The JWST Resolved Stellar Populations Early Release Science Program VII. Stress Testing the NIRCam Exposure Time Calculator

TL;DR

This study empirically evaluates the JWST NIRCam Exposure Time Calculator's accuracy using observations of resolved stars, highlighting the impact of non-ideal effects, source positioning, and crowding on SNR predictions and saturation limits.

Contribution

It provides an empirical assessment of the ETC's estimates, offering guidelines to improve its accuracy for different observational scenarios.

Findings

Poissonian SNR estimates match observations for bright stars

Sub-pixel dithering improves saturation limits by up to ~1 mag

ETC aperture strategy can affect exposure time estimates by up to a factor of 5

Abstract

We empirically assess estimates from v3.0 of the JWST NIRCam Exposure Time Calculator (ETC) using observations of resolved stars in Local Group targets taken as part of the Resolved Stellar Populations Early Release Science (ERS) Program. For bright stars, we find that: (i) purely Poissonian estimates of the signal-to-noise ratio (SNR) are in good agreement between the ETC and observations, but non-ideal effects (e.g., flat field uncertainties) are the current limiting factor in the photometric precision that can be achieved; (ii) source position offsets, relative to the detector pixels, have a large impact on the ETC saturation predictions and introducing sub-pixel dithers in the observation design can improve the saturation limits by up to ~1 mag. For faint stars, for which the sky dominates the error budget, we find that the choice in ETC extraction strategy (e.g., aperture size relative to point spread function size) can affect the exposure time estimates by up to a factor of 5. We provide guidelines for configuring the ETC aperture photometry to produce SNR predictions in line with the ERS data. Finally, we quantify the effects of crowding on the SNRs over a large dynamic range in stellar density and provide guidelines for approximating the effects of crowding on SNRs predicted by the ETC.