Column and Keyword Filtering Syntax
                                       
   The column filtering specifier can be used to create or modify table
   columns and/or header keywords in the input HDU. The presence of a
   column filter causes CFITSIO to create a new virtual file (usually in
   memory) that contains the specified modifications. Any other HDUs in
   the input file are also copied to the virtual file. The actual
   physical input file is not modified.
   
   The column filtering specifier must begin with the keyword 'col' and
   is enclosed in square brackets following the name of an input FITS
   file. The column filter can be used to perform the following types of
   operations. More than one operation may be specified by separating
   them with semi-colons.
   
Select a subset of columns from the input table

   The list of columns to be included in the virtual table should be
   separated by semi-colons. For example,
    file.fits[events][col X; Y]

   will create a virtual copy of the input events table that only
   contains the X and Y columns. The '*' and '$' wildcard characters may
   be used to match the names of more than one column to be selected. It
   is sometimes more convenient to list the columns to be excluded from
   the virtual table, as in:
    file.fits[events][col -DETX; -DETY]
 or
    file.fits[events][col !DETX; !DETY]

   These examples will copy all but the DETX and DETY columns to the
   virtual table.
   
Compute the values for a column

   Append a new column to the table or recompute the values in an
   existing column. For example,
    file.fits[events][col PI = PHA * 1.05; X; Y]

   will modify the values in the PI column, (or will create a new PI
   column if it does not already exist) using the arithmetic expression
   'PHA * 1.05' (where PHA is a column in the input table) to calculate
   the values for the column. The expression that is used to compute the
   columns or keyword values can be arbitrarily complex and may be a
   function of other header keyword values and other columns. The full
   syntax and available functions for the expression are described in the
   calc_express help file.
   
   In the above example, the virtual file will contain only 3 columns:
   PI, X, and Y. The '*' wildcard character may be used when specifying
   the name of the columns to be copied to the virtual file, as in these
   examples:
    file.fits[events][col PI = PHA * 1.05; X; Y; *DET]
    file.fits[events][col PI = PHA * 1.05; *]

   In the first example, the virtual table will contain the PI, X, and Y
   columns and any other columns whose name ends with the string 'DET'.
   In the second example, the virtual table will contain all the columns
   present in the input table, as well as the PI column.
   
   When a new column is created, an appropriate data type will be chosen
   by default based on the form of the expression used to calculate the
   values. Alternatively, one may specify the desired column data type in
   parentheses, followed the column name, as in '[col PI(I) = PHA *
   1.05]'. The datatype is specified using the same code letters that are
   allowed for the value of the FITS TFORMn keyword (e.g., 'I' for
   integer*2, 'J' for integer*4, 'E' for real*4, and 'D' for real*8, etc.
   for binary tables, and 'I8', F12.3', 'E20.12', etc. for ASCII tables).
   The full range of FITS data types is supported including character
   string, logical, bit, byte and complex data types. Vector columns are
   also supported in binary tables.
   
Create or modify a header keyword

   Create a new header keyword or modify the value of an existing keyword
   by prefixing the keyword name with a pound sign '#'. The expression
   must evaluate to a constant scalar (i.e., cannot have a column name in
   the expression). The comment string for the keyword may be specified
   in parentheses immediately following the keyword name (instead of
   supplying a datatype as in the case of creating a new column). For
   example:
    file.fits[events][col #VEL(radial velocity) = 23.4]

   will create the keyword
  VEL     =                   23.4 / radial velocity

Rename a column

   Rename an existing column or keyword with the syntax 'NewName ==
   OldName', as in:
    file.fits[events][col NEWPHA == PHA]

Using a text file

   For complex or commonly used operations, one can also place the
   operations into a text file and import it into the column filter using
   the syntax '[col @filename.txt]'. The operations can extend over
   multiple lines of the file, but multiple operations must still be
   separated by semicolons. Any lines in the external text file that
   begin with 2 slash characters ('//') will be ignored and may be used
   to add comments into the file.
   
SEE ALSO

   filenames, imfilter, rowfilter, binfilter.
   calc_express.