Each unit is composed of thirty nested coaxial and confocal mirrors having

ABSTRACT The imaging concentrators (4 units) are part of the payload of the italian satellite for X—ray astronomy SAX. They are operating in the energy range 0.3 — 10 Key and have an angular resolution of 1 arcmin HPR. The effective area of each unitis 80 cm2at 2 Key.The optics is composed of 30 confocal nested very thin double cone mirrors. To achieve a good optical quality and to allow the construction of several concentrators at an acceptable cost, a replica technique by electroforming the mirrors from masters is used. To verify the thermomechanical and imaging properties of the optical system, a development model has been built having a set of 29 of the 30 mirrors. The mirrorsare representative of the final ones except for the microroughness which is 3 nm instead of <1 nm because, in order to have an earlier verification, they have beenreplicated from mandrels that have not yet been superpolished.The optical system has been tested with low energy X—ray beam (E = 0.27 Key) and withvisible light. The results of these measurements are presented in the paper.