BINTABLE

BINTABLE is a Binary Table Extension in the FITS data format. It stores tabular data in binary form inside a FITS file, as an alternative to ASCII tables. BINTABLE is widely used in astronomy for catalogs, event lists, and other tabular data because of its compact size and efficient access.

An extension begins with a header, including XTENSION='BINTABLE', followed by a binary data block. The column definitions are listed in the header with repeated pairs of TTYPEn and TFORMn, which specify the column name and data format. Optional keywords such as TUNITn, TSCALn, TZEROn, TDIMn, and TNULLn describe units, scaling, array dimensions, and null values. The data area stores rows in the specified formats; variable-length fields can be stored with a heap mechanism, connected by pointers.

BINTABLE supports a wide range of data types (integers, floating point numbers, characters, and logicals) and can contain fixed-length or variable-length fields. Its binary encoding provides faster I/O and smaller files for large datasets compared with ASCII tables, making it well suited to large catalogs and time-series data.

It is supported by standard FITS readers and writers, including CFITSIO, and is widely used through Python libraries such as Astropy. BINTABLE remains a core component of FITS for tabular data in modern astronomical data workflows.