The performance of a new liquid Nitrogen cooled Fabry-Perot etalon for imaging at 3-5µm is described. Capacitance sensors monitor the etalon mirror spacing and parallelism, and error signals produced as a consequence of changes in these parameters are used in a feedback loop with piezoelectric actuators for active cavity control. These new cryogenic etalons are designed to be compatible with the Queensgate Instruments Ltd CS100/ET servo-stabilized Fabry-Perot system.
The cryogenic etalon has a clear aperture of 50mm and a nominal mirror spacing of between 5 and 60µm. It is coated for the 3 - 5µm spectral region, although coatings are also available for the 2 - 2.5µm and 8 - 13µm regions. Under servo-control at operating temperature the etalon has a response time of 30 msec and a minimum cavity tuning range of ±3µm about the nominal cavity length, corresponding to approximately 3 orders of interference at the midrange wavelength of 4µm.