Wide-field imaging at mid-infrared wavelengths: Reconstruction of chopped and nodded data

Ground-based astronomical observations at thermal infrared wavelengths (lambda greater than or equal to 2.4 mu m) face the problem of extracting the weak astronomical signal from the large and rapidly variable background flux. The observing strategy most commonly used, the "chopping and nodding" differential technique, provides reliable representations of the target uniquely in the case of compact sources, while extended and complex sources can be easily distorted by their negative counterparts. A restoration method, designed to remove the negative values and to provide reliable representations of extended sources, has been proposed by us in two previous papers and validated on simulated images. In this paper we apply our algorithm to real images taken at the UKIRT telescope with the MPIA camera MAX. We show that the algorithm successfully removes the distortions due to the negative counterparts and, in addition, provides noise reduction. In several cases an enlargement of the field is obtained, in the sense that the restored larger image provides reliable information on the source structure outside the central field of view. The restorations may be affected by artifacts, whose origin can be predicted theoretically. We suggest and demonstrate computational and observational procedures for their reduction. Once combined with the proper observing strategies, our inversion method can provide a viable solution to the problem of deep imaging of extended sources with large ground-based telescopes.