a test object including : an arrangement of optical elements defining a first partially reflecting surface and a second partially reflecting surface , at least one of the first and second partially reflecting surfaces being curved , wherein the first partially reflecting surface is arranged to receive a substantially collimated input beam and produce therefrom a first transmitted beam that passes on to the second partially reflecting surface , wherein the second partially reflecting surface is arranged to receive the first transmitted beam from the first partially reflecting surface and produce a collimated second transmitted beam and a first reflected beam therefrom , wherein the first partially reflecting surface is arranged to receive the first reflected beam and produce a second reflected beam therefrom , and wherein the first and second reflecting surfaces are configured to cause the second reflecting beam to converge onto a spot on a back surface to produce a diverging beam traveling in the same direction as the collimated output beam .