Removing the Pattern Noise from all STIS Side-2 CCD data

When HST/STIS resumed operations in July 2001 using its redundant “Side-2” electronics, the read-noise of the CCD detector appeared to have increased by ∼1 e− due to a superimposed and highly variable “herring-bone” pattern noise. For the majority of programs aiming to detect signals near the STIS design limits, the impact of this noise is far more serious than implied by a mere 1 e− increase in amplitude of the read-noise, as it is of a systematic nature and can result in ∼8 e− relative deviations (peak-to-valley). We discuss the nature of the pattern noise, and summarize a method to robustly detect and remove it from raw STIS CCD frames. We report on a Cycle 16/17 Archival Calibration Legacy program to (semi-)automatically remove the herring-bone pattern noise from all raw, unbinned Side-2 STIS/CCD frames taken between 2001 July and 2004 August — representing a gain in effective sensitivity of a factor ∼3 at low S/N. We also present some trends in the characteristics of the noise pattern. 1. Nature of the Pattern Noise The superimposed noise signal, due to analog-digital cross-talk or a grounding issue in the STIS Side-2 circuitry, is not a spatial signal, but a high frequency signal in time. That signal manifests itself as a spatial “herring-bone” pattern (Fig. 1) that can drift erratically — even during the relatively short time it takes to read the CCD. The pattern tends to be locally semi-coherent, however, and is best described as a modulated ∼14–18 kHz wave. The amplitude of that high-frequency wave is modulated by the superposition of three ∼1 kHz sinusoidal waves with phases that are shifted 120◦ from one another, and which have amplitudes of 3–5 e− (see Fig. 2a and b). Since a 14–18 kHz frequency corresponds to a spatial period of 2.5–3.2 pixels, the values of adjacent pixels along a row tend to be affected by offsets of opposite signs (Fig. 2a), resulting in relative deviations of up to ∼8 e− (peak-to-valley). Adjacent pixels along columns experience offsets that are shifted in phase by amounts that vary from region to region in a single frame, and also from frame to frame. The resulting impact on Side-2 CCD data is therefore far more serious than implied by a mere 1 e− increase in the amplitude of the read-noise, and is partly systematic in nature. 2. Removing the pattern noise Brown (2001) introduced a method to filter out the pattern noise by noting that the sequen-