Figure 16 shows a picture of the high order AO76 system at the DST (Rimmele, 2004a). Since the DST was retrofitted with AO, integration of the DM and tip/tilt mirror into the main telescope optics was not possible. Instead an AO bench had to be inserted between prime focus and the instruments. Since the DST has two instrument stations, two identical AO benches were installed that feed the diverse instrumentation. Figure 15 shows a comparison of a long exposure (3 s) image of granulation obtained with the AO76 operating (top) and with just tip/tilt compensation (bottom).
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Calibration of the AO system is extremely important to obtain optimal performance. Hence calibration tools are an integral part of the AO setup. A motorized aperture wheel placed at prime focus holds field-stops, a resolution target, a calibration pinhole, and a small mirror, which optionally feeds light from a single-mode fiber into the setup. The pinhole serves as artificial object for alignment of actuator and wavefront sensor grids. The pinhole also can be used to flatten the DM and co-align the focal plane of the WFS and prime focus. The laser feed is used to test and align the AO optics and to flatten the DM to very high precision using a interferometer (Ren et al., 2003). The laser interferometer can be placed at or near a instrument detector focal plane. Non-common path optical aberrations can be measured and calibrated out in this way.
A spherical collimator mirror follows prime focus and forms an image of the pupil on the 30 mm tip/tilt mirror. The tip/tilt mirror is mounted at a 45-degree angle and directs the light into the horizontal axis, i.e., onto the AO bench. Two off-axis parabolas (OAP) serve as collimator and camera mirrors, respectively. The collimator forms a 77 mm image of the pupil on the deformable mirror. The camera parabola forms an image of the Sun. This is a very common approach to implementing AO into the optical path between telescope and instruments. A cube beam splitter near the focal plane of OAP2 transmits about 5% of the light to the WFS assembly. The rest of the light is reflected to the science instrumentation.
Separate pupil imaging for tip/tilt and deformable mirrors is implemented to allow a significantly smaller tip/tilt device that achieves high bandwidth. The importance of high bandwidth tip/tilt correction has been pointed out by Conan et al. (1995). Because of the large variance contained in the tip and tilt modes it is extremely important to correct these modes efficiently with high gain and high bandwidth. For example, if a reduction of tip/tilt variance by a factor of ten is achieved, the residual tip/tilt variance is still of the same order as the variance in all the higher modes combined. A small tip/tilt device is also favorable in terms of cost.
The small fraction of light directed to the wavefront sensor path is split further by additional 5% cube beam-splitters to provide light for a video camera for visual performance control and target selection, and for the detector of the stand alone tip/tilt compensation system (correlation tracker (von der Lühe et al., 1989) that is implemented as an option in the AO system. A tip/tilt measurement can also be directly derived from the AO76 wavefront sensor.
Living Rev. Solar Phys. 8, (2011), 2
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