This window allows you to do three related jobs.
Flipping and shifting a spectrum allows you to assess the symmetry of a line. Shifting alone to determine how well spectra are aligned and redshifting to transform a spectrum or set of line identifiers to guess or confirm the redshift.
The transformation is applied to the spectrum shown in the Spectrum: drop-down menu. Initially this will show any editable spectra displayed in the associated plot (line identifiers are usually editable). If you want an editable copy of the current spectrum just press the Copy button.
One subtlety is that the offset value is either a redshift or a value in the coordinate units of the spectrum being modified, depending on whether the zRedshift checkbox is selected, or not. The coordinate units of the spectrum may not be the same as those of the current spectrum in the plot, which can lead to surprising results.
When applying a redshift it is assumed that the spectrum being modified has a wavelength coordinate system, of some kind, unless it has a proper coordinate system defined. In this latter case the only restriction is that you cannot have a standard of rest set to `Source' (as a transformation to and from Source is used to determine the redshift transform). In this latter case use the coordinate system toolbox to transform to another rest frame first.
The Reset button returns a spectrum to an offset of zero.
SPEFO options
The original motivation for creating this tool (suggested by Petr Skoda, see the acknowledgements section) was really to do flip comparisons of lines to determine radial velocities in the same fashion as the SPEFO program. This has some additional requirements beyond those required for simple shifting in a linear sense, as well as in redshift, and are enabled by selecting the Options->SPEFO options menu item. This reveals two text areas (you need to resize the window to actually see them first time). The upper one shows the flip corrected offset (that's half the standard shift) and the radial velocity for that flipped shift (calculated as c*Offset*0.5/(Flip centre)), assuming that the coordinate system is some wavelength.There's also an editable area for writing some notes about the measurement, both these areas are written the file SPEFO.log by pressing the Save to SPEFO.log file button.