Mirror Cell Design with 4-Point Collimation

by Ed Stewart <stargazerskymtn.com>

The rotating tube design of the Dobsonian described in the link on Installing the DobDriver II did not lend it self to a commercial mirror cell. Besides there were references in several back issues of Telescope Making that said there weren't any good ones available. Also it did not lend itself to the usual sling found in the traditional Dobsonian either. Since it was obvious that the cell would have to support the mirror in all dimensions as a single unit, I decided to try something different in confining the mirror in its side-to-side position. Also decided to use a square layout of four collimation points similar to that mentioned in Telescope Making #6 and to use push/pull bolts to make the collimation adjustments. Note:Telescope Making is no longer in print.

Side-to-side Mirror Alignment

Since I had a surplus of Formica laminate on hand, I decided to make use of it. Two disks of 1/2" or 13mm Baltic birch were cut on a router table to 1/8" or 3mm larger diameter than the mirror and glued together with very exact alignment of their edges. Work inside the cell such as the flotation plates and ventilation holes were completed. Then a strip of laminate was cut the length of the disks circumference and the a width sufficient to go from the full width of the combined disk to 1/4" or 6mm short of the mirror's surface-about 3" or 76mm in my case. To insure that the laminate would wrap around the disk accurately, I spaced above the disk a scrap plywood disk the same diameter, on a bolt through their centers (the hole used on the router table) in the position of the upper part of the mirror. The laminate was then glued and screwed around the disk forming a bowl-like container for the mirror. A 3" X 1/8" curved piece of aluminum was screwed in place over the laminate joint to strengthen it. A line of 1/8" holes had previously been drilled every inch along the length of the laminate and spaced across the width so they were centered along the mirror's thickness.

I rubbed silicone grease on the outside surface of a 5 mil plastic strip and then wrapped it around the mirror to create a small gap for the next step. After centering the mirror in the laminate circle, I squeezed silicone rubber adhesive or RTV through the line of 1/8" holes to form a pad of between the laminate and the mirror. Even though the silicone grease prevented the RTV from sticking to the 5 mil plastic, the mirror was very slow to dump out of the cell. Once out, I removed the plastic strip and put the mirror back into the cell by lowering the cell onto the upside down mirror. No mirror clips are necessary as the mirror only comes out in a completely face down position and then very slowly. In practice, this laminate/RTV method works beautifully after 10 years of use. Hint: for heavier mirrors, two or more layers of laminate could be glued around the cell for greater strength.

Four Point Collimation Adjustment

The usual triangular arrangement of collimation points does not allow for adjustments at a right angle to each other as you would have on an "X­Y" chart. An arrangement of four points in a square does provide for right angle adjustment, but the advantages are more than would first seem apparent-six axes of adjustment instead of three that comes with the triangular arrangement. By moving in or out with any side-by-side pair of adjustment points causes the cell to pivot on the axis running through the other two for a total of four axes (It would seem that the parallel axes are duplicated, but should one axis bottom out of inward adjustment, the other parallel axis can be used.). With diagonally opposite pairs, one is moved in while the other is out resulting in a pivot on the axis running through the other two diagonally opposite points for the other two axes. Example: pushing on point A and pulling on point C will cause a pivot on the Z Axis. A similar movement will occur on the K Axis by adjusting points B & D in opposite directions. In total, there are six axis of adjustment rather than the three with the standard 3-point collimation design.

In practice, a collimation adjustment bolt(s) is pulled (turned counterclockwise) to move the mirror spot image toward the direction desired. Turning an collimation bolt the width of screw driver slot or 3/32" will move the mirror spot about one­tenth of its dimension in the Cheshire eyepiece. The results are very predictable and long lasting. After trailering the scope for many thousands of miles, it normally passes the Auto Collimator test with no adjustments. Probably an equal share of credit goes the rigidity and strength of the Baltic birch plywood.

Making the Cell Removable

Note: The following is specific to my design, but with a little ATM thinking, you can devise a variation that will work for your scope.
Diagrams: Inside View   Outside View

Since my rotating tube design does not breakdown like the Serrurier truss tube design, I believed the heavy mirror and cell unit should be removed for transport to avoid over stressing the cell. And as it turned out, it would have been too heavy for me to lift into the trailer by myself. Using the drill jig for the tube's rings, blind (flat bottom) 1" holes were drilled 3/4" deep into the cell support 1" plate with a Forstner wood bit. When the cell is mounted onto the skeleton tube, the eight stubs of 1" tubing seat firmly at the bottom of these cell support plate holes and at the same time the support plate presses against 1/4" spacers attached to the last ring in the skeleton tube.

The use of the drill jig makes the fit onto the tubing stubs so exact that more than arm strength is needed to remove the cell. This was solved by using push/pull bolts around the outer edge of the cell support plate in line with the surface of the last ring in the skeleton tube. T-nuts were placed closed to the tubing stubs to apply the force near the points of resistance. A full set of eight push/pull bolts were installed but in practice only four are needed to pull the cell on and three to push it off. The bolts used to pull the cell on are removed at the end of an observing session and transferred to the push T-nuts rather than being duplicating.

  

Mirror Spotting

The instructions that came with the Tectron 3 piece collimation set said to put a 5/16" square black spot in the center of the primary mirror. The thought of putting something in the middle of that beautiful virgin mirror was intimidating to say the least. OK, I put some glue on a scrap piece of black plastic and now how do I determine where to put it precisely without sliding it around? One approach is to cut a circle of paper the diameter of the mirror. Fold it in half three times so that it is a pie-shaped piece. Cut the point of the pie-shape (the original center of the circle) off just big enough for your mirror spot to pass through when the paper is unfolded and laid on top of the mirror.

Note: If you might use a laser colliminator, then use a blacken circle with a hole in the middle like a paper re-enforcement instead of the black square. This way the laser can shine off the center of the mirror, but you can still use the optical tools to center the reflections. One source for a ring to spot the mirror is the unused position on the U.S. postage self-adhesive stamps--it has several concentric rings to choose from. I used a black felt-tip marker to blacken it. Also don't mark the diagonal's center or the laser won't work.