Storage Box 

by Frédéric Géa

Under David Vernet correction


Transport and storage of the mirror are often done in a separated case. One should not neglect this accessory but should include it as a part of the telescope. The mirror will have some of its existence in this box and is thus appropriate to comply with certain rules in order to avoid damage. These risks are as much of a physical nature as chemical too.

 

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The box must protect the mirror from the shocks, dust, and remain chemically neutral to avoid any premature deterioration of its aluminum coating. The usual materials in this case are untreated wood, but not any wood. It is to better avoid any exotic wood, which often contain silica used in its manufacture. If one makes a point of using this type of wood, check that there is no silica to avoid any risk of scratches following a fall of the mirror. To avoid these accidental scratches, it is best to sand the wood with fine sandpaper, which will prevent a grain of abrasive from “jumping” onto the mirror (a passage on the surfacing machine or planner is also enough). Those wishing a quite clean surface can certainly use Plexiglas by internal doubling of the walls, but that is really not necessary to protect the mirror. In the same way to minimize any risk of chemical aggression to the aluminum coating, it is better to avoid materials (however well meaning) like the majority of plastics, polystyrene and laminated surfaces. These materials are likely to release their solvents or other products being likely to damage the fine reflective layer prematurely, either by their own action over time, or by the adhesives, which are used in their manufacture. To help avoid the risks of chemical attack, it is not advised to seal the lid of the case so that these possible solvents remain in the confined atmosphere of the box. The question
of the treatment of wood with varnish or paint is often tackled, this solution is not necessary to preserve the wood in good condition because it is not exposed to aggressive atmospheric conditions, moreover, paints or varnish are also likely to release their solvents which can damage the coating.

If, despite everything you absolutely must make a point of taking this risk, take care to give several weeks or even several months of drying time before putting your mirror inside the box. Despite everything what is known, as before, the mirrors of 400mm and more need a different support. Indeed, the three-point support for the rear of the mirror is adequate for smaller mirrors but for the larger ones, one can use a foam disk of high density on which to rest the mirror. Neoprene foam has good mechanical and the chemical neutrality qualities necessary to support the mirror safely.

 

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Box view, with some dimensions for a mirror of 368mm of diameter. 

In this example, the 4 corner holds on the bottom (out of 15 or 18mm plywood) were cut from squares of about 128mm square and then cut diagonally to make two. The isosceles triangles thus obtained are reduced to leave only two edges of about 80mm length. This makes it possible to keep a minimal contact with the mirror, and should be sized to leave an appreciable space between the edge of optics and that of the box. The 4 blocks on the lid ensure stability of the mirror while carrying the box by the handle. These were made from fir 18 mm square and cut to 45° in a miter box and works well. A height of 10mm (minimum recommended to give some " breathing " room) between the internal face of the lid and the mirror, is set according to:

1) The height from the bottom of the box and the mirror (ex: 5mm is enough to slip the fingers under the glass)

2) The thickness of the mirror

3) The height of the sideboards (which determine the plan on which the lid will be closed).

3) The position of the blocks attached to the lid.

The layout of measurements must be done as precisely as possible to guarantee a minimum of play between the mirror and its supports. Allow for some felt to help cushion the glass while in transport.

 

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