Spherical surfaces are essential components in optical systems for different applications. Also, standard ‎spheres are typically employed to calibrate dimensional and mass-measuring instruments. Therefore, ‎measuring and eliminating the surface irregularities of these spheres represents a challenge for modern ‎industries. Surface irregularities can be measured by either the contact method (as stylus profilometers) ‎or the non-contact method (as interferometers). The Fizeau laser interferometer (GPI-XP, Zygo Co.) is ‎typically used to accurately measure surface irregularities for flat surfaces, lenses, and mirrors of 102 mm ‎diameter, and it is not prepared for testing heavy spheres. This work presents a modification for the ‎interferometric system to expand its capability for testing spheres of diameters up to 145 mm and masses ‎up to 2 kg. Also, the modification provides automatic rotation for the tested surfaces (an electric rotating ‎platform) to avoid the disadvantages of manual rotation (a tweezer). At different rotation angles, the ‎measurements are performed on a mass-standard silicon sphere (SiScKg-02-d, PTB Inst.). The average ‎measured value of the surface irregularities is 43.9 nm, with an improved repeatability of 2.8 nm and a ‎minimized measurement uncertainty of ±6.8 nm. The evaluation of the uncertainty budget is also ‎investigated in this study.‎