Line observations with the grating and in particular the F-P require good rest wavelengths to avoid scanning of wrong wavelength ranges. This may be a problem for many of the less well studied fine structure lines within the SWS range. A useful compilation of fine structure line data can be found in Kaufman & Sugar (1986, J. Phys. Chem. Ref. Data 15, 321).
For the line observation AOTs, observing wavelengths are specified by giving the rest wavelength and the source radial velocity . Thus, observing programs requiring observations of the same lines on a number of sources with differing radial velocities can be defined easily and without wavelength conversion errors. Furthermore, a correction of the radial velocity for the effect of the earth's motion is foreseen in the Fabry-Pérot AOTs, and will be implemented at the time of scheduling.