Fabry-P\'erot based Narrow Band Imager for Solar Filament Observations

We have recently developed a narrow band imager (NBI) using an air gap based Fabry-P\'erot (FP) interferometer at the Indian Institute of Astrophysics, Bangalore. Narrow band imaging is achieved by using an FP interferometer working in combination with an order sorting pre-filter. The NBI can be tuned to a different wavelength position on the line profile by changing the plate separation of the FP. The interferometer has a 50 mm clear aperture with a bandpass of $\sim$247.8 m\AA and a free spectral range of $\sim$5.3\AA at $\lambda$ = 656.3 nm. The developed NBI is used to observe the solar filament in the H$\alpha$ wavelength. The instrument is being used to image the Sun at chromospheric height and it is also able to scan the H$\alpha$ spectral line profile at different wavelength positions. We have also made Doppler velocity maps at chromospheric height by taking the blue and red wing images at $\pm$176 m\AA wavelength positions separately away from the line center of the spectral line. In this paper, we present a description of the NBI including lab test results of individual components and some initial observations carried out with this instrument.