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Webster Telescopes Forum • View topic - regarding placement of alt bearing.
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 Post subject: regarding placement of alt bearing.
 Post Posted: Sat May 19, 2012 5:32 pm 
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Joined: Sun Jan 15, 2012 3:54 am
Posts: 7
Is there a strict value regarding the angle of the alt bearing placement with respect to mirror box? I have read 30 degrees, and to place the alt pads at 70 degrees. but others have told me 45 degrees, not mentioning how to thus place the alt pads. I am confused on this, and also my pole blocks are large to the extent that I cannot fit the bearing to the top center of the mirror box and also fit the pole blocks. If I drop the bearing down about two inches from the center top, then everything fits ok, but I notice something else rather unnerving: when I tilt up the alt bearing, say at 45 degrees, well, it pushes more of that object of the center of the mirror box so that now it will become heavy on that side. Does this mean that I should not center the alt bearing at center of mirror box, but back a bit to equalize the bearing weight ? or is that not so important? And is it ok that my alt bearing are not centered to the top, but down a couple of inches? thanks again!!!!!


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 Post subject: Re: regarding placement of alt bearing.
 Post Posted: Sun May 20, 2012 2:34 pm 
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Joined: Thu Jul 02, 2009 11:48 am
Posts: 80
Location: Detroit Michigan
We have a few issues to work on here:

The bearings should be mounted so that they pivot on exactly the Center Of Gravity. If you have not mathematically calculated the COG, you can lay the assembled telescope on a piece of PVC Pipe (with your heaviest eyepiece/Paracorr/finder, ect. installed) and find the point where it balances.

Most people make the top of the rockerbox the COG, but there is no reason that it has to be. It's just easier to make it that way.

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As far as the bearing angle, there is not a set number.

You need enough angle to allow the telescope to aim all the way down to 0° (the horizon).

You can take a Rockerbox side piece (or a cardboard mock up) and lay it on the Mirrorbox to see at what angle the bearings have to be to give you the full range of motion.

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Once you have your Alt Bearing position, then you can figure out where to mount your truss clamps.

Post some pictures if necessary.

Thank you,

Eric

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Eric Webster
http://www.WebsterTelescopes.com


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 Post subject: Re: regarding placement of alt bearing.
 Post Posted: Sun May 20, 2012 3:00 pm 
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Joined: Sun Jan 15, 2012 3:54 am
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thanks so much. Did you know you have a "work from home link" at the top of your response? well if you are sponsored that's good.

So, now as far as the teflon pad placement on the rocker box side, doesn't exact placement depend on the angle of the bearing angle?

Do ATMers ever just give their best estimates of balance (COG), and however off they are, just add some dead weight somewhere ?
thanks again?


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 Post subject: Re: regarding placement of alt bearing.
 Post Posted: Tue May 22, 2012 4:31 pm 
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Joined: Thu Jul 02, 2009 11:48 am
Posts: 80
Location: Detroit Michigan
mantrain wrote:
thanks so much. Did you know you have a "work from home link" at the top of your response? well if you are sponsored that's good.


I do not see it on Firefox or IE, anyone else see it?

Links are disabled on the Webster forum so as not to attract spammers.


mantrain wrote:
So, now as far as the teflon pad placement on the rocker box side, doesn't exact placement depend on the angle of the bearing angle?


No, just use the standard 70° apart spacing and you will be fine. If you think about it, the 70° spacing dictates the final angle of the ALT bearings.

You can't run out of Teflon pad, before your scope makes it down to 0°, right?

mantrain wrote:
Do ATMers ever just give their best estimates of balance (COG), and however off they are, just add some dead weight somewhere ?
thanks again?



You can do some "fine tuning" with a little bit of weight, but 95% of your balance work math should be done before you cut the first piece of material.

Building a telescope is a many month journey, so spending a hour or two with a calculator (or a spreadsheet) to get the balance right is a worthwhile endeavor.

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Eric Webster
http://www.WebsterTelescopes.com


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 Post subject: Re: regarding placement of alt bearing.
 Post Posted: Fri May 25, 2012 1:21 am 
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Joined: Sun Jan 15, 2012 3:54 am
Posts: 7
It is really generous of you to offer this helpful information.

So, regarding the math, say my Mirror box now is 65Lbs, and my UTA is 15lbs, what calculations do I need to perform to figure out the dimensions of the rocker box?


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 Post subject: Re: regarding placement of alt bearing.
 Post Posted: Sat May 26, 2012 4:06 pm 
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Joined: Thu Jul 02, 2009 2:25 am
Posts: 122
Unfortunately, it would require about 20 paragraphs to fully explain the dynamics of telescope balancing calculations. Anything less and we would probably be doing you a disservice.

Look online and download "The Dobsonian Telescope" by Kriege. There are 2 chapters on balance that will save you many hours by not reinventing the wheel.


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