Abstract: 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 are firstly analyzed. Polishing experiments reveal that computer controlled small-tool polishing process does not worsen PSD2 error compared to the full-aperture process, while 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.