Enhancing the Spatial Resolution of Solar Coronagraph Observations Using Dynamic Imaging

Abstract

The Large Angle Spectrometric Coronagraph (LASCO) C1 coronagraph on board the Solar and Heliospheric Observatory (SOHO) is designed to image the corona from 1.1 to 3.0 R☉. The resolution of C1 is defined by the size of its CCD pixels, which correspond to 56, and not by the diffraction limit of the optical system, which may be as small as 3''. The resolution of C1 can be improved using the technique of dynamic imaging—the process of acquiring successive images of the same scene using subpixel displacements of the steerable primary mirror. We developed a technique we call the fractional pixel restoration (FPR) algorithm that utilizes these observations to construct an image with improved resolution. Simulations were used to test this algorithm and to explore its limitations. We also applied the direct co-addition and FPR algorithms to laboratory preflight images of a wire mesh grid. These results show that the resolution of the C1 coronagraph can be significantly enhanced, even in the presence of noise and modest differences between successive images. In some cases, the results can even reach the diffraction limit of the telescope.