**6. Precision metrology of in-situ and at-wavelength**

Most measurements for precision optics are made in controlled environment in order to verify compliance with specifications. However, the actual in-situ use of the optical components could be very different from the laboratory condition. Beam quality and position can be affected by temperature instability, distortion under high vacuum condition, a noisy vibration environment, and thermal distortion due to absorption of high power beams from synchrotron and FEL sources. In these cases, on-line figure measurement of bending mirrors and adaptive optics is highly desirable. The manufacturing of large astronomical optical components with nanometer accuracy requires in-situ testing without removing the optics from the polishing machine. These insitu situations present challenges to the metrology. Most of these metrologies are having increasing demands on nanometer level.

Owing to the restricted conditions for measurements on in-situ optics, very few measurements have been made, even though they are very important.
