Excitement is building over the total solar eclipse that will occur in the US on August 21, 2017. This is a rare opportunity to witness a visually stunning celestial event. Many of my astronomer colleagues will be traveling to places along the path of the eclipse, which runs from Oregon to the Carolinas. My wife and I will watch from Nashville.

For most of history, total solar eclipses—when the Moon blocks out the light from the Sun— were rare occasions when astronomers could study the Sun's atmosphere. This atmosphere— called the 'corona,' from the Latin word for crown—is much fainter than the Sun itself. That means the corona is impossible to see unless the Moon blocks out the Sun's glare, which is about a million times brighter than the corona.

We are lucky because the apparent size of the Moon on the sky is almost exactly the same size as the Sun. Thus, the Moon is almost perfectly able to block out the Sun when the alignment is just right. But this only occurs occasionally, and over small areas of the Earth. However, in 1931, the coronagraph was invented. This instrument allows astronomers to block the light from the Sun in order to study the corona at any time. Coronagraphs are inserted into a telescope to block the light from the Sun, but not from the corona, making an artificial eclipse. Recently, improved coronagraphs have allowed us to block the light from other stars in order to see the significantly fainter planets orbiting those stars. We call planets outside of our solar system 'extrasolar planets,' or more simply 'exoplanets'.

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.