5.4. Conventional coordinate types
In the first FITS paper, Wells et al. () listed a number of "suggested values" for CTYPE i. Two of these have the attribute that they can assume only integer coordinate values and that the meaning of these integers is only by convention. These two axis types are in wide-spread use and we wish to repeat their definition here to extend their definitions and to reserve their names and meanings.
The first conventional coordinate is CTYPEia = 'COMPLEX' to specify complex valued data. FITS data are limited to a single real number value at each pixel making this axis necessary to represent data which are weighted complex numbers. Conventional values of 1 for the real part, 2 for the imaginary part, and 3 for a weight (if any) have been widely used.
The second conventional coordinate is CTYPEia = 'STOKES' to specify the polarization of the data. Conventional values, their symbols, and polarizations are given in Table 7.
|1||I||Standard Stokes unpolarized|
|2||Q||Standard Stokes linear|
|3||U||Standard Stokes linear|
|4||V||Standard Stokes circular|
|-5||XX||X parallel linear|
|-6||YY||Y parallel linear|
|-7||XY||XY cross linear|
|-8||YX||YX cross linear|