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A SIMPLE MIRROR CELL

An optical element must be properly held or it may not perform well. Properly mounted achromatic objectives and even eyepieces should always rattle a little when shaken. If these optics are mounted tightly, they will sometimes become pinched and deformed. I remember a Kellner eyepiece I bought some years ago that gave a warped image. I defocused a star and saw a pinched and oval set of rings. When I shook the eyepiece I heard nothing. Then I released the lower retaining ring and all was well. The outer retaining ring on refractor objectives must also be loose. I gently set up the outer ring on my 6",_f/15refractor when I travel with it, but then loosen it slightly when I observe. It makes a whale of a difference. Mirrors are perhaps even more sensitive and require thoughtful mounting in carefully designed cells in order to perform to high standard; and thoughtful use as well. Despite it's apparent simplicity, a reflecting telescope is a delicate and subtly complex instrument that has the potential of yielding magnificent images, but it requires care in use and an intelligent, knowledgeable user.

The cell pictured above has been used in my telescope since 1994 and is for a 10", f/8 Newtonian. It is of very simple design and is constructed of two pieces of 3/4" plywood and a few easily acquired metal parts. You will notice that this cell also has the advantage of not having the usual crude and optically destructive mirror clips. These are a crass abomination and contribute to much of the diffraction seen in reflecting telescopes. Mirror clip diffraction can be seen as broad swaths of light emanating radially from a bright planet or star, unlike spider vane diffraction, which is a thin spike. The result, among other things, can be a washing out of delicate planetary detail. To operate properly, the pupilary opening of an objective (the mirror edge) must be clean and free of defects, not cluttered with three pieces of metal, often with square corners (shudder!). Often these mirror clips are not even padded and bear directly against the optical surface, scratching away the coating. We need to begin to build better reflectors in order to match the performance of refractors and there is no better place to start than the mirror cell.

In the form seen above, the base of the cell is a square piece of plywood that attaches onto the bottom of my scope with four bolts. The cut out center allows for a free flow of air for cooling (thermal stabilization) the mirror. The mirror rests on three pads. These pads consist of felt superglued to inner tube rubber squares that are themselves glued to the heads of the three carriage bolts that are used to hold and adjust the upper mounting plate. The rubber makes for a conforming surface and the felt allows the mirror to slide easily so as to avoid grabbing. Double sticky tape works as well as superglue. Note the four upright lateral supports. These are made of 1/4" X 1" mild steel bar stock cut to length with a hacksaw, drilled tapped and bolted into place as shown. I drilled holes laterally into the base plate to take the bolt and then drilled a hole through from the top in order to insert a nut.

Above, the cell is shown disassembled. At the left is the bottom plate. On the plate are the washers and compression springs. At the right is the top plate, showing carriage bolts and washers.

Here is seen the method of holding the mirror in place. Between the lateral support and the side of the mirror is a pad. This pad is not actually glued or firmly stuck to the mirror but only rests against it allowing for expansion and contraction of both the cell and the mirror without the possibility of constraining or "pinching" the mirror. The current practice of gluing mirrors to a particle board base with bathtub caulk makes me cringe. I'm all for simplicity, but not at the price of performance.

Here is a more detailed view of the lateral support and pad assembly.

The pad is made up of a 1" X 1" X .064" piece of hobby brass to which a steel washer has been soldered. This is in turn glued to a square of inner tube rubber with bathtub caulk. The screw is made from a piece of threaded rod. The rod is cut to length and then a cut is made in the end to form a screwdriver slot. The screw threads through the lateral support (which has been tapped to receive the screw) and the end fits into the washer hole, thereby holding pad in place while remaining loose against the side of the mirror. The face of the rubber was covered with caulk and pressed against the mirror and allowed to dry in order to form a curve to match the mirror side, but was then peeled off. The pads just lightly press against the sides of the mirror.

Another close-up showing the screw in the lateral support and the pad against the mirror side.

Here is a mirror that has had it's edge darkened with a black marker. This helps eliminate scattering and forms a clean entry pupil.

Oh, yes. To those who want to know what will happen if they turn their telescope upside down using this type of cell, I can tell you with certainty that the mirror will fall out. That's right, it will fall out and likely be damaged or broken. I make a practice of never turning my telescope upside down, or anyone else's.