5.2 Wavefront sensor

The wavefront sensor is a correlating Shack–Hartmann Wavefront Sensor (SHWFS). The SHWFS processes 76 subaperture images of 20 × 20 pixels. The 76 cm telescope aperture is sampled with 10 subapertures across the pupil resulting in d = 7.5 cm per subaperture. It has been demonstrated that a subaperture of about 8 cm is the smallest allowable subaperture size that delivers a theoretical granulation contrast of a few percent (Berkefeld et al., 2010Jump To The Next Citation Point). In comparison the photon noise on the subaperture images for a typical SHWFS detector is of order 0.5%. The wavefront sensor noise is discussed in more detail in Section 6.1.5. The condition r0 ≤ d results in additional wavefront sensor noise due to anisoplanatism effects within the SHWFS FOV (Section 6.1.6).
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Figure 17: Schematic implementation of a SHWFS. Adjustable components are motorized to automate alignment and calibration procedures (from Richards et al., 2010Jump To The Next Citation Point).

The optical design of the correlating SHWFS is also simple. Figure 17View Image from Richards et al. (2010Jump To The Next Citation Point) shows the main components of a SHWFS assembly. An adjustable square field stop is placed at the WFS focal plane. A lens is used to collimate the field and at the same time image the pupil onto the lenslet array. The lenslet forms the array of subimages. Lenslet arrays of different focal length can be used to vary the size of the WFS FOV. A typical FOV is 10” × 10”. The square field stop is needed to prevent overlap of the subfields in the focal plane of the lenslet array. In addition the stop contains a motorized pinhole mask that can be inserted at the WFS entrance in order to calibrate out aberrations internal to the WFS.

The WFS camera might be placed directly into the focal plane of the lenslet array. However, the subimages have to be matched in size and location to a fixed pixel pattern on the WFS detector. This is achieved by adding a re-imaging optical zoom system.

The relatively high read noise of about 60 electrons of the CMOS WFS camera is not an issue for the solar wavefront sensor since the noise is dominated by shot noise. The camera achieves a frame rate of 2500 fps for a 200 × 200 pixels imaging area. The camera is highly configurable. In its nominal configuration the AO camera reads out 76 subapertures, 20 × 20 pixel each. The 76 20 × 20 pixel subaperture images are processed by 40 DSPs. Ten parallel output ports of the camera, one for each cluster of DSPs, allow fast readout. The camera is programmable to accommodate different formats and frame rates and, thus, is usable for a variety of applications, including Multi-Conjugate AO (MCAO). The camera for the AO76 system is only one implementation example that was driven by the available technology at the time the system was developed. With the steady progression of detector development many off-the-shelf WFS camera options have become available, including interfaces to various processing platforms.

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