  We describe in this paper a new, public software for accurate "PSF-matched"
multiband photometry for images of different resolution and depth, that we have
named ConvPhot, of which we analyse performances and limitations. It is
designed to work when a high resolution image is available to identify and
extract the objects, and colours or variations in luminosity are to be measured
in another image of lower resolution but comparable depth. To maximise the
usability of this software, we explicitly use the outputs of the popular
SExtractor code, that is used to extract all objects from the high resolution
"detection" image. The technique adopted by the code is essentially to convolve
each object to the PSF of the lower resolution "measure" image, and to obtain
the flux of each object by a global chi2 minimisation on such "measure" image.
We remark that no a priori assumption is done on the shape of the objects. In
this paper we provide a full description of the algorithm, a discussion of the
possible systematic effects involved and the results of a set of simulations
and validation tests that we have performed on real as well as simulated
images. The source code of ConvPhot, written in C language under the GNU Public
License, is released worldwide.
