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

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.

Figures (12)

• Figure 1: The layout of WFC full-frame images, including parallel and serial readout directions, physical prescans, and virtual overscans acsihb.
• Figure 2: Data from the 2022-08-17 anneal period. Each point in a column refers to data from a difference image of two raw biases. Each plot with respect to amplifier was scaled the same to easily identify differences. The orange dashed-line separates the physical pre-scans columns (left) from the rest of the science columns (right).
• Figure 3: Data from the 2022-03-28 anneal period. All plot characteristics are the same as described in Figure \ref{['fig:202208anneal']}.
• Figure 4: Data from the 2017-09-21 anneal period. All plot characteristics are the same as described in Figure \ref{['fig:202208anneal']}.
• Figure 5: Data from the 2005-04-19 (Pre-SM4) anneal period. All plot characteristics are the same as described in Figure \ref{['fig:202208anneal']}.