Computing the Electronic Gain for Detectors Read Out Up-The-Ramp

TL;DR

This paper introduces a likelihood-based method for accurately estimating the electronic gain of detectors from up-the-ramp readout data, accounting for nonlinearities and providing consistent, nearly unbiased results, demonstrated on Roman Space Telescope data.

Contribution

A novel likelihood-based approach for gain estimation from up-the-ramp data that accounts for detector nonlinearities and improves accuracy over previous methods.

Findings

The method yields consistent and nearly unbiased gain estimates.

Application to Roman Space Telescope data reveals pixel-to-pixel gain variations.

Code implementation is publicly available for reproducibility.

Abstract

The electronic gain -- the conversion between photoelectrons on a pixel and the digital number recorded to disk -- gives physical units to an astronomical image and sets the relation between pixel value and photon noise. This paper presents a new, likelihood-based approach to derive the gain from images taken up-the-ramp, where the detector is read out nondestructively many times before being reset. Our method makes full use of the individual reads assuming an ideal detector subject to photon noise and Gaussian read noise. We extend the method to account for slight nonlinearities in the relation between photoelectrons and measured counts. We demonstrate that our likelihood-based approach provides a consistent (i.e. asymptotically correct) and nearly unbiased estimator of the gain both with and without fitting for nonlinearity. Finally, we apply this approach to a single detector from the Wide-Field Instrument on the Roman Space Telescope, and show how pixel-to-pixel gain variations describe much of the variations in pixel response seen in flatfield images. Code to compute gain and regenerate figures in this paper is available at https://github.com/RomanSpaceTelescope/SOCReferenceFileCode.