Many of the subroutines come in families which differ only in the datatype of the associated parameter(s) . The datatype of these subroutines is indicated by the last letter of the subroutine name (e.g., 'j' in 'ftpkyj') as follows:

x - bit b - character*1 (unsigned byte) i - short integer (I*2) j - integer (I*4) e - real exponential floating point (R*4) f - real fixed-format floating point (R*4) d - double precision real floating-point (R*8) g - double precision fixed-format floating point (R*8) c - complex reals (pairs of R*4 values) m - double precision complex (pairs of R*8 values) l - logical (L*4) s - character string

When dealing with the FITS byte datatype, it is important to remember that the raw values (before any scaling by the BSCALE and BZERO, or TSCALn and TZEROn keyword values) in byte arrays (BITPIX = 8) or byte columns (TFORMn = 'B') are interpreted as unsigned bytes with values ranging from 0 to 255. Some Fortran compilers support a non-standard byte datatype such as INTEGER*1, LOGICAL*1, or BYTE, which can sometimes be used instead of CHARACTER*1 variables. Many machines permit passing a numeric datatype (such as INTEGER*1) to the FITSIO subroutines which are expecting a CHARACTER*1 datatype, but this technically violates the Fortran-77 standard and is not supported on all machines (e.g., on a VAX/VMS machine one must use the VAX-specific %DESCR function).

The double precision complex datatype is not a standard Fortran-77 datatype. If a particular Fortran compiler does not directly support this datatype, then one may instead pass an array of pairs of double precision values to these subroutines. The first value in each pair is the real part, and the second is the imaginary part.

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