A metrology tool (1) for measuring the positions of structures (32) on a mask surface (31) is disclosed. On a measuring stage (33) a reflector (36) selective with respect to the wavelength is provided, which essentially reflects light within a first wavelength region emitted from a first illumination device (10), and essentially does not reflect light within a second wavelength region emitted from a second illumination device (20). The reflector (36) selective with respect to the wavelength preferentially is a dichroic mirror. By detecting the light within the first wavelength region reflected by the reflector (36) the position of predefined sections of outer edges (37) of the mask is determined. The light from the second wavelength region is used for determining the coordinates of structures on the mask. Due to the selectivity with respect to the wavelength of the reflector (36) this determination of coordinates is not perturbed by a reflection of the light within the second wavelength region from the reflector (36).