Measuring the Column Dependence of Read Noise in ACS/WFC Bias Frames

TL;DR

This study investigates the dependence of read noise on column position in ACS/WFC bias frames, finding no column dependence but identifying factors affecting read noise levels and their evolution.

Contribution

It provides a detailed analysis of read noise behavior in ACS/WFC bias frames, including effects of anneal periods and hot pixel masking, which was previously uncharacterized.

Findings

No column dependence of read noise was observed.

Read noise in pre-scans is about 0.5 e$^-$ lower than in science arrays.

Masking hot pixels reduces read noise in affected columns.

Abstract

The noise in bias frames for all four readout amplifiers in the Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) is dependent on row number. This is because dark current accumulated during readout increases across the detector, influencing and increasing the read noise as a function of row number. In this report, we investigate bias frames taken with the ACS/WFC to explore the column dependence of read noise for each of the amplifiers for different anneal periods. Analyzing the data, we find that there is no column dependence of read noise and that the read noise values for the physical pre-scans are approximately 0.5 e$^-$ lower than in the science arrays because there is no readout dark accumulated in this area. We further investigate 1) the evolution of read noise over an anneal period, 2) a linear decrease in read noise within the initial columns per amplifier, and 3) pixels in elevated read noise columns. We conclude that 1) there is no visual trend of read noise over an anneal period, 2) amplifiers A and C have an initial linear decrease of read noise in the science arrays, and 3) masking unstable hot pixels in a column will decrease its read noise values.