Optical Infrared Coordination Network for Astronomy
The construction of the next generation of 40 m-class astronomical telescopes poses an enormous challenge for the design of their instruments and the manufacture of their optics. The optics typically become larger while the tolerances needed to profit the most from the potential resolving power become ever more demanding. This challenge can be met by using the wider design space offered by freeform optics, by for instance allowing highly aspherical surfaces. Optical designs incorporating freeform optics can achieve a better performance with fewer components. This also leads to savings in volume and mass and, potentially, cost.
The adoption of freeforms in optical instrument designs is held back by the difficulty of manufacturing and aligning these components within the required tolerances. The proposal consortium (NOVA, ASTRON, UK-ATC, LAM and MTA CSFK-Konkoly Observatory) has previously developed the FAME (Freeform Active Mirror Experiment) concept to overcome these challenges: An array of actuators is used to correct deviations in the desired surface shape to high precision.
Enabling extreme freeform optical designs through the integration of an active mirror delivers three key benefits: Increased performance: The wavefront error can be corrected after assembly in the operational environment, correcting for alignment errors and thermal drift.
Reduced risk: In the initial operational phase, an active wavefront error control can quickly mitigate the impact alignment problems and critical components being out of spec. This is important as this is the phase when an instrument can typically make the biggest scientific impact.
Reduced cost: Freeform designs can be simpler, smaller, lighter and therefore less costly than classical designs. An active component enables these freeform designs by allowing higher tolerances on their polishing and alignment. This technology is now sufficiently mature, and its adoption so potentially beneficial, that the development of an operational Active Freeform Mirror (AFM) for use in E-ELT instruments and 3rd generation VLT instruments is the logical next step. A key component that is required to achieve this is however missing: A metrology and control system tailored to the specific demands of an active freeform mirror integrated in an operational astronomical instrument. The proposed activity will focus mainly on the development of such a mature system and test this with the existing FAME mirror. In parallel, a ready-to-build design for an operational AFM (FAME+) will be developed based on the experience gained through FAME. The FAME+ design will be compatible with operation in vacuum and/or cryogenic environments.
Michiel Rodenhuis (NOVA) -