5.3 Wavefront sensor and reconstructor processor unit
The DST AO76 uses a DSP system to perform all computations for sensing and reconstructing the
wavefront. The processing unit (Figure 18) is built from off-the-shelf components based on the ADSP-21160
SHARC DSP. Newer generation DSPs with much higher performance are of course available now. In
addition, CPUs and GPUs in the meantime have enough processing power to perform the processing
functions at the high update rates required and several solar AO systems currently operating use either a
high-end PC (Shand et al., 1999; Scharmer et al., 2000, 2003) or high-end workstations (von der Lühe
et al., 2003) to perform this function.
Figure 18: Left: functional block diagram of AO76 DSP based real time control (RTC) system.
Right: image of RTC (see Rimmele et al. (2004) for details).
The real time processing functions includes the following (see also Berkefeld, 2007):
- Reading the subaperture images into the processors.
- Apply flat and dark field corrections to the subimages.
- Optionally, an intensity gradient can be removed from each subaperture image using a bilinear
fit when the lock-target is, e.g., near the solar limb. This avoids a systematic bias in the shift
measurements as was described by von der Lühe (1983).
- The cross correlation between its two subapertures and a reference subaperture. The reference
subaperture in principle can be picked at random from the set of 76 subapertures. However,
apertures near the edge of the pupil that are occasionally vignetted are avoided. Partially
illuminated apertures are also avoided.
- The maximum of each cross correlation is located to subpixel precision by fitting a parabola
around the maximum pixel.
- Calibration offsets are removed from the x/y shifts.
- Global tilt is computed and removed from the x/y shifts. The global tilt measurements
determined in this way are used to drive the tip/tilt mirror.
- The x/y shifts are multiplied with the predetermined reconstruction matrix to compute actuator
commands. After applying a PI servo algorithm and gain and offset corrections for each actuator
the actuator commands are sent to the DM drive electronics.
The host computer serves as user interface and is not involved in any of the real time processing.