Satellite-based Coronagraphs
Coronagraphs in outer space are much more effective than the same instruments would be if located on the ground. This is because the complete absence of atmospheric scattering eliminates the largest source of glare present in a terrestrial coronagraph. Several space missions such as NASA-ESA's SOHO, SPARTAN, and Skylab have used coronagraphs to study the outer reaches of the solar corona. The Hubble Space Telescope (HST) is able to perform coronagraphy using the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), and there are plans to have this capability on the James Webb Space Telescope (JWST) using its Near Infrared Camera (NIRCam) and Mid Infrared Instrument (MIRI).
While space-based coronagraphs such as LASCO avoid the sky brightness problem, they face design challenges in stray light management under the stringent size and weight requirements of space flight. Any sharp edge (such as the edge of an occulting disk or optical aperture) causes Fresnel diffraction of incoming light around the edge, which means that the smaller instruments that one would want on a satellite unavoidably leak more light than larger ones would. The LASCO C-3 coronagraph uses both an external occulter (which casts shadow on the instrument) and an internal occulter (which blocks stray light that is Fresnel-diffracted around the external occulter) to reduce this "leakage", and a complicated system of baffles to eliminate stray light scattering off the internal surfaces of the instrument itself.
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