Many of us have made mirror cells for our scopes. Lets look at the part that moves...the part that controls mirror position and alignment. Here is a generic representation of the collimation bolt in a typical mirror cell, especially for a larger mirror, with open tailgate design. A threaded rod (red) passes through two tapped holes in metal square tubing (gray). A pivot triangle, such as you would see in a nine-point support (green) is held in place between two lock nuts (yellow). In order for the triangular element to pivot freely, to support the mirror equally at all points, some slop/space is needed between the two lock nuts. And if we remove the top locknut we will see....

 

...that there is also slop between the threaded rod and the hole in the triangular element. Again, this slop is to permit pivoting motion by the triangular element.

 

My initial plan is to attempt to use main mirror movement/focusing to keep the upper tube assembly simpler and lighter. (I feel I can more easily make a complex/moving system in the larger main mirror cell, as compared to attempting it in the far smaller spider/CCD holder.) However, to implement main mirror focusing, I can not use this typical 'sloppy' mirror cell design (which has served me well for years in my visual-only scopes).

What options do I have? Can I use rolling element bearings to reduce slop, yet keep friction low and stiffness high? A bearing to carry the pivoting element will need to allow a range of motion of at least several degrees. What bearing type(s) and configuration(s) will allow this, while keeping the mechanical deisgn simple and inexpensive? (I think the approach of Bruce Sayre, with ball joint bearings, is a workable idea, but these are teflon/sliding bearings, not rolling elements. Do rolling element bearings similar to this approach exist?)

What about controlling slop in the threaded rod as it passes through the square tube? It's an easy matter to use rolling element bearings to constrain a rotating shaft, but how can I use rolling element bearings to constrain a shaft that moves axially while it rotates? Can I merely drill and tap an undersized hole, and hope that a 'snug fit' of threaded rod in the square tube will acceptably reduce slop? Can I use ball bearings to constrain radial motion, and hope the threaded rod can move smoothly, in an axial manner, through the inner bore of the ball bearings? Will I be forced to use a more complicated approach, such as a lead screw to establish the motion, and a linear bearing arrangement to constrain motion in only one axis?

There appear to be several different tasks I need to address with main mirror focusing:

1. Collimating the mirror.

2. Moving the mirror up/back for focusing.

3. Constraining the motion so that it is only axial motion.

The typical amateur mirror cell performs task 1 pretty well, does not need to perform 2, and is a bit sloppy when performing 3. My design needs to accurately accomplish all three.

 

Compliant Hinges

Above I mention my preference for rolling element bearings. Maybe we should investigate compliant (bending or flexing) hinges. This may provide us some valuable design ideas that are better for our application than rolling element bearings.

Here is an application of compliant hinges from Tony Owens for main mirror focusing. The diagrams are directly applicable to many commercial Schmidt-Cassegrain telescopes, but the general principles are applicable to other designs as well.

What about if you have a big Newtonian with a square (truss) tube? No problem. Tony has modified the design a bit...and also made the clamping arrangement for the flexible disks somewhat simpler to fabricate.

 

Collimation

Here is a coaxial, push-pull collimator that Don Clement designed. It was intended for a C-14, and uses no springs (which is typical for a push-pull arrangement). Collimation controls are accessible from the rear of the telescope.

 

All feedback is encouraged!

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Last update: 21 Dec 2002