Secondary mirror for MRO

Thales Seso has manufactured (grinding, polishing and coating) the secondary mirror of the Magdalena Ridge Observatory (MRO). This is a challenging highly off axis convex aspherical mirror.

The main characteristics and specifications of this mirror are:

• Substrate: Lightened ULE structure with Corning fusion process
• Size : circular Φ690mm
• Shape: hyperbolic Convex with about 50µm asphericity from best fit sphere
• Optical quality required: WFE < 40nm for full CA • Coating: protected aluminium One of the main challenges of this mirror was the optical control because we are facing simultaneously different problems: a large mirror, an aspherical surface are, moreover, a convex one. Thales Seso has developed for this mirror a full aperture interferometric bench using a large sized Hindle corrector. Errors due to Hindle corrector itself and WFE distorsion induced by the natural gravity were managed and compensated thanks to a specific control iteration process.