A decadal survey of AO wavefront sensing detector developments in Europe
The success of the next generation of instruments for 8 to 40-m class telescopes will depend on the ability of Adaptive Optics (AO) systems to provide excellent image quality and stability. This will be achieved by increasing the sampling, wavelength range and correction quality of the wave front error in both spatial and time domains.
The modern generation of AO wavefront sensor detectors started in the late nineties with the development of the CCD50 detector by e2v under ESO contract for the ESO NAOS AO system. With a 128x128 pixels format, this 8 outputs CCD runs at a 500 Hz frame rate with a readout noise of 7e-.
A major breakthrough has been achieved with the recent development of the CCD220, also by e2v technologies. This 240x240 pixels 8 outputs EMCCD (CCD with internal multiplication), also developed by e2v, has been jointly funded by ESO and Europe under the FP6 programme. The CCD220 detector and the OCAM2 camera are now the most sensitive system in the world for advanced adaptive optics systems, offering an astonishing <0.2 e readout noise at a frame rate of 1500 Hz with negligible dark current. Extremely easy to operate, OCAM2 only needs a 24 V power supply and a modest water cooling circuit. This system is extensively described elsewhere in this conference (see Gach J-L. et al). An upgrade of OCAM2 is foreseen to boost its frame rate to 2500 Hz, opening the window of XAO wavefront sensing for the ELT.
Since this major success, new developments started in Europe. One is fully dedicated to Laser Guide Star AO for the ELT. The spot elongation from a LGS SH wavefront sensor induces an increase of the pixel format. Two detectors are currently developed by e2v. The NGSD will be a 672x672 pixels CMOS detector with a readout noise of 4e (goal 1e) at 700 Hz frame rate. The LGSD is a scaling of the NGSD with 1680x1680 pixels and 3 e readout noise (goal 1e) at 700 Hz frame rate. New technologies will be developed for that purpose: new CMOS pixel architecture, CMOS back thinned and back illuminated device, full digital outputs. In addition, the CMOS technology is extremely robust in a telescope environment. Both detectors will be used on the ELT, depending on the AO system considered.
Additional developments also started for wavefront sensing in the infrared based on new breakthrough in Avalanche Photodiode (APD) arrays. A 256x256 array is currently developed by ESO and Selex while a similar development started in France with the RAPID project. The latter should offer a 320x240 8 outputs 30 microns IR array, sensitive from 0.4 to 3.2 microns, with 2 e readout noise at 1500 Hz frame rate.