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Dynamically balance an 8" SCT with 80 mm finder

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#1 markr101

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Posted 07 February 2013 - 03:06 AM

I’m trying to mount an 80 mm Stellarvue finder on top of my U2K 8" SCT. I have a Stellarvue dovetail plate and rings on top, mounting the 80 mm finder. It is not heavy for an 80mm, just 2.2 lbs. The dovetail plate and rings are only another pound, or a little more. Underneath I have a Losmandy dovetail plate, with a five inch threaded bar and several 2.5 lb weights. When the counter weight is under the rear cell (as in the picture) the OTA balances horizontally, but not dynamically (by that I mean it won’t balance as the scope is rotated up to normal astronomical viewing angles). When pointed up the scope just sinks back to horizontal. I have tried using 1, 2 and 3 of the counterweights, moving them up and down on the threaded bar, and moving the bar forward and back along the OTA. Nothing is working. Celestron tech suggested I needed to place more weight at the front, on top of the OTA because the scope is “back heavy”. This doesn’t seem to help, since the scope seems in fact to be “front heavy” since it sinks back to horizontal when pointed upward.

And tips or ideas for me? I’m stumped. A picture of what I’m doing is attached.

Thanks,
Mark

Attached Files



#2 HowardK

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Posted 07 February 2013 - 06:30 AM

balancing can be a real pain.

Try moving the 80mm scope forwards and backwards along its dovetail whilst also moving the weights underneath to the front and rear of the main scope.

somewhere a good balance at all positions will be achieved

#3 Jon Isaacs

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Posted 07 February 2013 - 07:35 AM

Mark:

First let me say hello and welcome to Cloudy Nights... :waytogo:

Basically you have to balance the scope in two positions, vertical and horizontal. This is a teeter-totter problem. The basics:

If the tube is placed horizontally, then what matters is the distance the weight is from a vertical line through the center of the altitude bearing, i.e. the balance point. It's a center of gravity problem, so it's the weight times the distance from the pivot but only in the horizontal direction.

Think of the teeter-totter, it doesn't matter how tall each person is, only how much they weigh and how far they are from the pivot. If one person weighs 200lbs and the other person weighs 100lbs, then the person how weighs 100 lbs will have to be twice the distance from the pivot to balance.

Once you balance the scope front to back, then you place the tube vertically and adjust the distance the counterweights are from a vertical line through the center. You can move the weights in and out without affecting the horizontal balance but if you add or subtract weights you will have to rebalance the horizontal axis.

These adjustments should be done with scope free to pivot.

Since the scope is sinking at normal altitudes, it suggests that the front to back balance is heavy in front, it appears that way to me in the photo that is the problem. There are three choices:

- Move the finder rearwards, closer to the altitude pivot until the scope balances. If you can do it, this is the best solution.

- Add more counter-weights, you don't want to do this unless you absolutely have to.

- Move the counterweights reward, away from the pivot point. It appears they are as far back as possible.

That is a lot of weight you are adding.. it may result in vibration problems even when it is balanced.

Hope this helps.

Jon Isaacs

#4 markr101

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Posted 07 February 2013 - 11:33 AM

Thanks Jon. Very helpful. I pretty much have horizontal balance, now I see that I need to work on the vertical.

Mark

#5 Midnight Dan

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Posted 07 February 2013 - 01:59 PM

Here's a good link on the topic of balancing a fork mounted SCT:

http://starizona.com...ncing_fork.aspx

-Dan

#6 markr101

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Posted 08 February 2013 - 01:38 AM

Thanks to you as well Dan. I was doing it backwards - horizontal balance first. The article you referred me to cleared that up, vertical balance is done first.

Mark

#7 markr101

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Posted 09 February 2013 - 12:32 AM

Jon and Dan - thanks again. Attached is the a picture of success! The scope is balanced at any angle with the clutch disengaged. I went back to the stock counterweight rail and weights, in place of the heavier Losmandy dovetail rail that I was trying to use. To other SCT owners, the key was to position the OTA vertically (straight up), and balance the 80mm finder with counterweights. Then I rotated the OTA to horizontal and moved the counter weights back and forth along the counterweight rail until I achieved balance. Now as I change eyepieces I can simply slide the movable counterweight along the rail to fine tune. Much appreciated guys. The key was in fact to balance vertically first. Once that was done it was actually pretty easy. Now I think I'll try to piggyback my Televue Pronto (which is 7 lbs., significantly heavier than the 2.2 lb. Stellarvue finder). I'm quite sure I'll need the beefier Losmandy rail to do that.

Mark

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#8 Jon Isaacs

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Posted 09 February 2013 - 06:12 AM

Now I think I'll try to piggyback my Televue Pronto (which is 7 lbs., significantly heavier than the 2.2 lb. Stellarvue finder).


I would try this one first out under the night sky before trying anything new. You have added a lot of mass quite a distance from the pivot, you are putting a lot of extra load on the mount, vibration and instability are likely results.

jon

#9 StarmanDan

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Posted 09 February 2013 - 10:10 AM

I have almost the exact setup as you with a Meade 8" LX200 and an Orion ST-80. I use a 2-D counterweight system like this one:

Posted Image

It's very easy to balance any equipment I put on the scope.






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