Scientific detector workshop 2022 on-sky performance verification of near-infrared e−APD technology for wavefront sensing and demonstration of e−APD pixel performance to improve the sensitivity of large science focal planes

Near‑infrared adaptive optics as well as fringe tracking for coherent beam combination in optical interferometry require the development of high‑speed sensors. Because of the high speed, a large analog bandwidth is required. The short exposure times result in small signal levels which require noiseless detection. Both requirements cannot be met by state‑of‑the‑art conventional CMOS technology of near‑infrared arrays as has been attempted previously. A total of five near‑infrared SAPHIRA 320 × 256 pixel HgCdTe e−APD arrays have been deployed in the wavefront sensors and in the fringe tracker of the VLTI instrument GRAVITY. The current limiting magnitude for coherent exposures with GRAVITY is mk = 19, which is made possible with ADP technology. New avalanche photo‑diode array (APD) developments since GRAVITY include the extension of the spectral sensitivity to the wavelength range from 0.8 to 2.5 μm. After GRAVITY a larger format array with 512 × 512 pixels has been developed for both AO applications at the ELT and for long integration times. Since dark currents of <10−3 e−/s have been demonstrated with 1Kx1K e−APD arrays and 2Kx2K e−APD arrays have already been developed, the possibilities and adaptations of e−APD technology to provide noiseless large‑format science‑grade arrays for long integration times are also discussed.