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Old 10-10-2018, 09:26 PM
Dbroz (Daniel)
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How to balance my dob

Hi everyone

I am having a lot of trouble trying to balance my scope. When I bring it down to about 45 degrees it will stay in its spot but once I get a bit lower it will start to fall on its own, the when I bring it up if I have it almost vertical it will again move on it’s own and point up. I can’t seem to fix this problem. I have attached a photo, I’m not sure is the altitude bearings are too small. It’s really doing my head in. Any suggestions will be much appreciated Click image for larger version

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  #2  
Old 10-10-2018, 11:58 PM
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mental4astro (Alexander)
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Daniel,

The symptoms you describe are of a top heavy situation. This would have been solved with having the altitude trunnions located on the center of gravity of the OTA. Instead, the pivot point is located below the COG of the scope.

An easy solution to this is to add a counterweight to the mirror box. The main advantages being easy to do and you don't alter the quality of the action when you move the scope around - most important!

Poor solutions would be bungie/shock cords that act as a quasi counterweight. Problems with these include additional mucking around to get them to work properly and they are perishable. Another is some sort of braking mechanism, but the are difficult to retro-engineer and the stuff up the quality of the action.

Remember another thing, regardless of what system you end up using, if you end up using a full length shroud around the OTA, this will add more weight to the top end, and as the shroud becomes sodden with dew it adds more weight again. Different eyepieces also have a big impact - going from a wee 6mm plossl and swap to a 1kg lump of glass is a BIG difference for the loading of the secondary cage!

If you go down the counterweight route, you can use 1kg weights and work out the best counterweight amount.

Alex.
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Old 11-10-2018, 08:32 AM
Dbroz (Daniel)
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Hi Alex

Thanks for your advice. The problem is I have tried to put more weight on the mirror box, this fixed the problem of the scope falling to horizon, but it didn’t fix it pointing to the sky by itself. It moved both ways on its own when I get to a certain degree. Even without the extra weight it still falls both ways up and down I mean I would rather it go up on its own than down. But still very annoying. You mentioned about the bearings being centre of gravity. I’m guessing they are not in the correct spot, the bearings don’t sit centre of the mirror box, they are about 20mm more to the front of the mirror box. But the truss clamps I made don’t allow me to bring the bearings back 20mm. Would the bearing position be the problem here? Counter weights only work for the scope not dropping to horizon, but adding more weight to the mirror box brings the scope up faster by itself. So when I have the scope at about 85 degrees it will move on its own to vertical with or without the weights. I’m thinking I might need to make new bearings so I can move them to the centre of the mirror box. I really am having a hard time trying to balance this. I did try a elastic bungie cord, one that stretches very easily and it worked pretty well for keeping the scope from falling, but I still have the problem of it raising in its own when at about 85 degrees. Anyway I hope this all makes sense, and thanks heaps for your advice.

Daniel
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Old 11-10-2018, 09:46 AM
Billyboy78 (Bill)
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Yes, you can see in the pic. The centre of radius of the altitude bearing needs to be midway along the mirror box centreline. It doesn't matter where on the centreline you put it, you can fix that part with weights later. Extra weights and bungy cords now wont fix it.
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Old 11-10-2018, 10:01 AM
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mental4astro (Alexander)
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Now this is more info that's needed!

Ok, the bearings are forward. So you need to be careful WHERE you put the extra weight.

Let me guess, you've been putting the extra weight on the mirror box on its back - that is in the opposite direction from where the bearings have been moved to.

The best place to put the extra weight is more forward, best in front of the mirror box. Now I know this is not feasible because of the rocker box front panel too.

These are all problems many ATMers run into because of not enough planning or understanding of the mechanics at play. You need to think about where you are going to put the bearings along with their design, and plan the OTA around that. And you leave the rocker box design, especially its build, until after the OTA is finished so that you can work out the COG of the scope and only then can you design the rocker box to suit, and make weight distribution changes to the mirror box to suit again, ie more weight in mirror box means a lower COG.

I need to mention all of this not to put down or show mistakes, but to show the chain of errors that will help us all work out a solution.

The better solution is to look at putting the extra weight on th underside of the mirror box & again "forward" of the COG. This will stop the scope tipping down and falling up when at higher elevations. The COG has two dimensions - along the long axis and the short axis (ie towards the front and back of the mirror box). You've put the pivot point of the alt bearings too far forward so the scope us back heavy and why it tips up at high elevation.

Another thing that will help is changing the Teflon pads - make them smaller and space them as far as they can go. From your photo I see they are too close together. I also suspect they could be too large for the weight of the OTA. All of this means the action in altitude is too easy.

How did you come to working out the size of the Teflon pads and their spacing? This is important so we can solve the entire mechanical situation.

Hell of a lesson to learn about taking more time to study the dob design and plan one's scope a little more carefully

Alex.
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Old 11-10-2018, 10:20 AM
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mental4astro (Alexander)
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There is another more radical solution to this - a totally new rocker box and alt bearings.

Anything we try to do retrospectively is only stop-gap and still won't be a fix-it-like-it-should-work. No way of achieving this now.

You can still salvage the scope, but it will mean sucking it up and making a new rocker box and bearings that WILL suit the OTA. In a way it's making the rocker box and alt bearings in the order they should have been done - after the OTA was finished.

Larger diameter alt bearings will solve the issue with the clamp location, make it easier to match the alt pivot point to that of the COG of the scope, and still maintain a low profile rocker box. And it will also improve the balance of the scope when you change from small to large eyepieces and balance issues from a wet shroud (which is not mandatory and there are other alternatives).

You will end up with a scope that will work because you won't be needing to be second guessing everything you do when you use it. Piss around trying to fix things now and you will never really be able to relax while using it. There's just too many things going on with your scope right now.

Oh, and if you haven't done so already, rebuilding the rocker box will give you the chance to design for digital setting circle encoders from the start. I do this with all the scopes I design whether they have encoders fitted from the start or later on.

Alex.
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Old 11-10-2018, 10:28 AM
Dbroz (Daniel)
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Yeah I was thinking the same thing. Making a new rocker box is easy enough for me since my cousin works at a cabinet maker and can get everything cut on a cnc machine for me. I might try that. Thanks again for the advice.


Daniel
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