

[rat156]

The way I look at the figures, the underlying PE is 4" P/P, then add on 3" for seeing, so over the 20 minutes I was taking data I should get about 7" P/P. Each individual worm run may be less.

So the data shows the mount to be tracking well doe sit Fred? I have no experience with this, so shoot me some figures.

The other reports on the Ovision also state things like "PME performance", also comparing the mount to APs in the PE.

It was a bit cloudy at sunset last night, but cleared up later. I had been working around the house all day and was a bit stuffed, so didn't setup. Should have, I went out at around midnight, clearest skies I've seen in a while. Try tonight maybe.

Cheers
Stuart

[Bassnut]

Staurt

The 1st curve with original worms is 25 pixels p/p*.8=20 arcs/secs. Thats awefull, and worse, not regular for each pass (not just the 76 sec problem, generally rough) its impossible to get a pec curve with that, there is no periodic data there to average out.

I had somewhat better curves, without the 76 problem (or small), of about 7-8 arcsecs P/P. So I gave up generating a pec curve and found guideing without pec gave me a slightly lower p/p, but RMS of between 1 and 2 because the error, although not that periodic, was smooth enough to guide out at 2 sec guide exposures.

I found, as some others have, that error (periodic and not) actually smooths out a bit with the G11 slightly overloaded, so I was lucky and got away with no pec.

The Ovision worm graph shows a p/p of what, 4-5 arcsecs (ignor the fact its pushed up the scale, the 1 st blue line did that, you know all this, Im just rabbiting on ;-), periodic and very smooth, in fact I dont think ive seen a G11 raw PE curve this small and smooth, and with 3 arcsec seeing, a fair amount of that would be seeing (what were the exposure times, that makes a big difference).

The raw, out of the box PE of a PME is about 5 arcsecs P/P, and 1-2 RMS, although with PEC, P/P gets to 2 or so and RMS less than 1. Its a bit hard to tell exactly what it is, seeing makes a big difference, as I suspect would be with your curves.

If you expand a raw Ovision curve and compare it with the final curve with PEC, they look the same!. I maybe wrong, but I dont think PEC is on, or its not working (or the "raw" graph has PEC on). The final curves still show the same PE on each pass, which should cancel out with PEC.

If thats the case, and you can correct out the PE showing on the final graph, then your G11 will be well tuned indeed ;-)

[rat156]

Here's the result.

Quick and dirty processing, please excuse.

NGC 210.

10 x 10 minute Luminance frames, guided on the internal guide chip using the AOL.

CCDStack reports FWHM of 4 (must be pixels as it has no idea of the image scale), so that's 3.5".

Cheers
Stuart

[multiweb]

The RMS is a good indication of how tight your guiding is when you stack in the CCDIS popup window. From the stars on your pic it looks great already. Nice and round. :thumbsup:

[Bassnut]

Looks good Staurt, seems the upgrade was well worth it.

[telecasterguru]

I am looking to pick up a G11 next month so how easy is the ovision mod. I have no skills in using hammers, screwdrivers or any tools.

Frank

[multiweb]

You'll just need two allen keys. It's a straight swap.

[mick pinner]

what payment method did you use and are they easy to contact.

[telecasterguru]

Even though it only takes 2 allen keys to fit, is there any fiddling around to get it to work perfectly or is it perfect out of the box?

Frank

[wasyoungonce]

A very nice upgrade indeed. Thanks for the info.

[multiweb]

Just standard meshing. The only difference is that it's just one block instead of two end blocks. The nipple on the motor side is tight so it only rotates,on the other side the hole is slightly bigger so you can adjust your meshing. There's heaps of tutorials online to google on how to remove or set your G11 worms.

[rat156]

Hi Mick,

Paid with credit card, just fill out the order for non-EU countries from their website and send it to them by email. It took a couple of weeks from memory.

Cheers
Stuart

[rat156]

Now this got me thinking...

What I believe was happening was that the PE was in fact increasing. I inverted the curve and sent it back to the G11.

What a difference! I'll upload the data when I get it from the obs PC. I reckon +/-2" from the P-P data, so averaged out that should be about 2" PE.

WooHoo!!

I might even try an unguided exposure...

Cheers
Stuart

[netwolf]

I have had mine for quite some time now, but just have not had the time to install and test it out. We ordered three of them near the start of the year, and so far only Kal has had his installed. I still have mine in its box.

One concern i have had but i wanted to try it before sending it back. Is that I noticed when manually turning it very slowly i can notice for lack of a better word "bumps". Apoman has alse noted this on his and is hestant to install it. But turning it fast it seems very smooth.

Sturart did you notice this on yours?

[rat156]

Nope, smooth as silk. They are tested before they leave, install it, you won't regret it.

Cheers
Stuart

[Exfso]

You would think Scott Losmandy would have attended to this problem by now

[rat156]

He may have, I bought the mount secondhand from here, so I don't know the age.

The Ovision worm works in two ways, the mounting removes the 76 second error, but the worm is a really high precision part.

Cheers
Stuart

[multiweb]

What's the difference between the ovision and the losmandy high precision worm? Machining tolerances? Material used? Or is it the fact that the meshing area is narrower? Would that make a difference?

[multiweb]

Hi Stuart, I finally managed to get some data on my G11 tonight.
I couldn't figure out what camera to use as I only have a QHY5 and a QHY8 but I don't have Maxim or CCD Soft, etc... but I did have a Nexstar imager and it worked as a VGA-USB cam :)
Here's the result of 4 worm cycles on achenar so I'll be able to compare when I've replaced my worm block.
How do you load the PEC back into the mount with the gemini to smooth things out? :shrug:

[netwolf]

Marc
The Ovision is a single block with worm and bearings encased within. One of the problems with the Losmandy g-11 desing is that there are two independant block in which the worm sits and they can be out of alignment.

Interestingly the HGM Titatn i belive comes with a single block design. Why Losmandy has not done the same with the G-11 is anyones guess. The issue has been raised a lot in the Losmandy users yahoo groups. The first working solution i saw was developed by Rainer who i think is a member on these forums. He created modifications to his mount and that in turn i think has led up to these commercial single block developments.

There has been further discussions on replacing the couplers to also enhance performance further. Also there is the McLennan Gearbox which also have shown to provide further enahncment in performance.

This is not to say that the G-11 out of the box is not a good mount, it is a very good mount. However as always we can tweak it to perform even better, and many have achived fantastic results.

[rat156]

Hi Marc,

When you setup PemPro, specify your mount as G11 with Gemini (if you have it) then another tabe appears in the software which allows you to program the mount directly.

Looks good though. From my experience invert the curve before sending it to the mount.

Cheers
Stuart

[multiweb]

Thanks Stuart - yeah I realised that after chatting with Ray on the CCDware forums. I chose the wrong type of mount. I picked G11 instead of Gemini Lvl 4 which has the dialogs to upload PEC data. :doh:I'll have to do another PEC data run. It turned out better than I thought though, +/-4 arcsec out of the box and that's the standard SS losmandy worm. :) And I didn't get the 76s problem. I have one of the HP losmandy worm that I'll swap and see if it makes a difference.

I understand from the PEMPro docs that the PEC data is dependent on the guide rate used. What's the recommended guide rate? I think I use 0.5 sidereal from memory but I'll have to check. The range is from 0.3 to 0.8 I believe. What do you use?

[multiweb]

Yep, I'm aware of that. The one block set-up takes the standard two end block mis-alignment out of the equation and makes meshing way easier. Although you can use the aluminium cover as a traverse when aligning those blocks. That's what I did and they must be pretty well inline because my PEC data didn't show any bearing popping. Still the one aluminium block is the ONLY way to go IMHO.

They sure learnt from their G11 experiences ;) There is actually a one block worm design from Losmandy. I've seen it and touched it but it's not commercialised and I'm not sure when it will.:shrug:
Rainer has been of a tremendous help to me. :) I have based all my new blocks design on a lot of the work he had already done on his two mounts. He knows what he's talking about. :thumbsup:


Yeah I saw that on the internet. Not convinced. :shrug: I reckon changing the Oldham coupling for something more rigid is more beneficial than playing with the gearboxes.


It is an excellent piece of engineering very well designed. Every time I pull it apart and play with it I'm always amazed at how well thought it all is.:thumbsup: