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6.3 IEEE-754 Floating Point

Transmission of 32- and 64-bit floating  point data within the FITS format shall use the  ANSI/IEEE-754 standard [15]. BITPIX = -32 and BITPIX = -64 signify 32- and 64-bit IEEE floating point  numbers, respectively; the absolute value of BITPIX is  used for computing the sizes of data structures. The full IEEE set of number forms is allowed for FITS interchange, including all special  values. 

The BLANK keyword  should not be used when BITPIX = -32 or  -64; rather, the IEEE NaN should be used to represent an undefined value. Use of  the BSCALE and BZERO keywords  is not recommended.

Appendix H has additional details on the IEEE format.