Multiple images and the `Greenbank Convention'

In the general case of the binary table vector representation of images, the images contained in each row of the table will not necessarily have the same coordinate transformation keywords values. If, for example, the value of the reference pixel is different for each image/row then a single 1CRVLn keyword in the header is not sufficient to record all the individual values. Instead, this keyword should be replaced by a column with the same name (i.e., TTYPE n = '1CRVLn') which is used to store the reference value appropriate for each row of the table. This convention for expanding a keyword into a table column (or conversely, collapsing a column of identical values into a single header keyword) was originally developed and promoted at the Greenbank Radio Observatory, and hence this is commonly referred to as part of the Greenbank Convention for keywords.