The sensitivity limits shown in Fig. 8 have been derived assuming only photon noise of an isolated source, a homogeneous background and read-out noise of the detector. Especially for the longer wavelengths the structure of the background must be taken into account in order to derive the ultimate sensitivity limits for a specific observation.
In the galactic plane high source densities occur and the objects of interest are often embedded in a complex environment. Therefore, careful investigation of the IRAS Sky Survey Atlas plates for reference background positions, or mapping of these regions, is mandatory.
There may be considerable confusion noise due to celestial backgrounds, namely cirrus confusion and galaxy confusion, even outside the galactic plane up to high galactic latitudes. Cirrus confusion noise results from the difference of the background fluxes in the target beam and the reference beam due to spatial structures (e.g. filaments). Galaxy confusion noise comes from faint background galaxies which may lie in the beam but are not detected individually.
It turns out that galaxy confusion noise is only relevant in the darkest high
latitude regions. For the point-source flux limits (after 128s
integration time and S/N = 10) in the most sensitive PHT filters at
long wavelengths
(cf. Fig. 8, upper panel), the calculated 
level
of galaxy confusion noise is given in Tab. 7.
If fainter flux limits are achieved by exposure times longer than 128s the
noise level goes down. However, in the worst case (P3 detector, 
filter, 180" aperture) the
noise becomes as strong as the source flux below 1 mJy.
In regions with brighter backgrounds the cirrus confusion noise becomes
dominant and determines
the sensitivity limit of the observation. In Tab. 8 some
representative 
values are given. These values can be reduced
(by about a
factor of 2) by using two reference positions and minimizing the distance
between
the source and the reference field. In strong cirrus emission regions,
however,
mapping is necessary to detect faint point-sources.
