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Tabulated list of pixels

The third way to represent an image in a FITS file is as a list of pixel coordinates and other optional associated parameters in a binary or ASCII table extension. This representation is often used in high-energy astrophysics as a way of tagging the position and other properties of individual detected photons (similar to the QPOE format used in IRAF system). This is a compact way of representing sparse images since only the non-zero pixels need to be given in the list. At a minimum, this format requires that the FITS table have 2 columns which give the X and Y pixel coordinate of the corresponding event in the virtual 2-D image; any number of other columns may be included in the table to store other parameters describing the event, such as arrival time, or photon energy. One can of course convert this virtual image representation into a real image by computing the 2-dimensional histogram of the X and Y pixel columns.