DEVELOPMENT OF MIDIUM-SIZE COOLED LIGHT-WIGHT SILICON CARBIDE MIRRORS FOR INFRARED ASTRONOMICAL SATELLITES

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We have established the process to make a mirror blank of silicon carbide(SiC)porous core and SiC CVD coat based on the measurement of 160mm test mirrors at cryogenic temperatures. We have found that the CVD coat has to be applied on the entire surface of the mirror to prevent the polishing liquid from permeating into the blank. We have confirmed that the mirror made in this way does not show any appreciable deformation even when it is cooled down to liquid helium temperature. We have also examined the effects of the glue and found that it would not make significant problems if the metal part is well treated thermally in advance. We have fabricated a 700mm SiC mirror based on these processes and tested it at liquid helium temperature together with the supporting parts attached. We have found that the non-elastic deformation occurred due to the difference in the coefficients of thermal expansion between the supporting parts(metal)and the mirror at cryogenic temperatures. This will still not make a significant problem for use in infrared wavelengths and we have concluded that the main purpose of the present research has been achieved. However, details of the deformation process, particularly its repeatability, and its effects at room temperature in the adjustment process have to be investigated in detail in next step.