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Context: Deep photometric surveys for substellar objects in young clustersand for high-redshift quasars are affected by contaminant sources at differentheliocentric distances.If not correctly taken into account, the contaminationmay have a strong effect on the Initial Mass Function determination and on theidentification of quasars. Aims: We calculate in detail the back- andforeground contamination by field dwarfs of very late spectral types(intermediate and late M, L and T) in deep surveys and provide the data andtools for the computation. Methods: Up-to-date models and data from theliterature have been used: (i) a model of the Galactic thin disc by anexponential law; (ii) the length and height scales for late-type dwarfs; (iii)the local spatial densities, absolute magnitudes and colours of dwarfs for eachspectral type. Results: We derive a simplified expression for the spatialdensity in the thin disc that depends on the heliocentric distance and thegalactic coordinates (l, b) and integrate it in the truncated cone screened inthe survey. As a practical application, we compute the numbers of L- and T-typefield dwarfs in very deep (I = 21-29 mag) surveys in the direction of the youngsigma Orionis cluster. The increasing number of contaminants at the faintestmagnitudes could inhibit the study of the opacity mass limit at M <~ 0.003M_sol in the cluster.

Contamination by field late-M, L and T dwarfs in deep surveys