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

TL;DR

This paper evaluates the performance of a specialized EMCCD detector designed for the upgraded GPI 2.0 instrument's wavefront sensor, focusing on its noise, linearity, and calibration to enhance exoplanet imaging capabilities.

Contribution

It provides a detailed characterization of the HNu 240 EMCCD's performance metrics, supporting its suitability for high-contrast adaptive optics in GPI 2.0.

Findings

Low dark current and readout noise suitable for extreme AO

High quantum efficiency across key wavelengths

Fast readout capability up to 3000 fps

Abstract

The Gemini Planet Imager (GPI) is a high contrast imaging instrument that aims to detect and characterize extrasolar planets. GPI is being upgraded to GPI 2.0, with several subsystems receiving a re-design to improve the instrument's contrast. To enable observations on fainter targets and increase stability on brighter ones, one of the upgrades is to the adaptive optics system. The current Shack-Hartmann wavefront sensor (WFS) is being replaced by a pyramid WFS with an low-noise electron multiplying CCD (EMCCD). EMCCDs are detectors capable of counting single photon events at high speed and high sensitivity. In this work, we characterize the performance of the HN\"u 240 EMCCD from N\"uv\"u Cameras, which was custom-built for GPI 2.0. The HN\"u 240 EMCCD's characteristics make it well suited for extreme AO: it has low dark current ($<$ 0.01 e-/pix/fr), low readout noise (0.1 e-/pix/fr at a gain of 5000), high quantum efficiency ( 90% at wavelengths from 600-800 nm; 70% from 800-900 nm), and fast readout (up to 3000 fps full frame). Here we present test results on the EMCCD's noise contributors, such as the readout noise, pixel-to-pixel variability and CCD bias. We also tested the linearity and EM gain calibration of the detector. All camera tests were conducted before its integration into the GPI 2.0 PWFS system.