Adaptive Optics for Extremely Large Telescopes 4 – Conference Proceedings

Many branches of science require infrared detectors sensitive to individual photons.Applications range from low background astronomy to high speed imaging. Selex-ES Ltd inSouthampton, UK, has been developing HgCdTe avalanche photodiode (APD) sensors forastronomy in collaboration with ESO since 2008 and more recently the University of Hawaii.The devices utilise MOVPE grown on low-cost GaAs substrates and in combination with amesa device structure achieve very low dark current and near-ideal MTF. MOVPE providesthe ability to grow complex HgCdTe heterostructures which have proved crucial to suppressbreakdown currents and allow high avalanche gain in low background situations. A customdevice called Saphira (320x256/24μm) has been developed for wavefront sensors,interferometry and transient event imaging. This device has achieved read noise as low as0.26 electrons rms and single photon imaging with avalanche gains up to x100. It is used inthe ESO Gravity program for adaptive optics and fringe tracking and has been successfullytrialled on the 3m NASA IRTF, 8.2m Subaru and 60 inch Mt Palomar for lucky imaging andwavefront sensing. In future the technology offers much shorter observation times for readnoiselimited instruments, particularly spectroscopy. The paper will describe the MOVPEAPD technology and current performance achievements.