Sensitivity limits

To illustrate the operational performance we list in Table 3 the 10 flux limits for chopped measurements with =128 sec (measurement time = 256 sec) of some typical filters assuming typical background emission.

Table 3: Sensitivity Limits per pixel for typical filters and backgrounds, assuming S/N=10, 128 sec on-source integration time and chopped mode without detector transients.  

The figures in Table 3 have been confirmed by observing faint targets with known or well predicted fluxes. Whether the given S/Ns can be achieved during an actual measurement depends also on the cosmic radiation induced noise conditions during an orbit.

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, the calculated level of galaxy confusion noise is given in Table 4.

For on-target integration times longer than 128s the noise level goes down but then galaxy confusion noise starts to become the main source of uncertainty: in the worst case 280 mJy ( for the P3 detector, filter, 180'' aperture).

In regions with brighter backgrounds the cirrus confusion noise becomes dominant and determines the sensitivity limit of the observation. In Table 5 some representative values are given. Cirrus confusion can be reduced 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.