The lens assembly producing reference transmission spherical wavefront is important device for test of convex or concave spherical surfaces in an interferometer and it can offer a high-quality spherical wave and a reference spherical surface with peak-to-valley less than λ/20 (λ=0.6328 μm). According to their utility, the curvature center (C) of its last spherical surface, as a reference, should coincide with its focus. The mathematical form is deduced for showing the relation between the deviation of F′ and C and the F-number for a reference transmission sphere. The simulation of calculating the coincidence tolerance between F′ and C is shown with ZEMAX software (Focus Software Inc.). The setup to test the coincidence tolerance is designed with the reference flat in Zygo GPI interferometer. The interferograms have been given to show the coincidence value for three self-developed transmission spheres. All the wavefront errors related to the coincidence tolerance between F′ and C are less than five wavelength. The coincidence tolerance between F′ and C is less than 0.03 mm for F/1.5 transmission sphere, and 5 μm for F/0.75 transmission sphere.