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C. Strategies for Binning


The essential result to take from Table 7 is that one should always set the camera software to bin the images upon readout. The choice of what binning to use will be driven by the seeing, the use or not of the focal reducing lens, and the need for good spatial sampling. The seeing at Andreas Observatory is never good enough to take advantage of the unbinned spatial sampling of the spectrograph, even using the focal reducing lens. Ideally, one would like to have a pixel-scale that is about 1/3 of the seeing disk. The best seeing at Andreas Observatory is about 2''. It is not unusual to have seeing of closer to 3''. These numbers dictate a binning of 2 or 3 pixels in the spatial direction only! Binning along the spectral axis is usually a bad idea.

There are two reasons why one wants to have the images binned upon readout, rather than binning them during reduction. The first, and less important, is simple economy. A 1 x n-binned image is only 1/n as large as an unbinned image, and takes 1/n as long to read out. The second reason is that read noise is per pixel. This means that an image that is read out with 1 x n binning has less read noise per unit area than an unbinned image.


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Updated: 2011 August 18 [pbe]