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need advice on movement of mirror box

Started by BABOafrica, 09/16/2012 09:24PM
Posted 09/16/2012 09:24PM Opening Post
I'm in the middle of building a truss dob for a f/4.5 17.5" mirror.

I finished building the assembly of mirror box / rocker box / platform yesterday.

Without putting in the mirror yet (which will weight 29 lbs) the mirror box is heavy to push and pull. I did not detect any stickiness in the motion. It turns smoothly. But I was surprised that I need to use a bit of effort to turn the mirror box.

The mirror box alone weighs about 20 lbs (without the mirror) and the rocker weighs another 20 lbs. I guess it's just the inertia of the 40 lbs.

Perhaps it is all too heavy. At least the seconday cage is light: a mere 5 lbs (without eyepiece or finder scope).

I nothing to compare with (except pictures in books!!! and my own home-made 10" dob). I was wondering if people who have a big dobsonian have a similar experience.


In lumine tuo videbimus lumen.

8O Home-made 10” Dob / Home-made 4” refractor

EPs: Konig 32mm (1.25") / Zhumell WF 30mm (2") / Nagler 13mm T1 / Orion Sirius Plossls 25 & 10mm / Zhumell Plossl 9 mm / Meade MA 9mm
Posted 09/17/2012 07:40AM #1
You cannot finish the mirror box until you have built the whole "tube" structure.... (unless you are a really good engineer to know just what the weights are and do a whole lot of good calculations.)

THe ease of movement depends on the friction and stuff in the bearings, but-----it depends a lot also on the balance of the assembly. If you do not have the whole tube built, you have nothing to balance the weight of the box.

What happens is that there is this number (moment arm????? correct me you engineer types) where the five pounds of secondary cage/focuser/secondary/etc. has to balance the 20 plus 29 some pounds of mirror box/mirror. It does this by being put on a lever some distance away. The fulcrum around which this assembly rotates is the center of your bearing.

To balance a 50 pound weight one foot from a fulcrum point, you need to put those five pounds ten feet away from the fulcrum point. (50 x 1=5 x 10) Of course, the weight you are trying to balance is not at the fulcrum point, so you must be even further away. Multiply the weight of the secondary assembly by the distance from the fulcrum to get that magic number. Then multiply the weight of the mirror box from the fulcrum point (bearing center). These two magic nbumbers must be equal. YOu get them equal by manipulating the position of the bearing, thus the distance from the centers of weight to the fulcrum point.....(By the way, it is not that simple because the trusses themselves have weight--which weight is distributed along their length!!!)

THerefore, you need the tube assembled before you locate the bearings.

What you have to do is not put the bearings on.......
Assemble the whole tube, mirrors, trusses, finder scope and all, and test that the thing reaches focus as it should.

Then put the assembly on it side on a wooden dowel or pipe so that it will balance properly. (Find its center of mass.)

That center of mass is where the center of your bearing should be. ONce you know it, put the bearings on the mirror box.

By the way--it sounds like you may need to add something to the top of the secondary cage to change the balance point.

Posted 09/17/2012 11:36AM #2

My 17.5 inch truss dobb has a 34 pound mirror and 1 inch thick trusses.

I made my side bearings out of ply smaller than in the dobsonian handbook (kriege and berry)(18 inches)and to achieve balance they can be easily moved up or down on the mirror box.

I then use a chain on the back of the mirror box to get good balance. Works like a treat hope to get it out later this week.

I saved some weight with the mirror cell over the metal one using alu for the 18 point suspension and ply on the bottom of the box. This works well. My secondary cage is definetly over 5 pound with 2 inch focuser big secondary 50mm finder etc and a decent eyepiece.

I make this up with the heavy chain on the mirror box.

Posted 09/17/2012 11:38AM #3
you can improve the movement of the bearings with car polish its better to be stiff than too loose.

No backlash is the sign of success. My setup is formica on teflon and is very smooth.

That dobsonian handbook could be useful to you it covers balance etc very well.
Posted 09/18/2012 05:27AM #4
Many thanks for all the advice. However, I don't need help with the altitude change. It's the azimuth....

Unforuntately, it's my own lack of clarity at fault here.

My problem is not the change of altitude, which actually seems to be working quite well. My problem is with the azimuth.

As I was saying, changing seems smooth enough. I do not feel any stickiness. But it takes quite a bit of strength to make the whole mirror box / rocker box assembly turn, from north to west or north to east, etc.

Adjusting altitude is easy. I get a lot of torque because, as Alex mentioned, I can press up or down on the secondary cage, which is a good 40" elevated above the mirror box.

But I can't get the same torque, at least not when the OTA is pointed more or less straight up, because I don't have much distance from the pivot point. Instead applying force at 40" away from the pivot point, I'm applying it 10" away from the pivot point.

So, I suppose many other Dob users have noticed this problem long before I bumped into it.

I can explain further with a diagram if necessary.


In lumine tuo videbimus lumen.

8O Home-made 10” Dob / Home-made 4” refractor

EPs: Konig 32mm (1.25") / Zhumell WF 30mm (2") / Nagler 13mm T1 / Orion Sirius Plossls 25 & 10mm / Zhumell Plossl 9 mm / Meade MA 9mm