High contrast imaging (HCI) is one of the core research fields of the LEOPARD group. HCI aims to directly image and characterize exoplanets and circumstellar disks by suppressing the light of their host star that would otherwise ‘blind’ the observer. With HCI techniques the stellar light can be suppressed by orders of magnitude.
The goal of the LEOPARD group is to innovate and connect all components of an HCI instrument to characterize exoplanets which could possible harbour life. To this end, the LEOPARD group develops novel concepts for both ground-based and space-based HCI instruments. In addition, the LEOPARD group uses existing instruments in novel ways to discover exoplanets.
Directly imaged exoplanets orbiting the stars TYC 8998-760-1 (left) and PDS 70 (right). Three of the four have been discovered by the LEOPARD group. Credit: ESO/Bohn et al./Haffert et al.
High contrast imaging is challenging because the angular separation between the planet and the star is extremely small and the star is orders of magnitudes brighter. An HCI system therefore applies multiple optical techniques and data reduction methods to uncover the signal of exoplanets. There are four main components to a high contrast imaging instrument: the adaptive optics, the coronagraph, the imaging system, and the data reduction. The LEOPARD group aims to innovate and integrate these components to improve the performance on a system level.
When light from a planetary system enters the telescope, the wavefront is distorted due to atmospheric turbulence and optical aberrations from the HCI instrument itself. The wavefront consists of light waves with the same phase and is uniform before entering Earth’s atmosphere where it has become chaotic. The adaptive optics (AO) system uses a wavefront sensor (WFS), a deformable mirror (DM), and a control system to actively correct for the distortions and make the wavefront uniform again. A flat wavefront is crucial for coronagraphs to provide optimal stellar suppression.
Current AO systems are limited by their speed, the noise on the wavefront distortion measurement, and aberrations that occur after the wavefront sensor called non-common path aberrations. The LEOPARD group develops multiple types of wavefront sensors and control techniques to overcome these limitations.
Schematic of the vector-Zernike wavefront sensor developed by the LEOPARD group. A liquid-crystal focal-plane mask turns wavefront errors into intensity variations in the two pupils imaged onto the detector.
A coronagraph suppresses the star light by either directly blocking or redistributing the star light. It is placed in the light path after the AO system and consists of one or multiple phase or amplitude masks. The LEOPARD group uses liquid-crystal technology to create different types of coronagraphs with unique properties. Examples are coronagraphs with optimal performance over extremely broad spectral bandwidths, integrated high-resolution integral-field spectroscopy or extreme stellar suppression (>105) with unprecedented throughput of the planetary light. Many of these coronagraphs have been tested in the lab or are already installed on 8-m class telescopes around the world.
The APP coronagraph creates a dark region in the stellar point spread function with a phase mask.
The imaging system records the exoplanet signal through standard imaging or through other means like spectroscopy or polarimetry, enabling exoplanet characterization. Planetary properties like the possible presence of an atmosphere can be determined. The LEOPARD group has developed high-resolution integral-field spectrographs the size of a shoe box which will be installed in two high-contrast imaging systems (Magellan/MagAO-X and VLT/SPHERE). Moreover, the LEOPARD group has developed multiple concepts that integrate the imaging system with different components of an HCI instrument. These concepts include coronagraphic wavefront sensors that use the science camera to measure non-common path aberrations, coronagraphs that operate over the full bandwidth of integral-field spectrographs, and coronagraphs that make copies of the star light to improve astrometry and photometry.
The coronagraphic Modal Wavefront Sensor (cMWS), the integration of the APP coronagraph with holograms for focal-plane wavefront sensing.
Data reduction is the last d component of HCI. Data reduction is not simply a calibration step that minimizes the influence of noise factors like the thermal background or bad pixels. Post-processing techniques enhance the stellar suppression by exploiting differences between star and planet light like polarization state or spectral features. The LEOPARD group has developed a post-processing package for the SPHERE/IRDIS instrument that improves the polarimetric calibration. This package was used to discover the first polarized substellar companions, where their polarization signal likely originates from a circumplanetary disk
The detection of two polarized companions with the SPHERE/IRDIS instrument. The research was lead by the LEOPARD group. Credit: ESO/van Holstein et al.