We have implemented an Optical Parametric Oscillator (OPO) with an unstable cavity. The cavity is a telescope made of a concave mirror and a convex mirror. This latter is coated only in its center an is the pupil of the cavity. The nonlinear crystal is a 5 cm long LiNbO3. The OPO is pumped by a 20 ns Q-switch Nd:YAG laser and is operated at 1.9 micrometers for the signal and 2.4 micrometers for the idler. The OPO threshold is 7 mJ. For a pump energy of 30 mJ, the signal and idler energy produced is 9 mJ. Those results are comparable to those obtained with plane mirrors cavities. The near field of the output beam has an annular distribution imposed by the pupil. The distribution is not uniform but has 2 lobes located on opposite sides one from another. This uniformity can be improved increasing in the magnification factor from 1.2 to 1.33. The far field shows a central peak with side lobes in the vertical direction. This intensity distribution can be calculated from the diffraction of a plane wave by a uniform annular pupil similar to the near field intensity distribution. This indicates that the field is close to the diffraction limit in this direction. In the horizontal direction, there is peak which is twice as large as the previous one, and there is no side lobes. The field can be estimated to be 2 times the diffraction limit in this direction. This experiment shows that infrared OPO's with unstable cavities can produce beams close to the diffraction limit with a good energy conversion.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.