Therefore a proposer must be aware of the limiting source densities as a function of wavelength and the position on the sky. The most important contributors to the confusion limits in the ISO wavelength range are listed below. Extragalactic sources dominate the long wavelength end outside the galactic plane, Magellanic Clouds and nearby star forming regions. Small scale structure in cirrus clouds may dominate the longer wavelengths in intermediate galactic latitudes. The contribution depends heavily on the level of cirrus emission at the position on the sky.
Old stars dominate at the shorter wavelengths.
Young stars appear usually in groups. When the line of sight passes through such a group, young stars may dominate the confusion limit in the whole ISO wavelength range. However, the level of cirrus may easily change these limits by an order of magnitude depending on the location in the sky. Model dependent estimates of confusion limited flux levels have been presented by Franceschini et al. When the expected source flux suggests that confusion noise may be a significant factor, it is important to obtain observations with the highest available spatial resolution.