PLANETS will be an off-axis telescope combining several new technologies and instrumentation techniques. Off-axis telescopes can have far superior constrast 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 coronography 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 - coronography 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 will be constructed on Haleakala, a 3000m (10,000ft) volcano on the island of Maui, HI with excellent weather and seeing.
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, coronography, 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.
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.
High Precision Polishing with HyDra
UNAM has developed an incredible new tool to polish extremely smooth surfaces. The group has recently demonstrated 1/100th of a wavelength polish on several optics. This polish will further reduce the diffuse scattered light component from the telescope increasing the dynamic range of the telescope.
Another technique for suppressing scattered light from nearby bright sources is coronography. This image shows a simulation of a coronograph 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 coronograph 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 anglular 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.
Adaptive optics can be used to suppress atmospheric distortion and scattering. The Planets group has experience with several types of adaptive optics in design, construction and use. By combining adaptive optics with other techniques, greater contrast enhancement and dynamic range are possible.