Simplified block diagrams of SCIAMACHY (top) and SIM (bottom) spectrometers. External (internal) light source such as the sun (calibration lamps) are indicated on the left-hand-side. Detectors, i.e., photodiodes, which convert light to electronic signals, are indicated in the right-hand-side. For summary of SCIAMACHY and SIM instrumental parameters, see Tables A.1 and A.2, respectively.