Calibration of a liquid crystal etalon multispectral imager

Calibration of a liquid crystal etalon multispectral imager – Challenges and solutions Chhavi Goenka CEDAR 2015 With. Joshua Semeter 1, John Noto 2, Jeffrey Baumgardner 1, Juanita Riccobono 2, Mike Migliozzi 2, Hanna Dahlgren 3, Robert Marshall 4, Sudha Kapali 2, Michael Hirsch 1, Donald Hampton 5, Hassanali Akbari 1 1 Boston University, 2 Scientific Solutions Inc. (SSI), 3 KTH Royal Institute of Technology, 4 Stanford University, 5 Geophysical Institute, University of Alaska Fairbanks

What is a tunable filter? Anisotropic material Transmission Tunable filter Transmission peak External stimulus 7000 7200 7050 7100 7150 Wavelength (A) 6/7/2021 2

Simultaneous Multispectral Imaging of Aurora O(1 D-1 S) O(2 D-2 P) N 2+1 NG(0, 1) 6/7/2021 O(3 P-1 D) 3

Li. CHI – based on Liquid Crystal Fabry-Perot etalon 5577 Å emission (data from Poker Flat spectrograph) Passband of one channel of Li. CHI 6/7/2021 [Goenka et al 2013] 4

Wavelength calibration of the LCFP etalon Wavelength Calibration 5 [Goenka et al. , accepted, 2015]

Voltage calibration of the LCFP etalon 6

Effect of temperature on LCFP etalon 7

Challenges while operating in the field • Variation in temperature – change in voltage vs wavelength – Solution – use the aurora itself to recalibrate the filter [Goenka et al. , accepted, 2015]

Challenges for data analysis • Lower spectral resolution than standard techniques – Solution – a priori information about the auroral emissions along with post processing algorithms can be used to obtain the desired data [Goenka et al 2013]

Challenges to solve in the future • Cannot focus all channels equally perfectly with one front optical tube– a balance needs to be found – Solution : use four optical tubes in the front to be able to focus the channels individually and still one detector • Lower throughput than standard techniques – Light collection methods can be used to collect light discarded by polarizers – EMCCD cameras can be used with high gain 10

Thank you! 11