PLANETS will be an off-axis telescope combining several new technologies and instrumentation techniques. Off-axis telescopes can have far superior contrast because there are no obstructions in the beam such as secondary mirror supports. This limits the diffraction as well as scattered light from obstructions. The telescope will also be highly polished to minimize diffuse scatter from mirror roughness – a major source of scattering at large angles.
This telescope will be ideal for coronagraphy and other techniques requiring stable optical path as it will be seeing limited with very low instrumental scattered light. By combining expertise from various fields – coronagraphy and high contrast imaging from solar physics, polishing, polarimetry and adaptive optics from astronomical communities and the experience of each institutional partner, this telescope will make significant advances in several fields. The telescope is proposed to be constructed on Haleakala, a 3000m (10,000ft) volcano on the island of Maui, HI with excellent weather and seeing.
PLANETS is also a pathfinder project for hyper-telescopes dedicated to finding life and civilizations on planets around stars in our neighborhood. Its off-axis design is prototypical for the 74m telescope Colossus to be made of 60 8m off-axis telescopes.
The PLANETS telescope is optimized to study faint environments around bright sources. These include:
Exoatmospheres of terrestrial planets in the Solar system,
Atmospheres and surfaces of nearby bright exoplanets,
Circumstellar environment and protoplanets in circumstellar disks,
Biosignatures on potentially habitable exoplanets (path-finder).
The instrument suite will provide opportunities to investigate these environments using tools of polarimetry, spectropolarimetry, and coronagraphy. These instruments are being built by the consortium institutes.
The Overall Design
The Planets telescope is similar in design to the 1.6m “New Solar Telescope” or NST. A collaboration between IfA and Big Bear Solar Observatory is a few months from achieving first light on the 1.6m NST. This experience helped with the design of the 1.9m Planets telescope and will speed the completion of this project. There are a number of new off-axis or un-obstructed telescope designs such as the Advanced Technology Solar Telescope or ATST on Haleakala, High-Dynamic-Range Telescope (HDRT) as a replacement for the Canada-France-Hawaii Telescope on Mauna Kea, the 25m Giant Magellan Telescope (which partly adopted the HDRT design) and the NST. The main techniques will be high-dynamic range imaging, imaging polarimetry, aperture polarimetry, spectropolarimetry,
The main techniques will be high-dynamic range imaging, imaging polarimetry, aperture polarimetry, spectropolarimetry, coronagraphy, and adaptive optics assistance for many of these techniques.
There are a number of new off-axis or un-obstructed telescope designs such as the Advanced Technology Solar Telescope or ATST on Haleakala, High-Dynamic-Range Telescope (HDRT) as a replacement for the Canada-France-Hawaii Telescope on Mauna Kea, the 25m Giant Magellan Telescope (which partly adopted the HDRT design) and the NST. The main techniques will be high-dynamic range imaging, imaging polarimetry, aperture polarimetry, spectropolarimetry, coronagraphy, and adaptive optics assistance for many of these techniques.
Low Scattered Light
There are several contributions to the scattered light background of any telescope. Diffraction caused by the aperture, diffraction and scattering off obstructions in the beam, mirror roughness and figure error all contribute to the background of any image. For a 2m telescope, mirror roughness typical of today’s telescopes is comparable to the diffraction. In a conventional telescope with a secondary obstructing the primary and with support “spiders”, the diffraction off these spiders gives a much higher background than either diffraction or roughness at large angular separations.
Cutting Edge Off-Axis Design
Off-axis systems are not asymmetric systems, they are decentered systems. They provide an inherently low scattered light design because there are no obstructions in the beam. There are a minimal number of scattered light sources. All mirrors can be robustly supported and articulated because of the easy access allowed by this design. There are several myths about off-axis telescopes. They are more difficult to align, but are not inherently more “aberrated”. The telescope can be thought of as a section of a larger system where the full aperture is not illuminated. The blur in this system is only weakly dependent on the off-axis angle and the telescope will be entirely seeing limited.
Another technique for suppressing scattered light from nearby bright sources is coronagraphy. This image shows a simulation of a coronagraph mounted on a Keck-like segmented mirror telescope compared to a GMT-like telescope with a combination of 6 unobstructed off-axis mirrors. The top row of the image shows the simulated wave fronts phase incident on the mirrors under normal atmospheric conditions.
The coronagraph is efficient at suppressing the light from the central star, producing a “hole” in the middle of each image. However, the speckles formed at large angular separation are comparable to the intensity of the planet in the Keck-like design. The diffraction and scattering off the hexagonal segmented mirror edges produces a large scattered light background.