ESA Science & Technology - BAM OMA mirror

The parabolic mirrors of the Gaia Basic Angle Monitoring (BAM) cryogenic fiber collimators are made of sintered silicon carbide (SSiC) without CVD coating. They needed to be polished to a surface shape error of less than 12.5 nm rms and a surface roughness Rq 6 nm. The main difficulty of these mirrors is the small radius of curvature (R = 50.17 mm) over an effective aperture of 10 mm.

The strongly curved off-axis parabolic SSiC mirrors are designed by TNO Science and Industry in close cooperation with Astrium and were manufactured by Boostec. Polishing of these mirrors to demanding surface form and roughness requirements is performed by TNO in close cooperation with the Leibniz Institute of Surface Modification (IOM) applying a combination of 3D robot polishing and numerically controlled Plasma Jet Machining (PJM). See also the related image "Plasma Jet Machining on SiC mirrors".

Polishing of the Flight collimator mirrors has been completed successfully in September 2009. The achieved performances of the three mirrors are respectively:

Surface form:		4.4, 4.4 and 7.2 nm rms (requirement: ≤ 12.5 nm rms)
Surface roughness (Rq):		3, 6 and 5 nm (requirement: Rq ≤ 6 nm)