Balance point 12" truss

[Jone5y]

Hi All,
This is my first forum post so please bear with me.

I am currently in the process of rebuilding my 12" f5.2 dob to a truss design for greater transportability and hopefully better performance overall. The problem I currently face is that my balance point has ended up being some 150mm higher than the top of the box that supports the primary, trusses etc.
I had originally intended to use altitude bearings around 250mm diameter.

My question is whether or not I can (should) use larger bearings such that the centre of the bearing equals that of the balance point or counterweight the bottom end? I would need in excess of 7kg for this. Would a combination of the two be best.

Please let me know if more information is required to solve my dilemna

Thanks in advance
Steve

[tnott]

I would go with the oversize bearings and make them crescent or U-shaped like most of the new wave of atm scopes. You don't want to carry around an extra 7 kg for no reason. The only disadvantage is that they would not have a point of rotation to mount encoders on but that can be solved too. Also need to make sure that they do not get too long, thin and springy. Many people make them removable but for a 12" it's probably not necessary. Some examples:

http://www.fiskemiles.com/buildinga22inchdob/22-inch-dobsonian-telescope-finished.php
http://www.biophysik.uni-freiburg.de/Reiner/ATM/lowrider/lowrider_e.html
http://www.stathis-firstlight.de/atm/kyklopaseng.htm
http://freenet-homepage.de/atmpw/Bilder_20_Zoll/Bilder_Aufbau/html/default.html

A spring type "virtual counterweight" has also been used successfully.

Good Luck

[Jone5y]

Thanks for the prompt reply tnott, The option you suggested is also my preffered one and I have some Al tube leftover that I can use to prevent the thin springy issue.

Thanks again
Steve

[rmcpb]

Go for the oversize bearings. Have a look at the avator for jayscheuerle on CN to see an extreme example.

[jayscheuerle]

Hah! I never expected THIS scope to be shown as an example for anything worth doing. :P

For a scope with extremely large bearings that is functional as well, check out Jeff Morgan's http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/1140842/page/0/view/collapsed/sb/5/o/all/fpart/1/vc/1

The Frankenscope of mine was part experiment, part gift. It makes a better conversation piece than it does a functional piece of equipment. :D - j

[Jone5y]

Cheers for the advice guys,

I've gone with the oversize bearings as suggested (albeit slightly tamer than yours jayscheuerle :)). Final assembly should be complete tomorrow - I shall post some pics then. Hopefully I will have eliminated the probs I had with solid tube builds 1 and 2.

Steve

[Ian Robinson]

The balance point is easily moved lower - all you need some suitably sized cast lead weights attached as low in the mirror box as possible.

If you under the calculation of moments and the centre of gravity you will be able to work out how extra mass you need down there.