Visible emission line space solar coronagraph: science and optical design

Abstract

A solar coronagraph above the Earth’s atmosphere provides ideal conditions to study various properties of coronal plasma such as pressure, density, and temperature perturbation leading to the existence of waves, the topology of magnetic fields, dynamical nature of coronal loops & CME’s and space weather predictions. Keeping in view of these scientific objectives of coronal dynamics, a 20 cm coronagraph is being planned to be launched in space by ISRO. The solar corona will be imaged in two bright coronal emission lines (viz., the green and the red-lines at 530.3 nm and 637.4 nm respectively). The field-of-view (FOV) of interest in the solar corona is from 1.05 to 3.0 solar radii in order to maximize the science outcome of coronal dynamics and also to provide complementary data to the other existing/planned space missions. The optical design of the coronagraph has a 20-cm diameter off-axis parabolic mirror as its main optical imager. The mirror has a stringent requirement of 0.1–0.15 nm micro-roughness in order to provide a scatter light of less than 5-ppm. This mirror and other reflecting optics required for the coronagraph are being fabricated at LEOS, ISAC. The full optical design is optimized for the diffraction-limited imaging capabilities and is discussed in this paper along with the scientific objectives of the payload.