SKYSURF VIII -- Modeling SKYSURF Completeness Data for Comparison to the Hubble Space Telescope Exposure Time Calculator

TL;DR

This paper models the completeness of galaxy detection in HST images based on exposure time and background, comparing the results to the official Exposure Time Calculator to identify discrepancies.

Contribution

It introduces a method to empirically assess galaxy completeness using simulated objects and compares these results to the HST ETC, highlighting differences for extended sources.

Findings

ETC predicts 1-2 magnitudes fainter completeness limits for extended galaxies.

Discrepancies are due to ETC's simplified flat surface brightness profiles.

The method improves understanding of detection limits in HST imaging.

Abstract

Accurately assessing image source completeness is critical for interpreting measurements on telescope data. Using the Wide Field Camera 3 (WFC3) and Advanced Camera for Surveys (ACS) data from the Hubble Space Telescope ($HST$) archival project "SKYSURF", we model galaxy completeness as a function of the exposure time and background of an image. This is accomplished by adding simulated objects with varying magnitudes and sizes into these images, and determining the detection rate for each set of parameters. The fifty percent completeness results are then compared to the Exposure Time Calculator (ETC), in order to assess the differences between the STSCI ETC and our analysis of the archival data. Ultimately, we find that, for extended galaxies, the ETC predicts a 1-2 magnitudes fainter completeness limit than our data. We believe the difference is due to the ETC's flat surface brightness profiles being less accurate at predicting for extended sources compared to our more realistic profiles.