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The Gemini Planet Imager (GPI) is a high contrast imaging instrument thataims to detect and characterize extrasolar planets. GPI is being upgraded toGPI 2.0, with several subsystems receiving a re-design to improve theinstrument's contrast. To enable observations on fainter targets and increasestability on brighter ones, one of the upgrades is to the adaptive opticssystem. The current Shack-Hartmann wavefront sensor (WFS) is being replaced bya pyramid WFS with an low-noise electron multiplying CCD (EMCCD). EMCCDs aredetectors capable of counting single photon events at high speed and highsensitivity. In this work, we characterize the performance of the HN\"u 240EMCCD from N\"uv\"u Cameras, which was custom-built for GPI 2.0. The HN\"u 240EMCCD's characteristics make it well suited for extreme AO: it has low darkcurrent ($<$ 0.01 e-/pix/fr), low readout noise (0.1 e-/pix/fr at a gain of5000), high quantum efficiency ( 90% at wavelengths from 600-800 nm; 70% from800-900 nm), and fast readout (up to 3000 fps full frame). Here we present testresults on the EMCCD's noise contributors, such as the readout noise,pixel-to-pixel variability and CCD bias. We also tested the linearity and EMgain calibration of the detector. All camera tests were conducted before itsintegration into the GPI 2.0 PWFS system.

GPI 2.0: Performance Evaluation of the Wavefront Sensor's EMCCD