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Paper: CCD Charge Shuffling Improvements for ICE
Volume: 238, Astronomical Data Analysis Software and Systems X
Page: 515
Authors: Seaman, R. L.
Abstract: NOAO has been using IRAF at its telescopes since Unix workstations were first placed in the domes. At the Kitt Peak National Observatory, this has included data acquisition using the ICE (IRAF Control Environment) package that was developed in coordination with Skip Schaller at Steward Observatory. ICE continues to be used both inside KPNO and Steward and at other observatories. Improvements to ICE are described that support a dual exposure mode implemented via charge shuffling techniques. Charge shuffling involves repeatedly shifting the charge back-and-forth from side-to-side of a CCD while nodding the telescope alternately from an object to a blank sky position. The CCD is optically masked such that the sky pixels are kept dark while the object pixels are exposed and vice versa. The nodding and shuffling and opening and closing of the camera shutter occurs on a short enough time scale that the sky brightness variations are frozen. The output of this process is a dual exposure of contemporaneous object and sky spectra accumulated through the exact same optical path. This mode is beneficial, for instance, for multi-slitlet observations such that the width of each slitlet can be minimized to allow many more slits per exposure. New parameters added to ICE include the number of nods and the number of pixels to shift for each exposure. A variety of different nodding patterns are supported, such as a simple ABAB object/sky pattern and a bracketed pattern that begins and ends with a half-length sky subexposure. The on-object and on-sky exposure times may be specified separately.
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