Narrow-Band Imaging System for the Multi-application Solar Telescope at Udaipur Solar Observatory: Characterization of Lithium Niobate Etalons.

The Multi-application Solar Telescope is a 50 cm off-axis Gregorian telescope that has been installed at the lake site of Udaipur Solar Observatory. For quasi-simultaneous photospheric and chromospheric observations, a narrow-band imager has been developed as one of the back-end instruments for this telescope. Narrow-band imaging is achieved using two lithium niobate Fabry-Perot etalons working in tandem as a filter. This filter can be tuned to different wavelengths by changing either voltage, tilt, or temperature of the etalons. To characterize the etalons, a Littrow spectrograph was set up in conjunction with a 15 cm Carl Zeiss Coudé solar telescope. The etalons were calibrated for the solar spectral lines Fe i 6173 Å, and Ca ii 8542 Å. In this work, we discuss the characterization of the Fabry-Perot etalons, specifically, the temperature and voltage tuning of the system for the spectral lines proposed for observations. We present the details of the calibration set-up and various tuning parameters. We also present solar images obtained using the system. [ABSTRACT FROM AUTHOR]