The SUNRISE Telescope
With its aperture of 1 m and nearly 25 m focal length, the SUNRISE telescope is the largest and most powerful solar telescope that has ever left the Earth. It uses innovative mirror technologies, active in-flight alignment and image stabilization systems. Thanks to its high spatial resolution of less than one tenth of an arcsecond, the telescope is capable of resolving structures smaller than 100 km on the solar surface.
The SUNRISE telescope is a Gregory-type reflector consisting of a parabolic primary mirror (M1) with 2.5 meters focal length and an eliptical secondary mirror (M2). These mirrors are built into a light-weight and very stable carbon fibre structure. Two plane folding mirrors (M3 and M4) behind the primary mirror redirect the light from the telescope to the postfocus instrumentation (PFI) sitting piggy-back on the telescope.
Solar telescopes require special measures to protect their instruments from the Sun’s radiation. During the observations, nearly 1 kW solar radiation is concentrated on a disk of about 22-mm diameter in the first focal spot. Since the instruments analyze only a small area on the Sun, a great part of the incoming light needs to be deflected to prevent it from damaging the instruments. At this position a heat rejection wedge (HRW) acts as field stop. Two ammonia-filled heatpipes connect the HRW to dedicated radiators to prevent the HRW from overheating, thus avoiding Schlieren build-up, which could cause wavefront deformations.
The HRW has a central hole of about 2.5 mm diameter (about 10 % of the diameter of the Sun’s image). Only the light passing through this hole will hit the telescope‘s secondary mirror to be transmitted to the instruments.
The secondary mirror can be actively controlled to micrometer precision in order to provide diffraction-limited optical performance even in varying environmental conditions (telescope elevation and changing thermal environment due to passing over cloud decks, sea/land/ice).