Glass manufacturing – Processes – With pore forming in situ
Patent
1981-01-26
1982-12-21
Lindsay, Jr., Robert L.
Glass manufacturing
Processes
With pore forming in situ
65 55, 65106, 65107, 264 17, 264 454, 350310, C03C 1702, C03B 1906, G02B 508
Patent
active
043647634
DESCRIPTION:
BRIEF SUMMARY
The present invention is related to a method of making mirror blanks. The mirror blanks according to the invention can be utilized for making mirrors of optical quality, by what is understood that it is a matter of precision mirror, whose trueness to shape is better than ca 0.001 mm. The mirror blanks according to the invention can also be utilized in other reflecting systems, as e.g. solar mirrors for recovering solar energy, mirrors for radar purposes, etc.
Light optical mirrors are desired in many contexts, as mobile instruments, instruments subject to acceleration stresses, e.g. in airplanes and space vehicles, space telescopes, optical scanning systems, solar heating systems, etc. A great problem with such optical mirrors is the weight of the mirrors. A common rule of thumb is that the thickness of the mirror should not be below 10% of the greatest geometric extension of the mirror. A plane mirror having a diameter of 300 mm should thus have a thickness of 30 mm, and the weight of such a mirror is ca 6.4 kg. Such a mirror has great moments of inertia and makes great demands on a control equipment if such is used for adjusting the mirror. It is known to reduce the weight of a mirror by utilizing in making the mirror the so-called surface adhesion method, implying that the polishing is make in the usual manner to the best possible result. Then one of the polished surfaces is pressed against a master surface being very true to shape, which during the continued polishing gives support to the thin mirror material so that the same will not be deformed by the polishing pressure. This method of making mirrors is expensive, and the finished mirror has a low rigidity of shape.
Another known method of reducing the weight of a mirror is shown in the Swedish patent specification No. 70.08559-2, according to which the back of the mirror blank is provided with a plurality of recesses. The surface area of the recesses must not be too large, for the polishing pressure used in polishing the mirror causes a depression of the mirror face and leads by that to a non-planar face. The practical weight reduction is ca. 25% at its maximum. Another known manner of reducing the weight of the mirror is to connect front and back plates to each other by means of a plurality of rods or tubes of glass. The units are fused at ca. 900.degree. C. The enclosed cavities should be vented, since if not the pressure of the enclosed air, which varies with temperature, will influence the planar condition of the mirror face. As above, the area of the cavity must not be too large, for then the face will yield under the polishing pressure, and the underlaying structure can be traced in the polished face. Cf. in this context Swedish patent specification No. 71.00220-8. Another mirror having a low weight is the so-called replica mirror, which consists of a basic material of glass, metal or ceramic. The future mirror face is machined, e.g. by milling or grinding, to a plainess of ca. 0.01 mm. Through a casting process against this very precise master face the mirror face is moulded directly against the basic material. As casting material a polymer epoxy type material is used, and because of this the mirror quality does not normally satisfy the requirements of precision optics. The manufacturing process does not comprise any polishing, and accordingly relatively large recesses of the type shown in the first-mentioned Swedish patent specifications can be made. The weight reduction thanks to the recesses can be rather great, ca. 40%.
In U.S. Pat. No. 4,035,065 a mirror is disclosed including a cellular bed or substrate 4 laminated with a reflecting element. The reflecting element which consists of a glass slab having a silvering, is glued to the substrate. If the mirror is to have a concave form the substrate has a matching concave form, and the plane glass slab, the under-side of which is provided with a layer of glue, e.g. epoxy, is sucked against the substrate by means of vacuum until the desired contour is obtained. The vacuum is retained until the
REFERENCES:
patent: 1653053 (1927-12-01), Lyndon
patent: 2268251 (1941-12-01), Haux
patent: 3272686 (1966-09-01), Smith et al.
patent: 3505436 (1970-04-01), Krug et al.
patent: 4035065 (1977-07-01), Fletcher
Eslinger Lewis H.
Lindsay, Jr. Robert L.
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