Pushing the limits of the Cosmic Origin Spectrograph (COS) with an optimized background correction

TL;DR

This paper introduces ACDC, an optimized 2D background correction pipeline for the Cosmic Origin Spectrograph, significantly improving the signal-to-noise ratio in ultraviolet observations, especially for faint targets.

Contribution

The authors developed and validated a new 2D background correction method, ACDC, tailored for COS FUV data, enhancing data quality for low signal-to-noise observations.

Findings

Average S/N improvement of ~10% across spectra

Up to 60% S/N enhancement in some cases

Effective background correction for faint ultraviolet targets

Abstract

Observations utilizing the ultraviolet capabilities of the Cosmic Origin Spectrograph (COS) onboard the Hubble Space Telescope are of unique value to the astronomy community. Spectroscopy down to 900 A with COS has enabled new science areas. However, contrary to the situation at longer wavelengths, these observations are limited by detector background noise. The background correction currently applied by the standard calibration pipeline (CalCOS) is not optimized for faint targets, limiting the scientific value of low signal-to-noise observations. In this work we investigate a possible dependence of the variations of the dark rate in both segments of the COS far-ultraviolet (FUV) detector on time, detector high voltage (HV), and solar activity. Through our analysis we identified a number of detector states (on a configuration basis, e.g., HV and segment) characterizing the spatial distribution of dark counts, and created superdarks to be used in an optimized 2-dimensional (2D) background correction. We have developed and tested Another COS Dark Correction (ACDC), a dedicated pipeline to perform a 2D background correction based on statistical methods, producing background-corrected and flux-calibrated spectra. While our testing of ACDC showed an average improvement in S/N values of ~10%, in a few cases the improvements in S/N reached 60% across the whole wavelength range of individual segments.