A simple approach for measuring the piston error between two adjacent segments in a primary mirror of a telescope, based on the one dimension analysis of the diffraction pattern produced by a divided slit, is proposed. Using two wavelengths allows an increase of the dynamic range of the measurement. The main advantages are that even maintaining the correlation based scheme used by other authors, the time of processing should be reduced. Some experimental results are presented which show that for one wavelength a precision of 3 nm and a dynamic range of 316 nm are feasible for the red line of a He-Ne laser. For the two wavelength experiments a precision of 53 nm is obtained for lambda(eq)/2 = 1670 nm dynamic range.