  Ground-based astronomical observations at thermal infrared wavelengths face
the problem of extracting the weak astronomical signal from the large and
rapidly variable background flux. The observing strategy most commonly used,
the so-called ``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. We
show that the algorithm successfully removes the distortions due to the
negative counterparts and, in addition, provides noise reduction.
