As described in Section 2.9 there are a number of filters with different band widths dedicated to specific scientific goals. A filter set to determine the energy distribution of a source is obviously different from a set to detect molecular or dust features. To find the optimum combination of filters for a specific observation see Fig. 7 and the filter curves in the appendix.
Selection of apertures
For each filter Tab. 2 gives the fraction of the point spread function
entering the apertures. It should be emphasised
that a considerable portion of the flux is cut off, if an aperture
smaller than the diffraction limit is chosen.
Aperture sizes in the range of the Airy disc at the
wavelength(s) of interest or larger should be chosen.
It should be noted that in the case of diffraction
limited observations, i.e. if only the central maximum of the Airy disc falls
inside the aperture, about 70% of the full flux is observed, and the
remaining 
is cut off.
Chopper
General considerations for the use of the chopper are described in Section 2.11 and 3.1. The structure and the strength of the sky background mainly determine which mode to choose.
While chopping is very useful for faint sources against a relatively strong background, it is not recommended for use on bright sources, or where there is a high flux contrast between the source and background. For these latter cases the change in flux may be so high that strong detector drifts may occur (cf. Sect. 2.8).
Chopper avoidance region
Careful consideration must be given to chopped observations of extended objects like galaxies or bipolar outflows which are sensitive to the direction of chopping. The optimum direction of the chopper throw should be assessed. For example, the observation may be compromised if the reference beam coincides with a nearby object (see Fig. 10).
The chopper avoidance region is defined by the direction and the full angle to be avoided by the chopper beam (see Fig. 10).
Scan tolerance angle
Linear scans can only be performed along the spacecraft Y axis (cf. Sect. 3.5). Therefore, a tolerance angle for the actual scan line has to be provided to avoid strong scheduling constraints for the observation. Consider for instance a boundary of a given extended object (e. g. the boundary of an HII region or a spiral arm of a galaxy) which is to be scanned in perpendicular direction. In this case a small deviation of the actual scan line from its perpendicular orientation may be tolerable, whereas a completely different scan orietation (by e. g. 90 degrees) may be useless (see Fig. 11).
The scan tolerance angle is defined as the maximum angle the scan orientation can vary from its nominal orientation. The determination of the scan orientation and the tolerance angle is illustratred in Fig. 11.
Fluxes
To make best use of the detector sensitivity and to avoid underexposure or saturation, the detector dynamic range will be set for the expected flux and background levels. Therefore, the source flux and the background level should be estimated carefully. However, since accurate values of these parameters cannot be given a priori for all sources and wavelength ranges, the user should also give an uncertainty in his source flux estimate. This uncertainty should not exceed the sum of the source flux plus background flux by more than a factor of 20. From these three parameters - expected source flux, background flux and uncertainty in source flux - a valid range for detector operation will be determined, with the goal of avoiding detector saturation. Fig. 13 gives two typical cases of a background and a source dominated flux level.
Backgrounds
Like the source flux, the background level must be characterised for observations with PHT. In particular, for weak sources against strong backgrounds, it is the background level that determines the instrument settings. Fig. 14 illustrates the influence of the sky background level and its variation. For instance, a sky background significantly stronger than expected leads to saturation of the read-out electronics. As a consequence, the number of usable non-destructive read-outs will decrease which leads to a reduced S/N ratio. The observer should specify the maximum possible background level taking into account variations in the Zodiacal Light level. This depends on the Solar Aspect Angle of the object at the date of observation, which is not known before the observation has been scheduled. The variation of the Zodiacal Light is discussed in more detail in the ISO OBSERVER'S MANUAL.
Ground-based observations for the shorter wavelengths, IRAS maps and data bases, the Zodiacal Light History file, as well as model predictions, constitute data bases for the determination of the sky background flux levels. The observer should make sure the data bases used for background determination comprise the contribution of all background sources. For instance, in the ISSA maps the Zodiacal Light contribution has been subtracted.
Observations of Solar System Targets
For these objects only single pointing observations are allowed, and thus rastering, mapping or scanning are not possible. This limitation results from the fact that tracking is necessary for such observations (see also ISO Observer's Manual in the section on Solar System Targets).
