Planetary Society on WFIRST's Planned Coronograph

I have previously written about WFIRST (Summer 2015 Planetary Report). This NASA mission, planned to launch into space in the mid-2020s will have a coronagraph at least 1,000 times more powerful than any existing coronagraph. The power of a coronagraph is measured by the 'contrast ratio'—the ratio of the brightness of the central star to the brightness of the planet being studied. Current coronagraphs have a contrast ratio of about 100,000-1 million, which means that astronomers can see objects that are 100,000-1 million times dimmer (less bright) than the central star. WFIRST is being designed to achieve a contrast ratio of one billion to one! The primary difficulty in designing any coronagraph is in blocking all of the starlight. Once light has entered the telescope, it is extremely difficult to block it all with a coronagraph, partly due to a process called 'diffraction.' This is a process in which light is bent around corners or is scattered at the edges of objects. An analogy would be when you close the curtains, but there is a little gap and the light spreads as it passes through the small gap. In a telescope, this scattered light sometimes finds its way to the camera recording the telescope observations. So, achieving a contrast ratio of one billion to one requires us to keep that scattered light level very, very low.

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