Shackhartmann Wavefront Sensor Mode Shwfs

The timing for the three SHWFS CCDs is straightforward (see Fig. 1). All three CCDs operate with the same frame rate and binning factors (1x1 or 2x2). In principle, the three CCDs could be operated by a single clock driver board because their timing is identical. However, in practice the wiring is more complex and performance is reduced due to longer cable lengths and larger capacities. Therefore each CCD is now driven by its own clock driver board, which allows chip specific settings of clock levels.

The frame rate required for the SHWFS mode is 400 Hz (for a 64x64 window). A timing budget was established given the constraints of the CCD39-01 for maximum vertical and horizontal clocking rates and of the FIERA controller for maximum pixel rate. Table 2 shows this budget, assuming four horizontal prescans, one "pipeline" and two overscan pixels (to clean the horizontal register) per CCD quadrant. Similarly, two overscan lines for each half of the CCD are factored in.

Unfortunately, in executing the CCD clock patterns the FIERA controller created unexpected clocking overhead, which slowed the frame rate in SHWFS windowed mode from a theoretically possible 480 Hz to a much lower value of approx. 370 Hz. One kind of overhead was identified in the execution of loops, another was observed when executing very short clock patterns (i.e. vertical line shift with 2 ^s duration, but ~9 ^s of excess overhead). The loop overhead was reduced by unrolling the inner loops of the timing patterns which are executed 32 times in the case of windowed SHWFS mode. The second measure combined the short patterns for line shifting and skipping pre- and overscan pixels into one single pattern. After these modifications, the "execution" time of 4000 frames has been measured at 10.2 s, which corresponds to a frame rate slightly below 400 Hz.

Figure 1. Shack-Hartmann readout sequence.
Table 2. Timing budget for SHWFS and LOWFS modes.

Chip format

No. of outputs hor.

2

No. of outputs ver.

2

Total hor. pixels

94

Total ver. pixels

84

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