Alignment of Aspheric Optical Surfaces at Goddard Space Flight Center

Aspheric optical surfaces are becoming common in optical design. Fabrication efforts have improved, costs have reduced, and aspheres allow a designer to control more degrees of freedom using fewer surfaces. However, testing and alignment of aspheric surfaces is considerably more difficult than that of spherical surfaces. Testing requires expensive null lenses, and alignment requires numerous datums. Optomechanical alignment of aspheric surfaces began at Goddard Space Flight Center (GSFC) in the early 1980’s. Since then, many instrument designs have included aspheric optics, including COBE/DIRBE, HST/COSTAR , SIRTF/IRAC, Cassini/CIRS, HST/ WFC3, and IRMOS. We outline the alignment procedure using datums, testing of aspheric surfaces using computer generated holograms, and the independent verification of alignment via interferometry and image testing. Recently, GSFC has been developing the Infrared Multi-Object Spectrometer (IRMOS). This ground-based, imaging spectrometer is a MEMStechnology demonstrator for the James Webb Space Telescope, and is built in collaboration with the Space Telescope Science Institute and Kitt Peak National Observatory. The optical design includes four aspheric surfaces, one of which can be characterized as free-form. We detail the methodology in aligning the datum surfaces of the optics to the instrument bench, and the verification of optical performance.