After the liquid helium depletion of ISO, an engineering test programme was conducted on the different satellite systems. Besides functional tests of the various redundant systems, a lot of time was devoted to the testing of alternative pointing strategies in case of failure of one or more gyros.
During this test programme the temperature of the focal plane remained low
enough to operate the InSb band 1 detectors of the SWS. The
instrument could be operated in some timeslots available during the engineering
test programme. This time was used to record spectra of stars between
2.36 and 4.05 
m at medium resolution (
1500 - 2000).
As the temperature of the focal plane increased, changes in the position
readout of the grating assembly and changes in the response of the
instrument could be seen. These changes are detailed in Chapter
6.
The astronomical observation templates (AOT) of the nominal operations phase were not appropriate for the post-helium observations. A special observation mode was designed to avoid unnecessary energy dissipation by the long wavelength grating scanner and the heaters in the readout electronics of the detector blocks not used. The grating scanning ranges were limited to the angles corresponding to the wavelengths covered in AOT bands 1A to 1E. The observations were implemented and tested in the Calibration Uplink System (CUS). Therefore post-helium observations appear in the ISO Data Archive with observation type `SWS99'.
The observation mode exists in two flavours. The fast version had a continuous scanning strategy like the nominal SWS01 speed 4 observation mode with a dwell time of 1/4 sec. The on-target-time was 2352 seconds. The slow version was scanning like in an SWS01 observation but with a dwell time of 1/2 sec. It lasted for 4408 seconds. At the end of the observation a dark current measurement and a photometric check was done. The internal calibration source was used at its maximum brightness.
The complete timeline of an SWS post-helium observation is summarized below.
The SWS99 identifier was also used during the mission for non-standard calibration observations designed using the Calibration Uplink System (CUS). The CUS allowed Instrument Command Sequences (ICS) to be linked together in a flexible way to perform any kind of observation. While the CUS gave endless possibilities for how the observations would be performed, in practice most observations generated under this system were analogous to the AOTs. Consequently most CUS observations can be processed by the complete pipeline, yielding valuable and accessible scientific information on the sources observed.