  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 Coud\'e solar telescope. The etalons were
calibrated for the solar spectral lines FeI 6173 {\AA}, and CaII 8542 {\AA}. 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 parameters. We also present solar images obtained using the
system.
