The overall strategy is discussed in the SWS calibration papers. In summary internal calibration sources were used to tie together extensive ground based calibrations with in-flight observations of astronomical calibration sources. The three types of calibrations that were carried out are:
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.
In ground-based instrument level tests, the relation between grating position readout and physical grating angle was determined by measurement of wavelength references in the form of vapour absorption lines (H2O, NH3, 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 FP wavelength calibrator, which is known to high accuracy from fourier transform spectroscopy. H2O and NH3 vapour absorption lines have been used for additional checks and for determination of the variation of effective FP gap with wavelength.
In orbit, the grating position-angle relation and the FP position-gap relation was re-established during PV, first using the internal calibrators and then astronomical sources. It was then checked regularly during the mission.
The photometric sensitivity was 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 was used to monitor variations in the broad band sensitivity of the detectors, e.g. due to memory effects.
In orbit, an extensive program was executed to determine the photometric sensitivity on astronomical sources. It was started during PV and carried on at a lower level during the mission. The dark current measurements needed both on ground and in orbit were done with SWS shutter closed.