SOURCE CONFUSION

Source confusion is an additional noise factor closely related to astronomical background. However, there is a fundamental difference. The sensitivity limit due to confusion is determined by the telescope aperture, observing wavelength and position on the sky. After reaching the confusion limited level, sensitivity cannot be improved by increasing the integration time. With the ISO mission properties confusion limit can be reached within a relatively short integration time. 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. Their contribution is increasing toward the galactic plane and especially toward the galactic centre (bulge).
• 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. Groups of young stars exist throughout the galactic plane (the inner galaxy being more crowded than the outer galaxy), in the Magellanic Clouds and within nearby star forming regions (in Orion, Ophiuchus etc.).

If the estimated flux of the target object is less than about 1mJy (for an object away from the galactic plane at wavelengths below 25 m), then it is necessary to consider confusion. At galactic plane the situation is worse (i.e. the confusion limit is higher). At longer wavelengths the approximate limits are: 10mJy at 60 m, 50mJy at 100 m and 0.1Jy at 200 m. 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. (1991, Astron. Astrophys.\ Suppl. 89, 285) and by Gautier et al. (1992, Astron. J. 103, 1313). A discussion of the models and the consequences for observations with ISO can be found in Puget (1992, Les Houches Series on Infrared Astronomy with ISO, Th. Encrenaz and M. F. Kessler (eds.), p.278).

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. This can be achieved by setting the reference measurement (or chopping for PHT) as close as possible to the target position. The best observing strategy for sources with a flux level close to the confusion limit is to obtain fully sampled maps. This is, of course, very expensive as far as observing time is concerned.