A dual Fabry-Perot based narrow band imager for the National Large Solar Telescope

Abstract

A large solar telescope is usually equipped with several postfocus instruments deployed to study the dynamics of the solar features at di erent wavelengths. Indian Institute of Astrophysics (IIA) has proposed to build a 2-m class National Large Solar Telescope (NLST) to be located at a site which is suitable for high resolution observations of the sun. The narrow band imager (NBI) is proposed to be one of the back-end instrument for NLST to provide a spectral resolution of 40 mÅ or better. The NBI comprised of two Fabry-Perot interferometers kept in tandem. The instrument will be capable of observing the solar atmosphere at various wavelength positions of the spectral line with the expected temporal cadence of about one spectral image per second. The instrument will also have the additional capability of making Dopplergrams at very high cadence.The instrument can be combined with a high precision polarimeter to obtain the vector magnetic fields of the solar atmosphere (one or more levels) with good temporal cadence. Several simulations and numerical studies have been carried out to arrive at the optimal design of the instrument. In this paper, we present the important design parameters of the instrument such as wavelength coverage, optimum spacing ratio, parasitic light contribution, field-of-view, spectral and spatial resolution, signal-to-noise ratio etc. The theoretically estimated performance of the proposed NBI is also compared with similar instruments used around the world.