You can think of a coronagraph as having the same effect as a total eclipse of the sun where only its outer edge is visible. It is a telescopic attachment designed to block out the direct light from a star. The early coronagraphs were intended to view the corona of the Sun. But a new class of conceptually similar instruments called stellar coronagraphs are used to find extrasolar planets, host galaxies in quasars, and other similar objects.

A coronagraph is one of the instruments slated to go aboard a NASA space-based telescope that has been in the planning stages for so long that it’s name has changed. Originally called the Wide Field Infrared Survey Telescope (WFIRST), it is now dubbed the Nancy Grace Roman Space Telescope and is named for a former NASA Chief of Astronomy (not the TV personality of a similar name). Its coronagraph will directly image exoplanets and study their atmospheres. The masking will preclude the stars’ overwhelming brilliance from obliterating the image of the much fainter exoplanet. The Roman telescope will also have an extraordinarily large field of view that should reveal ever more exoplanets, possibly including life-bearing earth equivalents.

NASA says that the wide field will also play a role in unraveling the mysteries of dark energy and dark matter, and explore the evolution of the cosmos by peering back in time as it images celestial objects that are light years distant.

The Roman coronagraph will use starlight-suppression techniques so planets that are only 0.1 arc-seconds away from the host stars may be imaged. The Roman telescope is still in the design phase, so the exact configuration of its coronagraph is probably still in flux. Nevertheless, the designers at the Calif. Institute of Technology Jet Propulsion Lab have released a block diagram of how the coronagraph operates.

Stellar and solar coronagraphs are similar in concept but different in design because the Sun has an apparent size of about 1,900 arcseconds, while a typical nearby star might have an apparent size of 0.0005 and 0.002 arcseconds. Earth-like exoplanet detection requires 10⁻¹⁰ contrast.

The Roman coronagraph is a high-contrast device covering shorter wavelengths (0.5 to 0.8 μm) using dual-deformable mirror starlight-suppression technology. The deformable mirrors effect wavefront control for optical aberrations and suppress diffracted starlight from obstructions and discontinuities in the telescope pupil. The technique is intended to provide a part-per-billion suppression of starlight to enable the detection and spectroscopy of planets as close as 0.15 arcseconds away from their host stars.

In seeking to answer basic questions about dark energy, the Roman telescope will consider whether the accelerating expansion of the universe, now an accepted if enigmatic fact, is caused by a new energy component or by the decomposition of Einstein’s General Relativity on cosmological scales. A question that will be considered is: If it is the former, is the energy density temporally and spatially constant, or is it subject to variation?

To answer these cosmological questions, WFIRST will analyze baryon acoustic oscillations, record images of distant (in time and space) supernovae and use as a tool weak gravitational lensing.

Another area of inquiry will concern the hunt for extraterrestrial life. WFIRST will aspire to make a complete census of exoplanets in our galaxy, the Milky Way. The astonishing resolution and sensitivity of the instrument will make possible direct visualization of exoplanets that are only a few times the mass of our moon.