The Wide-Field Infrared Survey Telescope (WFIRST) is a NASA flagship space observatory expected to launch in the mid-2020s. The mission will probe dark energy as well as carry out broad infrared surveys. WFIRST will also image and spectrally characterize extrasolar planets with a coronagraph at an unprecedented level of sensitivity. The faint planet targets require photon counting detectors to meet the stringent signal-to-noise requirements. The CCD201-20 from Teledyne e2v, an electron multiplying CCD (EMCCD) sensor has been baselined for flight. However, the challenging WFIRST L2 radiation environment must first be considered. Radiation effects from protons and other high energetic particles can hinder the charge transfer efficiency (CTE) of EMCCDs – so-called displacement damage that causes ‘traps’ in the device – as well as affect dark current and other important performance. In order to investigate these effects, we established a photon counting laboratory at JPL which has been used to fully characterize pre- and post-irradiation EMCCD sensors following an extensive radiation campaign. This included studies of read out noise, dark current, clock induced charge and electron multiplication gain. Additionally, we built a scene generator capable of delivering flight-like images on the detector in order to probe degradation of CTE using realistic scenes at the single photon level. Beyond this characterization, we have implemented a technology development program currently underway between JPL and Teledyne e2v to modify the CCD201-20 for radiation hardness in space. These modifications are designed to greatly increase CTE in the presence of traps in addition to reducing the effects of cosmic rays under high EM gain conditions. In parallel to this development, we have also designed techniques that aim to identify damaged regions of the devices and mitigate their effects. I will speak about the challenges of photon counting with EMCCDs in space, with particular focus on WFIRST, in addition to the various technology development and mitigation programs that we have completed.
Leon K. Harding, "Deploying electron multiplying CCDs in photon starved coronagraph and spectrograph instruments for WFIRST (Conference Presentation)," Proc. SPIE 10639, Micro- and Nanotechnology Sensors, Systems, and Applications X, 106391M (Presented at SPIE Defense + Security: April 18, 2018; Published: 14 May 2018); https://doi.org/10.1117/12.2305187.5783297844001.
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