The Calibration Unit optics (new WLS part highlighted in yellow)

The demanding radiometric-accuracy requirements for the instrument call for in-orbit calibrations. The unit contains two light sources, one of which offers well-isolated spectral lines in the required wavelength range, and a quartz tungsten halogen lamp (White Light Source, WLS) for a broad-band continuum. The WLS is used to monitor the etalon that is present on the cooled Reticon detectors, due to freezing water vapour on the protective Silicon dioxide (SiO₂) layer. Although this etalon stabilises in vacuum, it is irritating during the ground calibration and for the mapping of key calibration data between the on-ground calibration and the in-orbit situation. The spectral light source is a hollow cathode lamp (Pt anode/Cr cathode) filled with a mixture of neon and argon. Adding argon to the gas mixture increases the number of spectral lines in channel 3 and reduces the very strong neon lines in the near-infrared, which would otherwise be saturated.

The Calibration Unit is complemented by a diffuser, which allows a solar calibration to be performed. Due to the orbital geometry, the Sun can be seen via the solar calibration port once per orbit. As with GOME-1, the diffuser is well protected against the hostile space environment and the harsh ultraviolet radiation by a mesh that attenuates the flux and a shutter that opens only for a Sun calibration. GOME-1 experience shows that one solar calibration per day is sufficient and no degradation of the Sun diffuser itself has been detected in 4.5 years. The beams of the three sources leave the Calibration Unit at different angles and the sources can therefore be separated by proper selection of the Scan Mirror position.