Once the geometrical transformation has been defined and all processing dependent on the original geometrical pattern of the image has been completed, it is a simple matter to apply the geometrical transformation to straighten the spectrum and align slit images with image lines.
The current geometrical transformations are applied to images in a dataset using the redgspec task by setting the transform flag. The current transformations are the transformations defined by the fit files located in the directory pointed to by the IRAF environment variable database$. The wavelength calibration is stored in the transformed file header.