The pixel coordinate system described above defines how to convert pixel indices into a set of continuous coordinates and therefore introduces a coordinate axis which runs along each dimension of the NDF, as follows:
![\begin{picture}(70,57)(-10,-7)
\par
\multiput(10,10)(0,3){11}{\line(1,0){45}}
\m...
...ut(0,0){\vector(0,1){45}}
\put(0,48){\makebox(0,0)[b]{\bf Axis 2}}
\end{picture}](img23.png)
The use of the pixel size to determine the units of these axes is rather restrictive, however, and in practice we may want to use more realistic physical units. This would allow a spectrum to be calibrated in wavelength, for instance, or the output from a plate-measuring machine to be related to axes calibrated in microns.
Of course, the pixel coordinate system is only the default choice, and is
intended to be used only in the absence of other information.
The NDF's axis components are designed to hold the extra information
needed to define more useful coordinate systems, so that realistic axes can
be associated with a NDF, along with labels and units for
these axes.
The method used also allows for the possibility that an NDF's pixels may not
be square and that they may not be contiguous (i.e. that they may
have gaps between them, or may overlap) when their positions are expressed
in axis units.
Statistical uncertainty in the pixel positions may also be represented, if
present.
NDF [1ex