Combining light from different apertures, like different telescopes or holes in a mask, is called interferometry. Interferometry improves the resolution compared to normal imaging. The LEOPARD group aims to leverage this advantage to study circumstellar disks, planets, and planet formation. The LEOPARD group is involved in the MATISSE instrument (Multi AperTure mid-Infrared SpectroScopic Experiment) of the Very Large Telescope and aperture masking with liquid-crystal optics at the Keck Telescope.
Optical interferometry is a technique in which light waves are superimposed, creating a wave of greater, lower or the same amplitude called a fringe. It is possible to extract information about the light source from this resulting wave. In astronomy, separate telescopes or mirror segments can be interfered with to make them work together as a single telescope with a higher resolution. The Very Large Telescope Interferometer has a resolution of the equivalent of a telescope of approximately 240 meters, 30 times larger than the single telescopes.
The MATISSE instrument combines the light of all four VLT telescopes and operates in the L (between 3.2 and 3.9 μm), M (between 4.5 and 5.0 μm), and N (between 8.0 and 13.0 μm) bands. MATISSE is capable of spectroscopy of the most inner regions of planet forming disks around young stars.
A single telescope can also be turned into an interferometer by masking parts of the telescope aperture, resulting in a gain of a factor two in angular resolution without the need for complicated optical systems. Each pair of holes in the mask creates a single fringe in the image plane with a frequency that depends on the distance between these holes, the baseline. If the holes are placed such that all baselines are unique it is possible to reconstruct an image of the source that is mostly independent of aberrations from Earth’s atmosphere or the telescope itself.
Masking the telescope pupil with a mask with unique baselines is called sparse aperture masking (SAM). The two advantages of SAM, a higher angular resolution and resistance against optical aberrations, make SAM a complementary technique to coronagraphy. When SAM is combined with adaptive optics, the sensitivity is greatly enhanced. Aperture masks are becoming a more common part of high-contrast imaging systems.
A downside of SAM is that a large part of the telescope aperture is blocked. For current designs for the James Webb Space Telescope or the Very Large Telescope these masks block around 80% of the light. SAM is therefore only useful for bright targets. The LEOPARD have developed a new type of aperture mask, the holographic aperture mask, with liquid-crystal optics with improved transmission and low-resolution spectroscopy. This type of masks creates holographic interferograms that provide additional information.
