1. Field of the Invention
The invention relates to a light-weight mirror, particularly for astronomical telescopes, comprising a mirror plate made of fused silica and a cellular or honeycombed supporting structure made of fused silica or a quartzose material, which are immovably welded or fused to each other, wherein there are a plurality of open passages which start at the interface between the two and perforate the supporting structure.
2. Discussion of the Prior Art
Light-weight mirrors of this type are known from U.S. Pat. No. 3,728,186. In the light-weight mirror there disclosed, the supporting structure comprises a fused-silica plate having a plurality of through bores or passages. In that mirror, the mirror plate is sufficiently thick for viscous heated fused silica to penetrate in the form of lenslike protuberances into the openings of the passages as the mirror plate is being welded or fused to the supporting structure. With attendant enlargement of the cross-sectional area of the passages, the exponentially increasing penetration results in such deformation of the free surface of the mirror plate that it cannot be remedied by grinding and/or polishing and the entire mirror has to be discarded.
The present invention has been prompted by the growing need to reduce the total weight of the mirror as much as possible without sacrificing its good mechanical and optical properties.
This object of the invention is accomplished, in the case of a light-weight mirror of the type outlined above, in the mirror plate, from 2 to 20 mm thick, while being welded or fused to the supporting structure is supported by means of supporting elements which extend through the open passages and are provided at least at the end facing the mirror plate with a seating member which at elevated temperature does not react with fused silica and whose surface facing the mirror plate is plane, curved or domed, said supporting elements being subsequently removed. Seating members made of graphite have proved suitable, However, seating members made of zirconium oxide may also be used.
The end of the supporting structure remote from the mirror plate is advantageously welded or fused to a backing plate made of fused silica and provided with openings which are in register with the open passages of the supporting structure. In joining the backing plate to the supporting structure, too, it has been found helpful to use supporting elements of the type employed in accordance with the invention in joining the mirror plate to the supporting structure. Portions of the supporting elements remaining in the passages after welding or fusing can readily be removed if said elements are made of graphite, as is preferred, by incinerating them in a stream of oxygen at about 800.degree. to 900.degree. C.
Generally the cellular or honeycombed structure has pores or cells measuring at least 30 mm, preferably 50 to 150 mm in the transverse direction, i.e., diameter in the case of cylindrically shaped pores or cells. The walls of the honeycomb or cellular support structures are generally 1 to 6 mm in thickness, preferably 2 to 3 mm in thickness.
When domed seating members are used, lens-like depressions form in the underside of the mirror plate as it is being welded or fused to the supporting structure, and these depressions enhance the mechanical stability of the light-weight mirror. Similar depressions may also be produced in the backing plate on the side facing the supporting structure.
Light-weight mirrors in accordance with the invention offer the advantage of a substantial reduction in the overall weight of the mirror since the thickness of the mirror plate can be considerably reduced and the cross-sectional area of the open passages of the supporting structure can be increased to several hundred millimeters without there being any likelihood that the free surface of the mirror plate will become uneven and that the mechanical stability of the light-weight mirror will be reduced to such an extent that its fitness for use is impaired.