Overall Strategy

1. Determination of focal plane geometry, array geometry and alignment. The relative positions of the various entrance apertures and detectors need to be known to ensure that one grating wavelength calibration can be applied to all combinations of entrance slit and detector. The respective offsets have been calibrated in instrument level tests on the ground. Beam profile measurements  and wavelength calibrations on astronomical sources were used for in-orbit checks - it was such calibrations that found the need for the virtual aperture 4.
2. Determination of wavelength scales . In ground-based instrument level tests, the relation between grating position readout and physical grating angle has been determined by measurement of wavelength references in the form of vapour absorption lines (H O, NH , HCl). The spectral features provided by the internal grating wavelength calibrator have been tied to that scale. For the Fabry-Pérot, the position-gap relation has been determined from the spectrum of the internal F-P wavelength calibrator, which is known to high accuracy from fourier transform spectroscopy. H O and NH vapour absorption lines have been used for additional checks and for determination of the variation of effective F-P gap with wavelength. In orbit, the grating position-angle relation and the F-P position-gap relation will be re-established first using the internal calibrators and then astronomical sources.
3. Determination of photometric sensitivity . The photometric sensitivity has been determined on ground by scanning the spectrum of a calibrated blackbody source within the test cryostat. These tests resulted both in detailed spectral response functions for the various AOT bands and in a first calibration of the signal created by the internal stimulators. The signal from the stimulators will be used to monitor variations in the broad band sensitivity of the detectors, e.g. due to memory effects. In orbit, an extensive program will be executed to determine the photometric sensitivity on astronomical sources. The dark current   measurements needed both on ground and in orbit are done with SWS shutter closed.