The FIRBACK survey is a deep imaging survey carried out with ISOPHOT
on board ISO, at 170 μm. The survey covers ≈4 sq deg in
three high galactic latitude fields (FIRBACK South Marano, FIRBACK
North 1 and FIRBACK/ELAIS North 2). In these fields, fluctuations
due to unresolved extra-galactic sources were isolated for the first
time [1]. We are using a P(D) (probability distribution of fluctuations) analysis to gain
information about the N(≥S) curve of the unresolved sources which
make up the background. Many planned
surveys (e.g. SIRTF, FIRST, PLANCK...) will pose the same analysis
challenges and we hope that the outcome of this study will be useful
in that context. As a preparation to the analysis of the FIRBACK maps we analyze
simulated sky maps. We create histograms of number of pixels per flux bin. The histograms are
binned such that there are enough pixels per bin to allow us to apply
Gaussian estimates for the errors. We then calculate the expected number of
pixels within the relevant flux ranges and
evaluate the χ2 statistic. We will use a maximum likelihood
algorithm to find the parameters of N(≥S).