Advances in ground-based and space-based reflecting coronagraph designs

A program to develop ground-based emission-line solar coronagraphs based on super-polished primary objective mirrors has been underway at the National Solar Observatory/Sacramento Peak over the last several years. The fundamental design requirements for effective coronagraph performance are discussed. The instrument currently under development has a 60-cm diameter objective with a 700-cm primary focal length. This design has been refined with an emphasis on achieving an extremely low level of instrumental stray light, suitable for both visible and IR operation and for carrying out high-precision polarimetry. It is seen as a possible prototype for a new generation of much larger aperture, low-scattered-light solar telescopes. Special applications include measurements of the signatures of waves in coronal loops, transient events such as coronal loop interactions, the spatial variation of magnetic fields in prominences allowing computation of electric currents, and the determination of the magnitude of coronal magnetic fields. For applications in space, a white-light reflecting coronagraph has been developed under the USAF program, SWATH (Space Weather and Terrestrial Hazards). This novel catadiopric design has a 10-cm diameter superpolished primary objective mirror, and a unique external occulter. It has a significantly higher throughput and resolution as compared with conventional space-borne lens-objective coronagraphs. Two basic modifications of this design are proposed that result in an in-line symmetric configuration, with a decrease in mass, while improving the scattered-light performance. Some special applications are described.