PROCEEDINGS ARTICLE | September 13, 2012
Proc. SPIE. 8447, Adaptive Optics Systems III
KEYWORDS: CMOS sensors, Avalanche photodetectors, Imaging systems, Cameras, Sensors, Quantum efficiency, Wavefront sensors, Adaptive optics, Infrared radiation, Charge-coupled devices
The purpose of this paper is to give an overview of the state of the art wavefront sensor detectors developments held in
Europe for the last decade.
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 development started in the late nineties with the CCD50
detector fabricated by e2v technologies under ESO contract for the ESO NACO AO system. With a 128x128 pixels
format, this 8 outputs CCD offered a 500 Hz frame rate with a readout noise of 7e-.
A major breakthrough has been achieved with the recent development by e2v technologies of the CCD220. This
240x240 pixels 8 outputs EMCCD (CCD with internal multiplication) has been jointly funded by ESO and Europe under
the FP6 programme. The CCD220 and the OCAM2 camera that operates the detector are now the most sensitive system
in the world for advanced adaptive optics systems, offering less than 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, commercialized by First Light Imaging, is extensively described in this paper. An upgrade of
OCAM2 is foreseen to boost its frame rate to 2 kHz, opening the window of XAO wavefront sensing for the ELT using 4
synchronized cameras and pyramid wavefront sensing.
Since this major success, new developments started in Europe. One is fully dedicated to Natural and Laser Guide Star
AO for the E-ELT with ESO involvement. The spot elongation from a LGS Shack Hartman wavefront sensor
necessitates an increase of the pixel format. Two detectors are currently developed by e2v. The NGSD will be a 880x840
pixels CMOS detector with a readout noise of 3 e (goal 1e) at 700 Hz frame rate. The LGSD is a scaling of the NGSD
with 1760x1680 pixels and 3 e readout noise (goal 1e) at 700 Hz (goal 1000 Hz) frame rate. New technologies will be
developed for that purpose: advanced CMOS pixel architecture, CMOS back thinned and back illuminated device for
very high QE, full digital outputs with signal digital conversion on chip. In addition, the CMOS technology is extremely
robust in a telescope environment. Both detectors will be used on the European ELT but also interest potentially all giant
telescopes under development.
Additional developments also started for wavefront sensing in the infrared based on a new technological breakthrough
using ultra low noise Avalanche Photodiode (APD) arrays within the RAPID project. Developed by the SOFRADIR and
CEA/LETI manufacturers, the latter will 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. The high QE response is almost flat over this wavelength range.
Advanced packaging with miniature cryostat using liquid nitrogen free pulse tube cryocoolers is currently developed for
this programme in order to allow use on this detector in any type of environment. First results of this project are detailed
here.
These programs are held with several partners, among them are the French astronomical laboratories (LAM, OHP,
IPAG), the detector manufacturers (e2v technologies, Sofradir, CEA/LETI) and other partners (ESO, ONERA, IAC,
GTC). Funding is: Opticon FP6 and FP7 from European Commission, ESO, CNRS and Université de Provence,
Sofradir, ONERA, CEA/LETI and the French FUI (DGCIS).
