报告题目：State of the art IR cameras for wavefront sensing using e-APD MCT arrays

　　报告人：Philippe Feautrier

　　Institut de Planétologie et d'Astrophysique de Grenoble – France

　　First Light Imaging SAS - France

　　摘要：The success of the next generation of instrument for large (up to 10 m) or extremely larges telescopes (up to 40 m) in the visible-infrared 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 and correction of the wave front error in both spatial and time domains. For example, advanced Shack Hartmann systems currently fabricated require 40x40 sub-apertures at sampling rates of 1-1.5 kHz as opposed to 14x14 sub-apertures at 500 Hz of previous AO systems. Beyond the e2v CCD50 developed for the ESO NACO instrument in the late nineties, new detectors of 240x240 pixels are required to provide the spatial dynamics of 5-6 pixels per sub-aperture. Higher temporal-spatial sampling implies fewer photons per pixel therefore the need for much lower read noise (<<1e-) and negligible dark current (<< 1e-/pixel/frame) to detect and centroid on a small number of photons. This detector development was jointly funded by ESO and the OPTICON European network in the Joint Research Activity JRA2, “Fast Detectors for Adaptive Optic". e2v technologies was chosen in 2005 to develop a dedicated detector based on an extension of their L3Vision EMCCD technology. Analysis showed that the sub-electron read noise of L3Vision CCDs clearly outperformed classical CCDs even though L3Vision devices exhibit the excess noise factor F of 21/2 typical of EMCCDs.

　　During these years, a revolution appeared for infrared HgCdTe avalanche photodiodes detector arrays providing outstanding sensitivity and speed at the same time.

　　Developed by First Light Imaging and based on the Saphira detector developed by Selex for ESO, the C-RED infrared camera is opening a new era in terms of sensitivity and speed in the SWIR scientific cameras domain and is particularly suited for infrared wavefront sensing in complex AO systems like MCAO.This is in strong contrast to what is observed in APDs made out of III-V material or Si, which requires high inverse bias and have typical noise factors of F~4-5 for III-V semi-conductors and F~2-3 for Si respectively. These exceptional characteristics of HgCdTe APDs are due to a nearly exclusive impaction ionization of the electrons, why these devices have been called electrons avalanche photodiodes, e-APDs. These results have inspired a large effort in developing focal plan arrays using HgCdTe APDs for low photon number applications such as active imaging in the range gated mode (2D) and/or with direct time of flight detection (TOF) (3D) and, more recently, passive imaging for infrared wave front correction and fringe tracking in astronomical observations. C-RED is using the SAPHIRA 320x256 2.5 microns cut-off 24 microns pixel pitch HgCdTe e-APD array allowing to obtain sub-electron readout noise, taking advantage of the APD noise-free multiplication gain and non destructive readout ability.

　　Another major AO wavefront sensing detector development concerns the RAPID project. Developed by the SOFRADIR and CEA/LETI manufacturers, the latter offers a 320x255 8 outputs 30 microns e-APD array, sensitive from 0.4 to 3 microns, with less than 2 e readout noise at 1600 fps. A rectangular window can also be programmed to speed up even more the frame rate when the full frame readout is not required. The high QE response, in the range of 70%, is almost flat over this wavelength range. Advanced packaging with miniature cryostat using pulse tube cryocoolers was developed in the frame of this programme in order to allow use on this detector in any type of environment. The characterization results of this device are presented here. Readout noise as low as 1.7 e at 1600 fps has been measured with a 3 microns wavelength cut-off chip and a multiplication gain of 14 obtained with a limited photodiode polarization of 8V. This device also exhibits excellent linearity, lower than 1%. The pulse tube cooling allows smart and easy cooling down to 55 K. Vibrations investigations using centroiding and FFT measurements were performed proving that the miniature pulse tube does not induce measurable vibrations to the optical bench, allowing use of this cooled device without liquid nitrogen in very demanding environmental conditions. In 2013, the partners delivered the first prototypes and, given the performance results of these prototypes, the decision was quickly taken to push for an on-sky demonstration on a demanding instrument. PIONIER was chosen as its interferometric combination of light requires a very fast detector to fight against atmospheric turbulence, and a minimum amount of noise in order to detect faint objects. The RAPID detector is now implemented on the PIONIER instrument on the ESO/VLTI interferometer in Paranal since December 2014. Since this time, RAPID observed more than 150 stars during more than 45 nights on the VLTI with a tremendous gain compared the previous camera based on conventional IR detectors.

　　报告题目：Herschel/HIFI observations of the diffuse interstellar medium

　　报告人：Maryvonne Gerin（巴黎天文台）

　　摘要：The Herschel space mission and the SOFIA airborne telescopes have provided a full access to the submillimeter spectral domain.In this talk I will present advances in the understanding of the diffuse interstellar medium, that were obtained thanks to Herschel and SOFIA. Absorption spectroscopy of hydrides and fine structure lines toward bright background continuum sources have led to a renewed view of the diffuse ISM physical conditions, and to the first step of molecule formation in the ISM. Diagnostics of the m olecular gas, cosmic ray ionization rate, dissipation of turbulence, elemental abundances, established for the local ISM, can now be used in external galaxies up to very distant ones

　　时间：2015年9月29日 星期二下午14:30

　　地点：（北京西路2号）紫台大楼619会议室

　　联系人：李子睿（83332207）

欢迎大家积极参加！

　　                                                紫金山天文台学术委员会