Telescopes are ultimately limited by atmospheric turbulence, which is commonly characterized by a structure function. The telescope optics will not further degrade the performance if their errors are small compared to the atmospheric effects. Any further improvement to the mirrors is not economical since there is no increased benefit to performance. Typically the telescope specification is written in terms of an image size or encircled energy and is derived from the best seeing that is expected at the site. Ideally, the fabrication and support errors should never exceed atmospheric turbulence at any spatial scale, so it is instructive to look at how these errors affect the structure function of the telescope. The fabrication and support errors are most naturally described by Zernike polynomials or by bending modes for the active mirrors. This paper illustrates an efficient technique for relating this modal analysis to wavefront structure functions. Data is provided for efficient calculation of structure function given coefficients for Zernike annular polynomials. An example of this procedure for the Giant Magellan Telescope primary mirror is described.