CFHT recently considered an upgrade of its curvature adaptive optics system to perform high dynamic range imaging. This requires high Strehl ratios and very stable PSF, which is usually achieved with high order AO systems providing a dense sampling and correction across the pupil. Such systems are conventionally thought of as Shack-Hartman/piezostack devices because, in theory the scalign law for noise propagation goes as N.log(N) where N is the number of actuators. However, it is difficult to demonstrate this behavior in practice, and as systems get more compelx, their efficiency drops to lower levels than expected. Simulations of such systems are usually optimistic because they fail to take the complexity of a real system's flaws into account.
Until now, only avalanche photodiodes (APD) have been used as the detectors in curvature wavefront sensors in astronomy. This is due to the strict requirements of very short integration time and very low readout noise. In 1999, Beletic et al. invented a new CCD design which should achieve the same performance as APDs but with higher reliability and lower cost. In addition, this CCD has higher quantum efficiency than APD modules and larger dynamic range, eliminating the need for neutral density filters on bright objects. The CCD was designed and fabricated by MIT Lincoln Laboratory in collaboration with ESO and IfA. R. Dorn extensively tested the CCD in laboratory at ESO and proved that it achieves the predicted performance. CFHT is currently implementing this CCD on PUEO, CFHT’s Adaptive Optics system, to assess its performance for the first time in real conditions on the sky for a direct comparison with the current 19 APD detector system. In this article we present the current implementation scheme and discuss the upgrade we foresee for PUEO NUI, a 104-element high-order curvature AO system envisaged to replace the current AO system at Canada-France-Hawaii Telescope.
In this paper, the results from a computer simulation comparison between curvature AO-systems using APDs and a special-purpose curvature-CCD, the MIT/LL CCID-35, are presented. The simulated AO-system is the MACAO 60-element curvature system, which is being developed by ESO for the Very Large Telescope (VLT) and VLT Interferometer (VLTI). The results indicate a difference in performance of 5% Strehl in K-band for a 15th magnitude guide star, 2% Strehl for a 16th magnitude star, and less than 0.6% Strehl for all other magnitude guide stars.