光学元件的中高频误差一般采用功率谱密度(PSD)表示，其划分为PSD1和PSD2两个频段。针对目前国内外研究较少的PSD2频段误差，分析和实验研究了其潜在的影响因素。采用沥青和聚氨酯盘抛光熔石英元件的实验结果显示: 小工具数控相比传统全口径抛光并未增大PSD2误差，而抛光盘材质对PSD2误差具有决定性的影响。沥青盘在抑制PSD2误差方向具有较好的优越性，工件表面的PSD2指标能够满足要求，而聚氨酯抛光元件表面的PSD2误差则较高。针对这一问题，提出采用固结金刚石丸片修整聚氨酯垫，通过细化金刚石颗粒获得了合格的PSD2指标。

Mid-spatial frequency wavefront error of optics is characterized with the power spectrum density (PSD) specification, and is divided into two parts: PSD1 (wavelength 2.5-33 mm) and PSD2 (wavelength 0.12-2.5 mm). Rather than the PSD1 error which has been researched in the literature, the present work focuses on the PSD2 range. Potential factors affecting PSD2 error was firstly analyzed. Polishing experiments revealed that computer controlled small-tool polishing process does not worsen PSD2 error compared to the full-aperture process, whileas the lap has a decisive effect. The optics polished by pitch laps shows a fair PSD2 error of 0.7 nm (RMS), which is much lower than the specified 1.1 nm. The polyurethane pad commonly results in a much higher PSD2 error of more than 1.5 nm (RMS). For this problem, we adopt diamond conditioner to dress the pad and decrease pad surface roughness. The validation of restraining PSD2 error by diamond dressing has been confirmed.