Fig. 1. Defect depth distribution after ultra-precision grinding of single crystal silicon. (a) Original surface, depth is 0; (b) depth is 1.5 μm; (c) depth is 2.5 μm; (c) depth is 3 μm

Fig. 2. Single crystal silicon magnetorheological polishing. (a) Polishing process; (b) after polishing

Fig. 3. Cylindrical smoothing process^[17]. (a) Practical equipment for cylindrical smoothing process; (b) semi-rigid cylindrical smoothing plate; (c) mid spatial frequency error before smoothing; (d) mid spatial frequency error after smoothing

Fig. 4. Traditional polishing and immersion polishing equipment. (a) Traditional polishing equipment; (b) immersion polishing equipment

Fig. 5. Surface defect suppression realized by isolating large particles. (a) Scratches on polished surfaces in air; (b) convex line on polished surfaces in air;(c) immersive polishing, without surface defects

Fig. 6. Ion beam processing equipment and physical sputtering process. (a) Ion beam processing equipment; (b) physical sputtering process

Fig. 7. Tilted incident experiments of ion beam^[23]. (a) Experimental photo; (b) surface error map

Fig. 8. Combined process route. (a) Immersion polishing; (b) ion beam figuring; (c) profile accuracy measurement

Fig. 9. Samples of plane mirror

Fig. 10. Picture of convex cylindrical mirror

Fig. 11. Processing results of convex cylindrical mirror. (a) RMS of low spatial frequency surface shape error is 13.537 nm; (b) RMS of middle spatial frequency surface shape error is 0.494 nm; (c) RMS of high spatial frequency surface shape error is 0.261 nm