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WFC3 Instrument Handbook for Cycle 24 |
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The WFC3 IR detector contains 1024в1024 square pixels of 18в18 micron physical size. The detector is divided into four quadrants of 512в512 pixels, each of which is read out independently from its outer corner, as illustrated in Figure 5.17. The outermost rows are read first, proceeding along each row from the outermost column to the horizontal mid-point of the detector, and then continuing inwards on subsequent rows to the vertical mid-point.In regard to bias drifts, the WFC3 IR class of detectors is the first to use reference pixels, configured as follows (see Figure 5.18). Of the 1024в1024 pixels, only the inner 1014в1014 pixels are light-sensitive. The five outer rows and columns of pixels all around the array use fixed capacitances to provide constant-voltage reference values. There are actually two types of reference pixels: (1) the pixels on the outermost columns/rows are connected to capacitors located outside of the unit cells. Their values follow a 4в periodic pattern, providing 4 sequentially increasing voltage levels all within the range of the detector output signal; (2) the 4 inner rows/columns are instead connected to capacitors created within their unit cells. These on-board capacitors are identical by design and all provide nearly the same reference signal. The current version of the WFC3/IR data reduction pipeline uses only the inner reference pixels, as they provide a more robust statistical estimate of the variable detector bias.5.6.2 Subarrays