A miniature cryogenic scanning Fabry-Perot interferometer for mid-IR to submm astronomical observations

We have designed and evaluated a Miniature Cryogenic Scanning Fabry-Perot (MCSF) interferometer which can be inserted into the optical path of a mid-IR camera to observe fine structure lines in the 25-40 μm wavelength regime. The MCSF uses free standing metal meshes as its filters and can scan over a length of ~2 mm. The short wavelength range in which the MCSF will be used requires very tight fabrication tolerances to maintain the parallelism of the meshes to within 0.15 μm and to obviate the need for dynamic parallelizing adjusters. A monolithic notch flexure design delivers these properties and minimizes the number of moving parts, maximizing reliability. The scanning mechanism includes a cryogenic stepper motor that drives a miniature fine-adjustment screw via a worm gear assembly. This allows for a step resolution of 1 step ~ 14 nm when operating in full step mode. Finite Element Analysis of the MCSF’s monolithic flexure guided the design and confirmed that the MCSF will remain within required limits over the course of operation. We developed the MCSF for use in the mid-IR camera FORCAST on the 2.5 meter SOFIA telescope.