Dobsonian not balanced on altitude axis

[gerardsheldon]

I would appreciate advice on balancing my dobsonian about the atitude axis. 

I have a skyliner 250p dobsonian flex tube.  When I release the altitude tension handles, the eyepiece end of the tube swings down towards the ground. 

It would be extremely difficult to move the centre of gravity to the correct position as it is a collapsible open tube assembly.     The centre of gravity lies where the three struts in the middle between top and bottom tube segments.  Please see picture at the following link:  https://www.rothervalleyoptics.co.uk/skywatcher-skyliner-250px-flex-tube-dobsonian-telescope.html

This is becoming more important as I want to build an equatorial platform. 

Has anyone got a good solution?

Gerard

 

 

 

[NGC 1502]

Hi Gerard.  One solution could be magnetic weights, attached as necessary to the tube.

I don’t know if a commercial version is available off the shelf.  One source of larger magnets can be old hi-fi speakers. Years back I sorted a Dob with lead weight in an old tobacco tin, attached with Velcro.  A tobacco tin can have a large Velcro contact area and mine worked well.  Roofing lead offcuts may be obtainable from a scrap metal merchant.  A posh version could be to bolt a dovetail to the tube, with a dovetail fit sliding weight, so easily adjustable. My local club’s 12” Lightbridge Dob has exactly that fitted by myself.

Hope you sort it, Ed.

Edited October 3, 2020 by NGC 1502
Spelling

[gerardsheldon]

Thanks Ed, where on the telescope did you attach the weights?  Behind the mirror, or on the top, bottom or side of the tube?

[Peter Drew]

Place the weights at the primary mirror end underneath the tube,  The combined weight of the focuser and the finder is off axis, placing the weights as described will help cancel that.   🙂  

[NGC 1502]

1 hour ago, gerardsheldon said:

Thanks Ed, where on the telescope did you attach the weights?  Behind the mirror, or on the top, bottom or side of the tube?

Exactly as  Peter Drew has described.   As well as balancing the tube horizontally, if done correctly this will also achieve radial balance.  If the radial balance is off, all may seem well until observing high overhead, when the tube can drift in unexpected ways.   The often mentioned “Dobson’s Hole”  ( the area of overhead sky ) is not hard to navigate without issues if it’s done right.   Of course this applies to all alt-az mounts.

A well set up properly balanced Dob with smooth movements is an instrument of joy.  A badly set up Dob is not.

Ed.

 

[westmarch]

Hi Gerard,

I agree with the advice above that the most efficient placement is opposite the viewing assembly but you may find it more convenient to put it on the top.  If you buy a magnetic kitchen knife holder and attach it to the primary mirror casing, you can attach iron weights to it and move them up and down until you achieve balance.

John

Edited October 3, 2020 by westmarch

[gerardsheldon]

Thanks very much for all your replies.  There are a number of options and I'll explore what works best.  Gerard   

[Peter Drew]

48 minutes ago, westmarch said:

Hi Gerard,

I agree with the advice above that the most efficient placement is opposite the viewing assembly but you may find it more convenient to put it on the top.  If you buy a magnetic kitchen knife holder and attach it to the primary mirror casing, you can attach iron weights to it and move them up and down until you achieve balance.

John

I'm afraid this would compound the issue.     🙂

[banjaxed]

https://www.amazon.co.uk/gp/product/B003MWG7BO/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

I bought 2 of these from Amazon and stuck 2 strips of drought excluder on the OTA    so they don’t scratch the paint.

[John]

7 minutes ago, Peter Drew said:

I'm afraid this would compound the issue.     🙂

That's where I have my counterweights and they work fine.

I realize that the optimum position for them would be on the lower tube opposite the focuser and finder but on my scope the weights would hit the mount uprights so I've put them on top of the tube and they work well:

 

 

[Stu]

55 minutes ago, John said:

That's where I have my counterweights and they work fine.

I realize that the optimum position for them would be on the lower tube opposite the focuser and finder but on my scope the weights would hit the mount uprights so I've put them on top of the tube and they work well:

 

 

I do the same as you John, with my 8”. When observing near the zenith I remove the weight and it remains in balance, then pop it back on again when I need to point the scope lower. Seems to work well.

[Peter Drew]

Of course this can work, specially if you have well designed mounts.  I was suggesting the optimum placement which is worth it if convenient.    🙂

[Rusted]

I'm 100% with Peter on this one. The weights don't have to be exactly opposite the focuser + finder in the radial sense.
"Underneath" the lower tube would be 95% while still almost compensating for radial imbalance as well. 
Moment = Mass x Distance. So the lower they are placed, the lighter the weights can be. Less to carry about.
No reason for them to hit the uprights if placed correctly. I wouldn't want a reflector with a nose heavy balance! 
Forget to add the weights when you are tired and preoccupied and in the dark and you may well regret it!
I've seen primary mirrors miss inadequate restraining clips
Then roll edge over edge right down the tube at high speed and straight into the secondary!
Having a heavy camera doing a nose dive as well could be an expensive mistake.

[johninderby]

I’ve fpund the size of the alt bearings has a big influence on the sensitivity of balance of the OTA. The small dia. bearings used on the SW do make it more sensative. With my Bresser changing eyepieces doesn’t affect the balance. Only if changed the finder setup would I need to rebalance and sliding the tube up or down in the rings only takes seconds. 

 

Edited October 4, 2020 by johninderby

[NGC 1502]

1 hour ago, johninderby said:

I’ve fpund the size of the alt bearings has a big influence on the sensitivity of balance of the OTA. The small dia. bearings used on the SW do make it more sensative. With my Bresser changing eyepieces doesn’t affect the balance. Only if changed the finder setup would I need to rebalance and sliding the tube up or down in the rings only takes seconds. 

 

That looks an excellent Dob mount.   Simple, strong, fuss free, bunch of eyepieces and a star chart = bliss.  Did I mention dark sky and no clouds.......😳