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Modifying a Mirror Cell to Support a Royce Conical Newtonian Mirror

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The Royce conical mirrors represent a great advance in Newtonian mirror technology. An embedded central nut allows stress-free mounting of the mirror, and the shape can be conical because there is zero stress induced on the thin edges by the cell. The mirror can be half the weight of a standard mirror, and also cools much more rapidly and without interference from support structure. This report shows how to modify a typical mirror cell designed for a standard mirror by adding a central mounting plate and struts. The resulting cell is extremely stiff and completely open in the back, allowing rapid cool- down both through direct convection and by conduction of heat into the supporting structure.

Here is a photo of the disassembled cell with the central mounting components already installed:

The cell is that supplied with the (discontinued) Astrozap Newtonians, and is identical to that found on the Meade SN-10 optical tube assembly. In these cells there is a central plastic plate that has been removed from the caddy ring. The idea was to create a structure that would attach to the caddy ring in a reversible fashion, should the cell ever be needed for a standard mirror. After much thought and rough calculation I decided on the following scheme – a central plate 4” in diameter supported by three-sided aluminum box struts tangentially – here is my working diagram:

Prototype drawing:

Here we are looking at the ring caddy from the back side. The struts are mounted with 10/32 machine screws and lock washers so that the closed side of the box strut is held tightly against both the caddy plate and the central plate. This greatly stiffens the plate itself as well as provides a rock-solid central support for the conical mirror. In my original conception there were two screws on either end of each strut but this proved to be unnecessary. There is a complication in that one of the locking posts on the cell body must now bite on the inside of the box strut rather than on the caddy plate itself. This required careful measurement and assembly so that the locking post would not impinge on the edge of the strut. Here are some pictures of the assembled caddy plate.

Mirror side:

Cell side with adjuster posts and spring retainer bosses showing:

Close up of mounting screws, mirror side:

Close up of mounting screws, cell side:

Central plate, cell side:

Here is the assembled cell – mirror side, with ½” nylon mirror mounting post – note the flat-head countersunk screws here:

Assembled cell – rear view:

Note how the locking post closest to the camera nestles between the open sides of the box strut. Here is the mounted mirror in the telescope from the back:

The central plate itself was made from an outdoor electrical outlet 4” steel cover purchased from Ace Hardware stock. Two holes were provided and two needed to be drilled. To ensure that the central hole was dead center of the entire caddy plate assembly, I placed the assembled structure on a hi-fi turntable and nudged it until it was perfectly centered as it rotated, then used a Sharpie to mark the location of the center on the plate. I drilled this out successively to larger and larger sizes until reaching the required ½ “ size. The resulting hole is centered to within the accuracy of my digital micrometer.

The materials were purchased for under $12. Assembly is very simple but great care must be taken to measure and drill as accurately as possible using small gauge bits and a variable speed drill to create pilot holes. Note that each mirror cell will have to be addressed on its own merits, and this method may not be suitable for all.

Here is the mounted mirror resting comfortably!

And finally one of the assembled telescope with Royce 10” f/4.5 mirror installed, number 509!


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