Photon Counting and Precision Photometry for the Roman Space Telescope Coronagraph

The Nancy Grace Roman Space Telescope will include, as one of its two instruments, the highest contrast coronagraph ever attempted, with sensitivity down to Jupiter class planets. With flux ratio below 1e-8, these planets will be exceedingly dim, so that signal rates are as low as 0.01 electrons per second at the imaging detector. These rates necessitate ultra-low noise detectors and methods. For its science imaging camera, the Roman Coronagraph will employ an electron multiplication CCD (EMCCD), to achieve near-zero read noise. EMCCD's, however, deliver the low read noise at the cost of amplification of all other noise, because of the stochastic nature of the electron multiplication process. To circumvent this next-order challenge, a thresholding technique called photon counting can be used. The resulting image has no read noise and no excess noise factor (ENF). The remaining challenge, for precision photometry, is to account for the undercount and overcount effects inherent to photon counting. These arise primarily from the inefficiency of thresholding itself, and coincidence loss, where multiple-electron events are not distinguished from single-electron ones. Here we present a detailed description of the photon counting algorithm and the corrections necessary to achieve photometric accuracy below 0.5%.