BITPIX

BITPIX is a header keyword in the FITS (Flexible Image Transport System) file format that specifies the number of bits per data value in an image or binary table. It informs software how the pixel values are encoded and how to interpret the data array that follows the header.

The BITPIX value is an integer with typical valid values of 8, 16, 32, -32, and -64. Positive values indicate integer data: 8 denotes unsigned 8-bit data, 16 denotes 16-bit signed integer data, and 32 denotes 32-bit signed integer data. Negative values indicate floating-point data: -32 corresponds to 32-bit IEEE 754 floating point, and -64 corresponds to 64-bit IEEE 754 floating point. The choice of BITPIX, together with the NAXIS and NAXISn keywords, defines the data type and shape of the pixel array.

The data size in bytes is determined by the number of pixels times the bits per pixel specified by BITPIX, with the data section organized into 2880-byte FITS blocks. Endianness in FITS is big-endian, so readers may need to convert to the host machine’s byte order when necessary.

BITPIX is a mandatory descriptor for image data in FITS files and is also relevant for binary table extensions that store numeric arrays. It remains a fundamental indicator used by data processing software to correctly read, interpret, and manipulate astronomical data within FITS files.