The data acquisition package (ICE at KPNO) must synchronize and manage both the instrument control (camera shutter operation), the charge shifting on the CCD, and the telescope nodding.
The mountain programming group (MPG) implemented a modified version of ICE to enable the N&S mode. Several possible designs were discussed between the MPG and IRAF programmers, and the final choice was to fully implement a TCS-level offset function.
The IRAF group (primarily Rob Seaman) was responsible for modifications to the ICE CCD data acquisition software. The nodding/charge shuffling exposure mode requires that only the middle third of the chip be unmasked. After each object or sky subexposure is taken, the shutter must be closed, the charge shift to one side or the other and the telescope nodded to the other offset position. This is repeated through many exposures, back-and-forth.
Given the requirement to repeat this on time scales of a minute or quicker for an hour or more at a time for each exposure, any successful implementation of charge shuffling must parametrize the most useful exposure parameters while hiding the complexity of the multiple offset exposures from the observers and the telescope operator. New parameters added to ICE include the number of nods to include in each exposure and the number of pixels to shift for each exposure. It is now possible to turn the N&S mode on and off at will and to easily change the parameters with each new exposure.
In addition, since preferred NOAO observing recommendations for charge shuffling are still being evaluated, access was provided to a variety of different nodding patterns (a simple `alternate' ABAB object/sky pattern versus a `bracket' pattern that begins and ends with a half-length sky subexposure; see below). A second evaluation feature allows the on-object and on-sky exposure times to differ. Whether these options are preserved as user parameters or are rather implemented as hardwired defaults will depend on scientific evaluation of their utility. The added complexity of the shuffling mode also required revamping various abort procedures and other such infrastructure and the addition of several new keywords to the image headers (see table .
A fully supported implementation of charge shuffling as a KPNO facility class instrument will require further discussions and work on both observing run preparation (to better generate slit masks) and the data reduction facilities for these new image types.