Correlation Wave-front Sensor

The pointing system of the gondola was designed to reduce the wind-induced pendulum motion of a few degrees amplitude to about 1000 RMS. As a second stage, the Correlating Wavefront Sensor (CWS), provides for high-frequency precision image stabilization and guiding as well as alignment control for the telescope (low frequency). The CWS unit consists of a camera system, the tip/tilt mirror, electronics and the control software. It is located close to ISLiD in the central compartment of the PFI structure.

The high-speed camera system has a field of view of 12 × 12 arcsec² on the sky. A lenslet array in a pupil plane has six subapertures arranged in a concentric ring, forming six independent images on the detector. The information derived from the six independently analyzed images of the same solar scene are used to measure the local wavefront tilt per subaperture.

The coefficients for tip and tilt, defocus, and Seidel coma are used as error signals. A control loop time-integrates these error signals and converts them into actuation signals to drive the telescope secondary mirror M2. The fast read-out of the CWS camera (>1 kHz) allows detecting correlated image motion of the six separately generated images on the detector caused by residual uncompensated gondola movements and vibration as well as by the slow drift of solar features due to solar rotation.

Fast software routines convert the correlation signals to actuator signals for the tip/tilt mirror, performing the pointing correction, image stabilization and guiding. The closed-loop control of the image stabilization system has a bandwidth of about 60 Hz (at the 6 dB level) with a sensitivity of better than 0.003 arcsec. It provides very efficient damping of low frequencies where the pointing control cannot be provided solely by the gondola.

During its successful flight in 2009, the SUNRISE telescope had by far the highest pointing stability ever achieved on a balloon-borne telescope.

The CWS unit was developed by the Kiepenheuer-Institut für Sonnenphysik, Freiburg, Germany.