Design and construction of a 20 cm Fabry-Perot interferometer

A 20 cm diameter Fabry-Perot interferometer has been designed and constructed to achieve Doppler shift and Doppler broadening measurements of three thermospheric nightglow/twilight emissions of 630 nm OI, the doublet emission of O+ at 732 nm, and the 1083 nm He 1s 2p multiplet states, respectively. The selection of an etalon gap of 0.8724 cm and a focal length of 300 mm for an apochromatic multi-lens assembly combined with the application of ultra-narrow 20 cm diameter filters (bandwidth typically 0.35 nm) make possible the high resolution observations of these emissions without contamination from adjacent OH emissions. A large format (2048x2048) silicon-based CCD camera enabled the field of view to be increased to achieve 29, 23, and 16 orders for these three nightglow/twilight emissions respectively corresponding to a half-angle field of view of ~2.5 degrees. The science goals to be achieved by this instrument called MaxFPI are to search for possible speed and temperature differences between OI 630 nm and the He 1083 nm emissions, to observe plasma drift speeds during evening and morning twilights, and to observe neutral density fluctuations in the 630 nm nightglow with high temporal resolution of a few tens of seconds and an accuracy of 2-3 ms-1. This instrument has been tested successfully in the laboratory and first light observing the mid-latitude nightglow emissions at Jenny Jump State Park is expected later this summer.