This is a review of the demonstrated capability to produce ultra-smooth super-polished surfaces (< 2 angstroms rms) when a reaction bonded silicon carbide (RB SiC) mirror is coated with a SiC coating. Also covered is the viability of this design choice for large scale lightweighted mirror blanks. A super-polished surface finish on bare RB SiC is not attainable due to the difference in hardness of the two phases, SiC and up to 30% silicon. Therefore, deposition of an additional layer of SiC onto the figured RB SiC blank is required. This technique has been demonstrated and can result in meter class optics with a surface roughness in the regime of less than 2 angstroms rms. The blank and optical fabrication techniques are described along with performance characterization data.
Steven G. Johnson,
"SiC coatings on RB SiC mirrors for ultrasmooth surfaces", Proc. SPIE 2018, Passive Materials for Optical Elements II, (6 December 1993); doi: 10.1117/12.165233; https://doi.org/10.1117/12.165233