INTEGRAL/IBIS 7-year All-Sky Hard X-ray Survey I. Image reconstruction

This paper is the first in a series devoted to the hard X-ray whole sky survey performed by the INTEGRAL observatory over seven years. Here we present an improved method for image reconstruction with the IBIS coded mask telescope. The main improvements are related to the suppression of systematic effects that strongly limit sensitivity in the region of the Galactic plane (GP), especially in the crowded field of the Galactic center (GC). We extended the IBIS/ISGRI background model to take into account the Galactic ridge X-ray emission (GRXE). To suppress residual systematic artifacts on a reconstructed sky image, we applied nonparametric sky image filtering based on wavelet decomposition. The implemented modifications of the sky reconstruction method decrease the systematic noise in the similar to 20 Ms deep field of GC by similar to 44%, and practically remove it from the high-latitude sky images. New observational data sets, along with an improved reconstruction algorithm, allow us to conduct the hard X-ray survey with the best currently available minimal sensitivity 3.7 x 10(-12) erg s(-1) cm(-2) similar to 0.26 mCrab in the 17-60 keV band at a 5 sigma detection level. The survey covers 90% of the sky down to the flux limit of 6.2 x 10(-11) erg s(-1) cm(-2) (similar to 4.32 mCrab) and 10% of the sky area down to the flux limit of 8.6 x 10(-12) erg s(-1) cm(-2) (similar to 0.60 mCrab).