The mechanism by which charge transfer from a CCD is performed lends itself to on-chip pixel binning. To quickly review charge transfer in a CCD, the following diagram is shown. As light (photons) falls on the surface of the CCD, charge (electrons) accumulates in each pixel. The number of electrons that can accumulate in each pixel is referred to as Well Depth. For the KAF-0400 and KAF-1600, this is 85,000 electrons. Some CCDs, such as the SITe 502AB have well depths exceeding 350,000 electrons. Once the exposure is complete, this charge must be transferred to a single output and digitized. This is accomplished in two steps. First, an entire row is transferred in the vertical direction to the horizontal register. Second, charge is transferred horizontally in this register to the output amplifier.
Most CCDs have the ability to clock multiple pixel charges in both the horizontal and vertical direction into a single larger charge or "super pixel." This super pixel represents the area of all the individual pixels contributing to the charge. This is referred to as binning. Binning of 1x1 means that the individual pixel is used as is. A binning of 2x2 means that an area of 4 adjacent pixels have been combined into one larger pixel, and so on. In this instance the sensitivity to light has been increased by 4 times (the four pixel contributions), but the resolution of the image has been cut in half. The following figure illustrates the effect.
Binning can be a very useful tool. It can be used to effectively increase the pixel size while also increasing sensitivity. It is a good method for focusing, because image acquisition speeds up greatly while giving greater sensitivity to lower out-of-focus light levels. A Kodak KAF-0400 having 796x512, 9µ pixels can appear to have 398x256, 18µ pixels with a binning of 2x2. With 3x3 binning, the sensor appears to have 265x170 pixels with a size of 27µ and 9 times the sensitivity! At 3x3 binning, a KAF-1600 will appear as 512x341 pixels each 27µ in size.
Depending on your focal length and optics, the optimum pixel size for the type of observing you are doing can change. Having the ability to do at least 3x3 binning will give you greater flexibility. Binning also has an impact on well depth. While each pixel on a Kodak KAF-0400 has a maximum well depth of 85,000 electrons, the output node which collects binned charges has a capacity of greater than 140,000 electrons. This allows greater charge to be collected for each super pixel than was possible with unbinned pixels.