Lightweight mirrors

One way to categorize applications for lightweight mirrors is to use the altitude of the system as a criterion. This leads to three different categories, each being characterized by specific cost tradeoffs. 

Space applications obviously offer the highest cost savings if a lightweight mirror is used instead of a solid substrate when considering the cost of launch at several 10 000 kg (depending on the type of spacecraft and type of orbit for satellites). In addition, the feasibility and maintenance of a fully active or semi-active support is certainly much more complicated for a space-based mirror than for a ground-based or an airborne system. Examples of space-based systems with mirror substrates include weather, mapping, and surveillance satellites as well as optical and X-ray astronomical satellite telescopes and space probes. 

The cost savings offered by the applications will determine which method of lightweighting is applied in a specific case. While in certain applications weight savings of up to 70% achievable by machining may be sufficient, other applications may require lightweighting factors of 85% and more, which can be obtained with fused mirrors. 

Assuming a common shaft diameter of at least approximately 15 mm, the ratio of the cored hole to the undercut borehole is about 1 1.5. 

CNC-processed lightweight mirrors are integral parts in a number of advanced technology telescopes. Table 4.4 gives a short overview. 

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The SiC/aluminum (SiC/Al) MMCs are reinforced with SiC particles (P) (Fig. 4), whiskers (W), fibers, or monofilament (MF). In 1988, an aluminum tube in the catamaran Stars and Stripes 88 was replaced by a lighter SiCp/Al MMC tube having a 30.5-cm outer diameter [3]. Lightweight mirrors for telescopes have also been fabricated from SiCp/Al MMC foam [47]. 

Diirr, J.K. Honke, R. von Alberti, M. and Sippel, R. Development and manufacture of an adaptive lightweight mirror for space application. Smart Mater. Struct., 12 (2003), pp. 1005-1016... 

Fig. 4.45. Zerodur lightweight mirror produced by grinding - study with different kinds of holes...

Apart from the fabrication of lightweighted mirror substrates, the fusion technique is also well suited to fabricating other complex components, with cavities in particular, when Zerodur is the material of choice but conventional processes cannot achieve the proper geometry. Weight reductions of more than 85% can be achieved by using the fusion technique. 

Fig. 4.46. Zerodur lightweight mirror sample of 485 mm in diameter and 12 mm in height, manufactured by fusing single ribs, faceplate, and backplate. The weight reduction is 80%...

Fig. 4.65. A lightweighted mirror substrate made by the R.E.O.S.C. Company in France, for the Earth observation satellite SPOT...

Apart from the fabrication of lightweighted mirror substrates, the fusion technique is also well suited to fabricating other complex components, with... 

Fig. 4.51. Zerodur lightweight mirror sample 664 mm in diameter and 72mm in height, manufactured by direct casting by the use of a ceramic fiber mold. The weight reduction is 57% (faceplate thickness 12 mm, rib thickness 10 mm, cavity depth 60 mm, cavity width 50 mm)...

Applications. Telescope mirrors (currently up to 8 m diameter), lightweight mirrors (for space deployment), precision instrument stages, temperature-inert mountings... 

One usually considers a lightweight mirror blank to reduce costs or to improve thermal control. This is particularly true for large telescopes. Reducing mirror weight... 

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