Coronagraph

At view angles close to the Sun, the sky is much brighter than the background corona even at high altitude sites on clear, dry days.

Ground-based coronagraphs, such as the High Altitude Observatory's Mark IV Coronagraph on top of Mauna Loa, use polarization to distinguish sky brightness from the image of the corona: both coronal light and sky brightness are scattered sunlight and have similar spectral properties, but the coronal light is Thomson-scattered at nearly a right angle and therefore undergoes scattering polarization, while the superimposed light from the sky near the Sun is scattered at only a glancing angle and hence remains nearly unpolarized.

Either way, the instrument design must take into account scattering and diffraction to make sure that as little unwanted light as possible reaches the final detector.

Several varieties of optical vortex coronagraphs exist: This works with stars other than the sun because they are so far away their light is, for this purpose, a spatially coherent plane wave.

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.

[6][7] The primary payload, Visible Emission Line Coronagraph (VELC), will send 1,440 images of the sun daily to ground stations.

The VELC payload has been developed by the Indian Institute of Astrophysics (IIA) and will continuously observe the Sun's corona from the L1 point.

Specifically, it is easier to obtain images when the planet is especially large (considerably larger than Jupiter), widely separated from its parent star, and hot so that it emits intense infrared radiation.

However, in 2010 a team from NASAs Jet Propulsion Laboratory demonstrated that a vector vortex coronagraph could enable small telescopes to directly image planets.