The column or keyword filtering specifier is used to modify the column structure and/or the header keywords in the HDU that was selected with the previous HDU location specifier. It can be used to perform the following types of operations.
The column filtering specifier is enclosed in square brackets and begins with the string 'col'. Multiple operations can be performed by separating them with semi-colons. For complex or commonly used operations, you can write the column filter to a text file, and then use it by giving the name of the text file, preceded by a '@' character.
Some examples:
[col PI=PHA * 1.1 + 0.2] - creates new PI column from PHA values
[col rate = counts/exposure] - creates or overwrites the rate column by
dividing the counts column by the
EXPOSURE keyword value.
[col TIME; X; Y] - only the listed columns will appear
in the filtered file
[col Time;*raw] - include the Time column and any other
columns whose name ends with 'raw'.
[col -TIME; Good == STATUS] - deletes the TIME column and
renames the STATUS column to GOOD
[col @colfilt.txt] - uses the filtering expression in
the colfilt.txt text file
The original file is not changed by this filtering operation, and instead the modifications are made on a temporary copy of the input FITS file (usually in memory), which includes a copy of all the other HDUs in the input file. The original input file is closed and the application program opens the filtered copy of the file.
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