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 wave-front sensor, mounted on its light-weight
carbon fiber sandwich plate. Two fixation points are visible on the sides.

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.

Sub-images (left) and correlations functions(right) of the wave-front sensor during closed-loop tracking. Screen shot taken from the flight-control system.

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.

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