Photometry from Schmidt plates
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With the advent of fast measuring machines such as COSMOS, there has been a revival of interest in photometry of wide field Schmidt plates. The very large datasets which have now become available are typically used for two types of programme, large scale statistical surveys of the distribution of various astronomical populations, and searches for very rare objects. In each case, good photometry is essential, and a number of techniques have been developed to attain acceptable accuracy over the large fields covered. A number of different approaches are used for detecting objects or defining samples, including magnitude, colour index and variability, in addition to non-photometric methods such as proper motions and objective prism spectra. Each detection method presents its own particular problems, and a number of calibration algorithms have been developed to optimise photometric accuracy.