MIRROR CELL

Jack Schmidling Productions, Inc.
18016 Church Road ~ Marengo IL 60152
Phone:815 923 0031 ~ Fax:815 923 0032 ~ Email:arf@maxx.mc.net

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Overview

One of the frist decisions that confronts the telescope builder, whether he makes or buys the primary mirror is how to mount it. There are as many ways of doing it as there are telescope makers and a wide variety of commercial products to choose from also. The mirror cell, as it is called can be knocked together from scraps our designed on a computer and provide a sewer through which all sorts of time and money can be spent. The design I use is closer to the former but works as well as the latter, at least for smaller mirrors.

The design requrements of the cell are uncompromising but not very difficult to achieve, at least again, for modest size mirrors. The larger the mirror is the more difficult it gets to meet the requirements which are as follows:

1. The cell must hold the mirror rigidly so that it can not change position when the telescope is move to different positions in the sky.

2. It must not apply any pressure or pinching to the mirror which could in any way distort the very precise surface figure.

3. It must allow for expansion and contraction of the glass while fulfilling the above requirements.

4. It must allow for changing the tilt of the mirror in order to align it with the rest of the optical system.

5. It must allow for air to circulate freely through the tube and around the mirror.

6. It must be easily removable for cleaning in a completed scope and the mirror itself must be easily removable from the cell for further polishing and figuring if star testing is part of the mirror making procedure.

The following design does all of the above in addition to costing next to nothing to build.


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This view shows the cell without the mirror mounted. It consists of only two pieces. A round disk of 1/8" Masonite, the same diameter as the mirror and a triangular piece of 1/2" wood or plastic whose outside diameter is the same as the inside diameter of the telescope tube. Three screws, springs and plastic knobs or wing nuts complete the parts list.




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The screws are glued into the Masonite disk and pass through holes in the mounting base after also going through appropriate springs. The knobs on the back of the base will then draw the respective quadrant of the mirror back when tightened and the spring will pull it forward when loosened. (The toothpick is optional)


I use threaded brass rod for the screws but any screw will do if you hacksaw the head off. The hole in the Masonite should be a bit smaller than the screw so it can be screwed in with a drop of Super Glue. When the glue sets, file or sand the surface smooth.

The holes through the base should be enough larger than the screw to allow freedom from binding on the threads. If wood is used for the base, plywood should be avoided because it does not like screws into the grain which are later required for mounting the cell in the tube. The holes are of course, 120 degrees apart. The ends of the base can be filed or sanded to more or less conform to the radius of the tube if desired.


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Cell With Mirror Mounted and Ready for Star Testing

The topic which gets the most heated is the method of mounting the mirror to the cell. I find running plastic electrical tape around the mirror and Masonite disk hard to improve on. It's easy to remove for testing, does not block any portion of the surface of the mirror and seems about as permanent as necessary.

For larger mirrors, blobs of silicone glue on the Masonite in strategic locations will provide load equalization and possibly some peace of mind if the glue has not set when the mirror is placed on but removing it becomes a real pain. My inclination would be to put pieces of wax paper between the glue and the mirror so that the mirror can be lifted off when the glue sets and rely on the tape to hold the mirror.


The cell is mounted in the tube with three wood screws into the ends of the triangular base

Most everything else is common sense and one just needs to use what works.

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