Telescope Reviews: Dob mount completed, now having trouble balancing

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kcgolden
sage

Reged: 04/01/07

Loc: Austin, TX

Dob mount completed, now having trouble balancing new

new

#2880355 - 01/22/09 12:21 AM

Thanks to everyone who helped me with my various questions while building my mount. It was a big help. Now my mount is relatively complete and has smooth motion on all axis. That's the good news...

The scope itself is an Orion 8" Reflector which was originally intended for use on an EQ mount. I basically built a dob mount for it and I incorporated the tube rings into the design in order to make it easier to slide the scope back and forth for balancing.

The bad news is that I can't seem to balance. If the scope is pointed near the horizon, it acts as though the front end is heavy. However, when I point it closer to the zenith, it acts as though the rear is heavy. In between those two points, the scope pretty much stays put when you let go.

Any ideas what could cause this?

Edited by kcgolden (01/22/09 12:36 AM)

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sixela
Postmaster

Reged: 12/23/04

Loc: Boechout, Belgium

Re: Dob mount completed, now having trouble balancing new

new

[Re: kcgolden]
#2880494 - 01/22/09 03:34 AM

You have two dimensions to balance: for good balance, you want the centre of gravity -- three coordinates -- to lie on the rotational axis of the bearings (an axis is a line, which gives you one degree of freedom less to cater for).

Sliding the tube in the rings achieves only tuning for one dimension.

Balance problems like this suggest that the centre of gravity is (with the tube horizontal) above the rotational axis of the bearings. If you can, move the bearings up, or install a counterweight at the bottom (as far from the bearings as possible, to make it small).

If you have rings, you can usually rotate the tube until this problem no longer appears (when the COG is again on the rotational axis, albeit not in the middle between the two Alt bearings).

Chances are that the focuser won't be in a comfortable position, though

. Usually, it'll be pointing down (which is more than a bit dangerous for eyepieces).

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Vincent Becker
sage

Reged: 09/16/08

Loc: France

Re: Dob mount completed, now having trouble balancing new

new

[Re: sixela]
#2880706 - 01/22/09 08:42 AM

Could the problem be the focuser and eyepiece temselves? Maybe you could try to turn the tube so that the focuser is horizontal (then parallel to the altitude axis of your mount) and see what hapens.

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rboe

Reged: 03/16/02

Loc: Phx, AZ

Re: Dob mount completed, now having trouble balancing new

new

[Re: Vincent Becker]
#2880722 - 01/22/09 08:56 AM

Sixela nailed it. The tube's center of gravity is to the upper side of the tube and needs some mass on the otherside, probably for the reasons Vincent mentions.

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kcgolden
sage

Reged: 04/01/07

Loc: Austin, TX

Re: Dob mount completed, now having trouble balanc new

new

[Re: rboe]
#2880957 - 01/22/09 11:39 AM

In that case, I basically want to move the bearings so that they rest on the pads at some point above the under side of the tube?

I already took the bearings off but when they were on, the bottom of the tube was about equal with where the bearing was resting.

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DSalters
scholastic sledgehammer

Reged: 03/03/07

Loc: High Ridge, MO (St. Louis)

Re: Dob mount completed, now having trouble balanc new

new

[Re: kcgolden]
#2881081 - 01/22/09 12:50 PM

Kelsey, I am in the same boat as you. Last night, I took my rebuilt 6" Dob. and noticed all of the same problems you mentioned. Even when I strap on a 3 lb. counterweight underneath and to the very end of the tube, it still wants to rise when pointing near the zenith.

I took the bearings off last night and will reinstall them higher on the tube (if the tube is resting horizontally) and see what happens. I figured that this is what needed to occur.

Please continue to post your progress. I'll do the same.

Glad I found this thread!

Silexa,

For future scope-building reference, how can we know how high (relative to a horizontal tube) to place the bearings? Is it best to try and place them as high as possible since most focusers and finders are on "top"?

Thanks!
Daniel

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oldtimer
Pooh-Bah

Reged: 11/13/08

Loc: Lake County Illinois

Re: Dob mount completed, now having trouble balancing new

new

[Re: kcgolden]
#2881663 - 01/22/09 06:26 PM

Forget all that X and Y stuff. Lets keep it simple. Balancing the tube when it is at or near the horizontal position is pretty straight forward. Now point the tube straight up and look at it. Does it have equal weight on both sides of the tube, in your case probably not.

If you have a 2" focuser and use 2" eyepieces (and maybe a finder) these add weight to just one side of the tube. The best place for an equal counterweight is at the rear of tube on the opposite side of the focuser.

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sixela
Postmaster

Reged: 12/23/04

Loc: Boechout, Belgium

Re: Dob mount completed, now having trouble balanc new

new

[Re: kcgolden]
#2881783 - 01/22/09 07:35 PM

Quote:

In that case, I basically want to move the bearings so that they rest on the pads

The pads aren't relevant here. What's relevant is that the line connecting the centre of the bearings needs to cross the tube more on the top (tube horizontal) of the tube (or the tube should be rotated until the COG is on the existing line connecting the centre of the bearings, but that might not be what you want if the focuser location is at a pleasant place right now).

In other words, you move the bearings vs. the tube.

Then you put the thing back on the pads and let the tube rest where it does.

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DSalters
scholastic sledgehammer

Reged: 03/03/07

Loc: High Ridge, MO (St. Louis)

Re: Dob mount completed, now having trouble balancing new

new

[Re: oldtimer]
#2881787 - 01/22/09 07:37 PM

True, I guess it is straightforward in all reality. Sometimes, though, it may take trial and error. In the case of building a truss-scope (which is a completely different topic than this thread but still relevant nonetheless), one can't afford three attempts at bearing placement before too many holes are drilled in your mirror box. Clamps would certainly apply here!

I raised my bearings by around 3/4" and the balance has definitely changed. I still use a 3 lb. counterweight because I intentionally built my rocker too short to allow the bearings to go any farther up the tube. However, it is now much more evenly balanced with the counterweight on the SIDE of the tube (but more on top than bottom...kinda a two o'clock position if looking up the tube from the mirror end), at the end (to compensate for the heavy EP and focuser on the other side).

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sixela
Postmaster

Reged: 12/23/04

Loc: Boechout, Belgium

Re: Dob mount completed, now having trouble balanc new

new

[Re: DSalters]
#2881791 - 01/22/09 07:41 PM

Quote:

For future scope-building reference, how can we know how high (relative to a horizontal tube) to place the bearings? Is it best to try and place them as high as possible since most focusers and finders are on "top"?

No, because with the rotational axis too high you'll have the complementary problem (a scope which nosedives when you point it close to the zenith but not when you point it lower). It has to be *just right*.

If you can't make it just right, then you really have to increase the friction in some way: moving the teflon pads, changing their size, adding a friction brake,...

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sixela
Postmaster

Reged: 12/23/04

Loc: Boechout, Belgium

Re: Dob mount completed, now having trouble balancing new

new

[Re: oldtimer]
#2881794 - 01/22/09 07:44 PM

Quote:

Forget all that X and Y stuff. Lets keep it simple.

If the X and Y stuff lets you do things with less counterweights (or do it at all -- see above), perhaps it's not such a bad investment to try to understand the issue

.

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DSalters
scholastic sledgehammer

Reged: 03/03/07

Loc: High Ridge, MO (St. Louis)

Re: Dob mount completed, now having trouble balancing new

new

[Re: sixela]
#2882044 - 01/22/09 10:13 PM

After I played around with mine tonight, I realized the error in suggesting that perhaps the bearings should be as high as possible. Obviously that would create a complimentary situation as you mentioned. I should have known that sooner but I just wasn't thinking at the time.

I guess large diameter bearings make up for needing to find the perfect balance point?

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sixela
Postmaster

Reged: 12/23/04

Loc: Boechout, Belgium

Re: Dob mount completed, now having trouble balancing new

new

[Re: DSalters]
#2882376 - 01/23/09 03:48 AM

Larger diameter bearings are indeed more tolerant.

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HandyAndy
scholastic sledgehammer

Reged: 01/11/08

Loc: West Midlands and around

Re: Dob mount completed, now having trouble balancing new

new

[Re: sixela]
#2883301 - 01/23/09 03:47 PM

Hi,

To find the vertical balance position of a truss tube with typical eyepiece/camera I would use a wooden plank and make spacers so I could lay the 'OTA' horizontal on it. You can then use a broom pole OWHY as a pivot and roll the plank along it till you find the balance. Use a square and make a line across the rocker box on each side in line with the rollers axis.

Similarly make a plate and stand the 'OTA' on vertically and place the broom handle so its at right angles to the secondary-focuser axis and again roll it along till you find the balance point. The balance point will be a small way from the centre-line on the opposite side to the focuser. To make things look symmetrical add a weight at the bottom of the rocker box opposite the estimated centre of gravity of the focuser and finder. Try again. Again use a square to mark a line on the rocker box side.

Centre the bearing on the cross the lines make or the average from each side.

A larger diameter bearing helps and setting the teflon pads the bearings sit on further apart increases the stiction.

I have built a 12" and 14" dob for my societies with solid OTA's and there was no need for balancing weights with light eyepieces.

Cheers. Andrew.

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rutherfordt
sage

Reged: 05/07/06

Loc: Northeast Tennessee USA

Re: Dob mount completed, now having trouble balancing new

new

[Re: kcgolden]
#2883498 - 01/23/09 05:46 PM

When I had this problem with my Dob, I put a C-clamp on the back, underside of the rocker box-- didn't look great, but it worked like a charm!

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kcgolden
sage

Reged: 04/01/07

Loc: Austin, TX

Re: Dob mount completed, now having trouble balancing [Re: rutherfordt]
#2885060 - 01/24/09 01:14 PM

My dob is balanced now. I moved the bearings up as high as they would go and then I had to put two C-Clamps on the back in order to balance since the mount didn't allow enough clearance to move the mirror back far enough.

I'm kind of dissapointed that I used my favorite clamps for this but they can be replaced.

I'll post pictures after I get it stained.

Thanks everyone who helped.

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