The study of exoplanets is often hard because the light of the host star is many times brighter than that of the planets. The coronagraph is the optical device in a high-contrast imaging instrument that blocks or modifies the stellar light while transmitting most of the light coming from the planets around it.
The LEOPARD group has invented multiple coronagraphs, including a coronagraph with extreme stellar suppression and unprecedented throughput. In addition, the LEOPARD group has developed several types of liquid-crystal coronagraphs with capabilities beyond starlight suppression. The coronagraphs incorporate focal-plane wavefront sensing, are optimized for high-resolution spectroscopy, or have additional photometric and astrometric reference points. These are examples of improving the performance of a high-contrast imaging instrument by looking at it on a system-level instead of optimizing the performance of a single component.
The layout of the Phase-apodized-pupil Lyot coronagraph that uses a phase mask and two amplitude masks to create a D-shaped region of extreme stellar suppression where planets can be found. Image credit: E.H. Por.
The coronagraph is named after the first application of such a device, masking the disk of the Sun to image the solar corona, and was invented by Bernard Lyot in 1931. Nowadays, there are many types of coronagraphs used for exoplanet detection. Generally, they place a physical mask in different locations in the HCI instrument to remove the stellar light by modifying its amplitude or phase. It can be shown that using phase masks results in an improved performance compared to only amplitude masks. The LEOPARD specialises in designing and implementing phase masks coronagraphs created with liquid-crystal technology.
Examples of four types of coronagraphs that modify the stellar light (red) with either phase or amplitude masks. These masks can be located in the focal plane, where an image of the star is formed, or the pupil plane, a version, where an image of the telescope pupil is formed.
Coronagraph designs
Below some of our coronagraph designs are discussed in more detail.
The most widely implemented coronagraph that is developed by the LEOPARD group is the vector-apodizing phase plate (vAPP) coronagraph. A vAPP coronagraph can operate over spectral bandwidths more than three times larger than other coronagraphs. This is ideal for integral field spectrographs, which can resolve a spectrum for one pixel in an image. In addition, it can be adapted to include focal-plane wavefront sensors or photometric and astrometric reference holograms. Since 2015 seven vAPPs have been installed in six different instruments, most of which on 8-m class telescopes, like the Large Binocular Telescope. The vAPP will also be installed in two instruments on the 39-m Extremely Large Telescope.
The Large Binocular Telescope. Image credit: NOAO
An artist impression of the Extremely Large Telescope. Image credit: ESO
The Single-mode Complex Amplitude Refinement (SCAR) coronagraph is optimized for high-resolution integral field spectroscopy, capable of retrieving very detailed spectra of exoplanets. Similar to the vAPP coronagraph, it consists of a phase plate that changes the stellar point spread function in a way that suppresses the light close to the star. For the SCAR coronagraph the light is changed such that it can not couple well into single-mode optical fibers, cables that transport light only if it has a specific shape. The fibers then transport the exoplanet light to a spectrograph, which creates a spectrum for multiple pixels in the field. The SCAR coronagraph has three main advantages compared to existing coronagraphs. First, it provides good stellar suppression closer to the star. Second, the use of single-mode fibers can reduce the size of the spectrograph by orders of magnitude, from the size of a bedroom to the size of a shoebox. Lastly, spectroscopic data reduction provides an additional contrast gain for exoplanet atmosphere characterization.
Left: The layout of the SCAR coronagraph. The light is modified by a phase plate and imaged onto a microlens array to couple light into a single-mode fiber array. Right: The phase pattern (top), the modified point-spread function of the starlight with the hexagonal microlenses in red (middle), and the stellar suppression (bottom) of the SCAR coronagraph. Image credit: S.Y. Haffert and E.H. Por
The Phase-apodized-pupil Lyot coronagraph (PAPLC) is a pairing of the apodized-pupil Lyot coronagraph and the apodizing phase plate (APP) coronagraph. With a simple knife-edge in the focal plane, it is capable of extreme stellar light suppression of 1010 at the smallest angular separations, while having an unprecedented exoplanet throughput. Moreover, the coronagraph can be optimized for any telescope aperture, which is more difficult for other coronagraphs and is capable of more precise focal-plane wavefront sensing. Simulations of exoplanet yield with future space telescopes, like LUVOIR-A, demonstrated that the PAPLC performs better than other considered coronagraphs.
Graph showing the expected amount of planets found with a mission of LUVOIR-A. Left: a total of 52 Earth-like planets will be detected and characterized using different coronagraphs. Right: if a PAPLC is installed in LUVIOR-A, 13 more Earth-like planets will be detected and all detected exoplanets will be characterized with the PAPLC coronagraph.
The Multi-grating vector-vortex coronagraph (MGVVC) combines the VVC with multiple polarization gratings to overcome manufacturing limitations of a classical VVC. The performance of the classical VVC is limited by the liquid-crystal recipes which are not yet good enough and create polarization leakage. This polarization leakage produces an image of the star at ~0.1% of the stellar intensity, which is still much brighter than the exoplanets. By combining the VVC with a polarization grating (PG) pattern directly followed by one or several separate PGs, it is possible to suppress the polarization leakage terms by additional orders of magnitude by diffracting them out of the beam. This multi-grating approach enables the manufacturing of MGVVC with a leakage suppression of 108 for a 60% spectral bandwidth or 105 for an octave of spectral bandwidth.
Left: A microscopic image of the MGVVC pattern taken through crossed polarizers. Image credit: ImagineOptix.
Polarization leakage of a MGVVC (solid), which is the combination of three individual components (dashed). Compared to a single component, the gain is five orders of magnitude. Image credit: D. S. Doelman