Aiming at the improvement of edge effect in CNC polishing. A new polishing method based on the surface extension is proposed. The basic idea and workflow are presented. The availability of this method is verified by simulation. Experimental study was carried out on 420mm×420mm caliber fused quartz optical element. The experimental results show that this method can restrain the collapse and warped edge surface. PV less than λ/3，GRMS less than 7.7nm/cm and PSD1 less than 1.8nm can be obtained combining the small scale smoothing technology.
The influence of polishing parameters such as particle size, pad material and pressure on the surface roughness of glass optics were investigated and analysed. It reveals that the surface roughness will get worse with increase of the polishing particle size. The surface roughness would remain stable in a certain period of polishing pressure, but get worse with increase of the pressure beyond the period. The surface roughness is getting better when using smooth pitch polishing pad than polyurethane pad with lots of micropores. By optimizing the polishing parameters, the surface roughness of large aperture fused silica window is improved to 0.46nm before band-pass filtering and 0.084nm after band-pass filtering.