HOW IT WORKS: CORONAGRAPH

Project Blue will use a device called a coronagraph to take pictures of any planets around Alpha Centauri. A coronagraph is a device that blocks the bright light from a central object but allows the faint light from its surroundings to enter the telescope.

It was invented in the 1930s by French astronomer Bernard Lyot. He wanted to study the Sun’s faint outer atmosphere which became visible only during a total eclipse when the Moon blocked the Sun’s bright disc, so he designed and built a device that could mimic the action of the Moon and create an artificial eclipse inside his telescope. Since the Sun’s outer atmosphere is called the corona, Lyot’s device was christened a coronagraph.

Looking for exoplanets is another good example of wanting to see something faint next to something bright. Planets do not generate their own light, instead they simply reflect that given out by their parent stars. It has been compared to trying to see a firefly on the rim of a searchlight.

In space, coronagraphs have principally been used to view the Sun with spacecraft such NASA-ESA’s Solar and Heliospheric Observatory (SOHO) mission. The Hubble Space Telescope did include a coronagraph in its Near Infrared Camera and Multi-Object Spectrometer (NICMOS). In 2011, it was used by astronomers to take an image of the four exoplanets around star HR 8799. Taking repeated images of planets allows their motion around their star to be seen.

The NASA James Webb Space Telescope, which will launch next year will also include a coronagraph in it Near Infrared Camera (NIRCam).