2. Description of objects to be measured

The mirrors were made available for intercomparison by some members of the WG. The aim was to address different issues:

• analyze the quality of spatial mid-frequency reconstruction. To do this, we use a "medium quality" mirror with low-frequency defects (LAM-Flat mirror);

• push the measurement capabilities of the instruments to their limits. To do this, we need a high-quality mirror whose shape defects are smaller than those of the reference mirrors used, in order to determine their impact on measurement results (LCF-Flat mirror);

• evaluate the results for the analysis of spherical mirrors. Only participants who had spherical references (commonly known as spherical calibrators) were able to perform the measurement. Only the LCF and the LAM measured this mirror (LAM-Sphere);...