**4. Current development and trends in nano-accuracy surface figure metrology**

Precise metrology is the basis for enabling fabrication of precision optics. The rule-of-thumb requires that the metrology accuracy should be at least 3-fold (< 50 nrad) better than the specification of the optics. The great challenge for metrology and manufacturing is that nanometer and nano-radian accuracy is needed for spherical or aspheric mirrors with large surface slope angles.
