Science observations were carried out with the LWS in four operating modes or `astronomical observation templates' (AOTs). Additional modes were used for calibration observations where more flexibility in operation was required. Two AOTs were commissioned for the grating mode and two for the FP mode. The four AOTs were all operated with calibration measurements made before and after the main source observation and this involved measuring the response of the detectors to a set of standard illuminator flashes and the dark signal. The operation of each mode is explained in detail in the LWS handbook (Gry et al., 2002). All ISO observations were identified with a Target Dedicated Time (TDT) number. This was an eight digit number where the first three digits specified the ISO orbit (ISO revolution number), the middle three digits specified the observation number and the last two digits the sequence number.
The grating was used in the `L01' and `L02' modes. The L01 mode was used to produce observations covering a wide spectral region at medium resolution. This was achieved by scanning the grating in angle and recording the photocurrent from the 10 detectors simultaneously. The full spectral region could then be recovered by stitching together the trace of the individual detectors. Both forward and backward scans were made with the grating. The L02 mode involved using the grating to make short scans, either around known spectral lines or as a single photometric point on each of the 10 detectors.
FP observations were made using the `L03' and `L04' modes. High resolution, long spectral surveys, covering a wide wavelength range were made using the L03 mode. The FP and grating were both scanned to create a sequence of mini-scans. This was very time consuming for wide wavelength ranges and had much lower signal to noise than grating mode measurements. In theory full wavelength coverage could have been achieved in a single observation but in practice many individual observations were made, each covering part of the spectrum. The L04 mode was used to record narrow ranges in wavelength, usually with a single grating angle. This allowed the observations to be targeted to known lines, reducing the required observing time. This saving in time gave the possibility of increasing the signal to noise by making many more repeated scans.
There were also several other modes of operation used for calibration observations that allowed more flexibility. One of these was the Fixed-FP mode. This involved keeping the FP gap fixed but scanning the grating angle. With this configuration the infinite series of FP orders sampled the grating response function creating a series of peaks corresponding to the convolution of FP and grating responses (these observations are described in more detail in Section 3.6.3).