4.3.2 Primary Data Array

In FITS format, the   primary  data array shall consist of a single data array of 0-999 dimensions. The random groups convention in the primary data array is a more complicated structure (see §7). The data values shall be a byte stream with no embedded fill or blank space. The first value shall be in the first position of the first primary data array record. The first value of each subsequent row of the array shall be in the position immediately following the last value of the previous row. Arrays of more than one  dimension shall consist of a sequence such that the index along axis 1 varies most rapidly, that along axis 2 next most rapidly, and those along subsequent axes progressively less rapidly, with that along axis m, where m is the value of NAXIS , varying least rapidly; i.e., the elements of an array $A(x_{1}, x_{2}, \ldots, x_{m})$ shall be in the order  shown in Figure 4.1. The index count along each axis shall begin with 1 and increment by 1 up to the value of the NAXISn  keyword (§5.4.1.1).

  If the data array does not fill the final record, the remainder of the record shall be filled by setting all bits to zero.