We examined the sky confusion noise in 40 sky regions by analysing 175 far-infrared ( 90–200 \mu m ) maps obtained with ISOPHOT , the photometer on-board the Infrared Space Observatory .
For cirrus fields with \langle B \rangle > 5 MJysr ^ { -1 } the formula based on IRAS data ( Helou & Beichman , ( 1990 ) ) predicts confusion noise values within a factor of 2 to our measurements .
The dependence of the sky confusion noise on the surface brightness was determined for the wavelength range 90 \leq \lambda \leq 200 \mu m. We verified that the confusion noise scales as N \propto \langle B \rangle ^ { 1.5 } , independent of the wavelength and confirmed N \propto \lambda ^ { 2.5 } for \lambda \geq 100 \mu m. The scaling of the noise value at different separations between target and reference positions was investigated for the first time , providing a practical formula .
Since our results confirm the applicability of the Helou & Beichman ( 1990 ) formula , the cirrus confusion noise predictions made for future space missions with telescopes of a similar size can be trusted .
At 90 and 170 \mu m a noise term with a Poissonian spatial distribution was detected in the faintest fields ( \langle B \rangle \leq 3 - 5 MJysr ^ { -1 } ) , which we interpret as fluctuations in the Cosmic Far-Infrared Background ( CFIRB ) .
Applying ratios of the fluctuation amplitude to the absolute level of 10 % and 7 % at 90 and 170 \mu m , respectively , as supported by model calculations , we achieved a new simultaneous determination of the fluctuation amplitudes and the surface brightness of the CFIRB .
The fluctuation amplitudes are 7 \pm 2 mJy and 15 \pm 4 mJy at 90 and 170 \mu m , respectively .
We obtained a CFIRB surface brightness of B _ { 0 } = 0.8 \pm 0.2 MJysr ^ { -1 } ( \nu I _ { \nu } = 14 \pm 3 nWm ^ { -2 } sr ^ { -1 } ) at 170 \mu m and an upper limit of 1.1 MJysr ^ { -1 } ( \nu I _ { \nu } = 37 nWm ^ { -2 } sr ^ { -1 } ) at 90 \mu m .