Optics of telescope makers Torricelli, Divini, and Campani dating back to the 17th century have been tested interferometrically. It was found that the optical polishing technology had progressed to thorough control over the surface figure and finish, while the nearly paraxial conditions of the aperture remained constant. The instruments that were examined exemplify the knowledge of optics of that time, particularly with respect to the early use of erector units in terrestrial telescopes.