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STIS has been operating with some significant changes in its performance since the release of the STIS Instrument Handbook (IHB) version 5.1 (July 2001). This update provides the latest information regarding STIS performance and policies which are relevant to the preparation of Cycle 11 Phase II proposals.
The items presented here include:
ISR 2001-005: STIS CCD Read Noise During Side-2 Operations
Due to a failure of the primary (Side-1) STIS electronics, the STIS has been operating on its redundant (Side-2) electronics since July 2001. CCD data taken on Side 2 show an increase in effective read noise of approximately 1 e- pix-1 for gain=1 and 0.2 e- pix-1 for gain=4. The increased read noise manifests as a herring-bone pattern that can be seen easily in short-exposure CCD images. When the 2-D CCD images are converted to 1-D time series, Fourier transforms demonstrate that the read noise patterns are temporally correlated. The frequency of the read noise pattern ranges from 15.5 to 18 kHz, which corresponds to a spatial frequency on the detector of approximately 3 pixels (horizontally, in the direction of serial clocking). The gross variations in this frequency are correlated with the temperature of the CCD housing, but the correlation is too loose to use this temperature to predict the frequency. Simple filtering of the data can sometimes mitigate the pattern noise, but the filter must be carefully tuned to a narrow frequency range (tens of Hz) centered at the precise frequency of the pattern, to avoid introducing artifacts in the data. A code is provided to analyze and filter the pattern noise in STIS CCD data.
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