Perhaps the most profound questions in astronomy are about life: Are we alone? What makes a planet habitable? Is our Solar System unique? To answer these questions, astronomers have developed powerful telescopes to search for planets around other stars, called exoplanets. Notable examples include the Kepler mission, the James Webb Space Telescope, and upcoming initiatives such as ESA’s PLATO and Ariel missions.
So far, over five thousand planets have been discovered. Astronomers estimate that nearly every star hosts at least one planet. Many earth-sized planets have been discovered orbiting nearby stars within their habitable zones, where surface water could appear in liquid form. To study the atmospheres and surfaces of these potentially habitable worlds, we must continue improving the sensitivity of our detection methods. To this end, SRON is working on optical components using liquid-crystal technology, together with NOVA.
Figure 1. The image of a star swamps the planetary system around it. Blocking the starlight creates a dark area at the position of the parent star (left) and the planetry system becomes visible (right).
Coronagraph
Exoplanets are outshined by their host stars by one in a million or even one in ten billion, while maintaining a close distance. The method that we apply to study them nonetheless is therefore rightfully called high-contrast imaging. We use coronagraphs to block most of the light from the star while transmitting the light coming from the planet. SRON is working on manufacturing and improving coronagraph masks with a focus on extending the spectral bandwidth and polarimetric capabilities.
Figure 2 (left): Picture of a liquid-crystal vortex coronagraph. Figure 2 (right): microscope image of the center of the coronagraph.
Liquid Crystals
Our coronagraphs consist of liquid crystal masks that manipulate the phase of light waves. We directly write patterns on the masks with micron-sized pixels and a phase accuracy of one degree. These masks are flat and lightweight and manipulate the phase light independent of its wavelength. This makes them ideal for operating across a large spectral bandwidth for analysing the spectra of exoplanets. SRON collaborates with NOVA and the company ColorLink Japan to develop these liquid crystal masks.
Figure 3: Comparison of the impact of polarisation leakage on the standard vortex coronagraph and the double-grating vortex coronagraph.
A fundamental limitation of liquid-crystal masks is their ‘polarisation leakage’, where light is transmitted through the optic without phase manipulation, reducing the performance. We are working on a solution based on stacking multiple masks with grating patterns to diffract the unwanted light away. The main advantage of this multi-grating approach is that in theory we don’t need to filter out unwanted polarisation.
Research question
Our expert
-
Read moreDavid Doelman-Oldenburger
Scientist
