A FITS file shall be composed of the following FITS structures , in the order listed:
Each FITS structure shall consist of an integral number of FITS logical records. The primary HDU shall start with the first record of the FITS file. The first record of each subsequent FITS structure shall be the record immediately following the last record of the preceding FITS structure. The size of a FITS logical record shall be 23040 bits, corresponding to 2880 8-bit bytes.