A broad band imager for the European Solar Telescope

We report on the results of the conceptual design study of a broad band imager for the European Solar Telescope (EST), a joint project of several European research institutes to design and realize a 4-m class solar telescope. The EST broad band imager is an imaging instrument whose function is to obtain diffraction limited images over the full field of view of EST at multiple wavelengths and high frame rate. Its scientific objective is the study of fundamental astrophysical processes at their intrinsic scales in the Sun’s atmosphere. The optical layout foresee two observational modes: a maximum field of view mode and a high resolution mode. The imager will have a 2'x2' corrected field of view in the first mode and an angular resolution better than 0.04" at 500nm in the latter mode. The imager will cover a wavelength range spanning from 390nm to 900nm through a number of filters with bandpasses between 0.05nm and 0.5nm. The selected optical layout is an all refractive design. To optimize optical performances and throughput there will be two arms working simultaneously: a blue arm (covering the 380nm – 500nm range) and a red arm (600nm – 900nm). The blue arm will have two channels while the red arm only one. Each channel will be divided in three subchannels: one will host narrow band filters for chromospheric observations, another one, in focus wide band filters used as reference for speckle reconstruction and photospheric observations, and the last one, out of focus wide band filters for phase diversity reconstruction of photospheric observations.