Design

The spectroscopic axis information is not quite complete with only label and units in the axis structure. For a frequency scale you have to know which reference frame it refers to, for wavelengths you might also be worried about the refractive index of the medium in which the wavelength is measured. And for radial velocity scales you need to know the rest frequency of the spectral line. Another thing most useful in spectroscopy would be to store continuum and line fits along with the data. The concept of extensions to an NDF provides a good means to store such information. The design of Specdre's own extension is outlined here.

The Specdre Extension is the structure <myndf>.MORE.SPECDRE and is of type SPECDRE_EXT. <myndf> is the creation name of the main NDF, the one with the data, axes etc. <myndf>.MORE contains all the extensions to this main NDF. The Specdre Extension may contain other NDFs, thus we have to distinguish between the ``main NDF'' and the ``Extension NDFs''.