An Installation for concentrating cosmic radiation originating from a celestial object with a non-zero apparent angular radius, comprising - a concentrating optical surface able to reflect incident cosmic radiation toward a target surface O'X'Y' so as to form an image of said celestial object here, and liable to contain local surface errors and aiming and orientation errors, - a system for inspecting the reflective optical surface, characterised in that the inspecting system comprises: - means for acquiring images of the optical surface from various viewpoints M' mn (x' mn, y' mn) that are located inside the image mark of the celestial object in the target surface, m varying from 1 to M, and n varying from 1 to N, so as to obtain M x N images of the optical surface illuminated by the cosmic radiation, with M viewpoints along X' and N viewpoints along Y', where M>1, N>1 and M.N≥30, - and a unit for processing the M.N acquired images, which unit is suitable for: ∘ calculating the slopes δΔ /δx and δΔ /δy for each point P(x,y) of the reflective optical surface, where: ∂ Δ P ∂ x = g X ε 0 ∑ m = 1 M ∑ n = 1 N sign x ′ mn L M ′ mn, P ∑ m = 1 M ∑ n = 1 N L M ′ mn, P ∂ Δ P ∂ y = g Y ε 0 ∑ m = 1 M ∑ n = 1 N sign y ′ mn L M ′ mn, P ∑ m = 1 M ∑ n = 1 N L M ′ mn, P L(M' mn, P) being the luminance at a point of the image corresponding to the point P(x,y) of the reflective optical surface observed from the viewpoint M' mn, ε 0 the apparent angular radius of the celestial object and g x and g y pre-set coefficients, and ∘ determining from these slopes δΔ /δx and δΔ /δy, a local surface error ΔP(x,y) at the point P(x,y) of the reflective optical surface. - a concentrating optical surface able to reflect incident cosmic radiation toward a target surface O'X'Y' so as to form an image of said celestial object here, and liable to contain local surface errors and aiming and orientation errors, - a system for inspecting the reflective optical surface, characterised in that the inspecting system comprises: - means for acquiring images of the optical surface from various viewpoints M' mn (x' mn, y' mn) that are located inside the image mark of the celestial object in the target surface, m varying from 1 to M, and n varying from 1 to N, so as to obtain M x N images of the optical surface illuminated by the cosmic radiation, with M viewpoints along X' and N viewpoints along Y', where M>1, N>1 and M.N≥30, - and a unit for processing the M.N acquired images, which unit is suitable for: ∘ calculating the slopes δΔ /δx and δΔ /δy for each point P(x,y) of the reflective optical surface, where: ∂ Δ P ∂ x = g X ε 0 ∑ m = 1 M ∑ n = 1 N sign x ′ mn L M ′ mn, P ∑ m = 1 M ∑ n = 1 N L M ′ mn, P ∂ Δ P ∂ y = g Y ε 0 ∑ m = 1 M ∑ n = 1 N sign y ′ mn L M ′ mn, P ∑ m = 1 M ∑ n = 1 N L M ′ mn, P L(M' mn, P) being the luminance at a point of the image corresponding to the point P(x,y) of the reflective optical surface observed from the viewpoint M' mn, ε 0 the apparent angular radius of the celestial object and g x and g y pre-set coefficients, and ∘ determining from these slopes δΔ /δx and δΔ /δy, a local surface error ΔP(x,y) at the point P(x,y) of the reflective optical surface. ∘ calculating the slopes δΔ /δx and δΔ /δy for each point P(x,y) of the reflective optical surface, where: ∂ Δ P ∂ x = g X ε 0 ∑ m = 1 M ∑ n = 1 N sign x ′ mn L M ′ mn, P ∑ m = 1 M ∑ n = 1 N L M ′ mn, P ∂ Δ P ∂ y = g Y ε 0 ∑ m = 1 M ∑ n = 1 N sign y ′ mn L M ′ mn, P ∑ m = 1 M ∑ n = 1 N L M ′ mn, P L(M' mn, P) being the luminance at a point of the image corresponding to the point P(x,y) of the reflective optical surface observed from the viewpoint M' mn, ε 0 the apparent angular radius of the celestial object and g x and g y pre-set coefficients, and ∘ determining from these slopes δΔ /δx and δΔ /δy, a local surface error ΔP(x,y) at the point P(x,y) of the reflective optical surface.