Design and testing of an occulter for a terrestrial solar coronagraph

COSMO Large Coronagraph (COSMO LC) is a telescope dedicated to the measurement of the Sun magnetic field. This project is currently at a design phase funded by the National Science Foundation under the technical direction of the High Altitude Observatory. COSMO LC is a refractive telescope whose objective lens has a clear aperture of 1.4m, it will be the largest refractive telescope in the world. This telescope can observe the Sun corona thanks to the internal occulter which is able to obscure the solar disk. This device needs to accomplish two main functions: 1) adapt its diameter to the Sun apparent size, 2) reject all the incoming heat to not start any air turbulence which leads to the degradation of image quality (seeing). Diameter change is accomplished by means of a cam mechanism which actuates 14 petals arranged azimuthally while the occulter cooling is obtained through cold water running through internal channels and forced air convection. This article describes the mathematical models employed to quantify the seeing effect on image resolution and the technical solutions adopted to implement the above-mentioned functions. In addition, the tests performed on this device are described along with the results.