6.12 FP Wavelength Calibration

Although the FP wavelenth calibration accuracy meets the specifications (see Section 5.12), the velocity difference observed between different lines in a given object is substantially higher than the velocity dispersion measured on repeated observations of the same line, as illustrated in Figure 6.12. This aspect shows that the wavelength measurements by the Fabry-Pérot are very stable but the absolute wavelength accuracy is hampered by an unknown effect, which could be the uncertainty on the theoretical wavelength of some fine-structure lines, as discussed in Section 5.12.5.

Figure 6.12: This plot illustrates the good wavelength stability of the Fabry-Pérots together with the uncertainty in the absolute calibration. Central velocities are plotted for each line and each object as a function of revolution number. It shows that although for the same line/object the velocity is very stable, the velocities derived from different lines are different. This evidences a systematic uncertainty in the calibration which is not yet understood but which in any case is never higher than half a resolution element.