10" f/8 Next comes the primary mirror cell.

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Automated Mirror Cell Optimization or 'PLOP'

A 10" mirror of 40mm thickness does not need a complex, multi-point, support cell. Though it does need gentle support and restraint. Thinner mirrors may well need 9 or more balanced and carefully calculated support points to avoid flexure. 

The need for fan cooling and avoiding poor mirror cell practice does not require greater complexity. Two pieces of 3/4" plywood plywood will form the simple, primary mirror cell. I have found some suitable coil springs, coach bolts and matching wing nuts. The three domed, coach screw heads will have rubber glued to them where they protrude through the mirror support board. Then they will be covered in felt glued on top of the rubber. The screws will provide 3-point collimatable support at the required radius. Avoiding the necessity of adding separate support points.

It is important that mirrors can slide freely within the restraints of the side tabs. Even if the distance they move is almost theoretical. The felt will provide this freedom to move. I shall not be following the modern practice of gluing the mirror to the cell with silicone aquarium cement.  If the plywood bends or there is any differential expansion between the mirror and its backboard then the mirror may be temporarily strained or warped.

The difference between a fine figure and a poor optical figure is less than the change in figure due to warping. Astigmatism is likely too. ie. Bending of the mirror blank across a particular diameter. Causing the surface to no longer be a figure of revolution. The shape of the supporting mirror blank could become a pretzel or a potato crisp, optically speaking. Only millionths of an inch separate the finest parabolas from the run-of-the-mill optical surface.

It is important that the side restraining tabs do not project over the mirror surface. Which is an all too common practice. Light obstructing tabs will cause diffraction. Provided the main tube isn't pointed downwards then the mirror is quite safe from falling out of its cell. The side restraining tabs will be rubber covered and reach almost to the front surface of the blank. This will ensure the blank remains seated against its support pads. The centre of gravity of the blank will remain behind the support tabs even with the tube horizontal. Again, the rubber will be cut from an old, car inner tube. The side tabs themselves will be right angle brackets screwed to the mirror support board beneath the rear edge of the mirror.

The cell base board (or perhaps two laminated boards for greater thickness) will be round and fit the telescope tube nicely. The circle will have a central perforation for the mirror cooling fan. Since it is otherwise closed to the tube end, the backboard will direct the airflow from the fan efficiently. A bare fan hanging in fresh air has very poor efficiency. The air can easily curl around and around the fan edges (like an invisible doughnut) instead of being directed cleanly forwards and away from the fan to form a proper airflow.  

The mirror support board will also be perforated to allow the draught from the fan to reach the back of the mirror blank. The air will then curl naturally around the mirror sides and move on up the open tube. The draught will suck the boundary layer from the front of the mirror. (where it really matters) Once clear of the mirror the airflow will scour the inner walls of the tube before reaching the secondary mirror. Where it will scavenge any warmth from the elliptical secondary blank and its spider. Spider vanes which are warmer than the surrounding air will have their own boundary layers. Causing diffraction effects out of all proportion to their actual thickness.

The smallest air temperature differences, in the optical path, will have different refractive indices to the cooler air in the tube. As is easily visible if you look at the background beyond the top of a bonfire. The airflow from the fan is to prevent stagnant, warm air from clinging to the mirror surface, inner tube walls, the secondary mirror and its supporting spider. If air did not have a different refractive index when at slightly different temperatures then there would be no need for a fan. The Newtonian would then be an almost perfect telescope. Unfortunately life is not that simple and the warm air must be removed by using a degree of force. The trick is to know how much air movement is required under different temperature regimes. More is not necessarily better when it comes to fan speed and and quantity of the airflow required.  

Pictures will follow:

The flexible plywood sources, I hoped I had found, were dead ends. 8mm was the thinnest available from the builder's merchants. The other did not deal with the public. I may have to try and find full sheets of marine or aircraft ply and roll a tube in two parts. Or, rather, overlapping halves. I have never seen anything but 1.5m x 1.5m. Which is only about 5' square. The length joint could be hidden by a mounting ring. Ideally I would like to use epoxy to laminate two layers but it will be months before it is warm enough for normal use.

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