Linear optical model for a large ground based telescope

A linear optical model for 30 m class astronomical telescopes is discussed. Our approach to modeling the optical performance of the telescope is an extension of the line-of-sight analysis to incorporate low order image aberrations. The model describes the optical path difference at the telescope exit pupil as a linear combination of primary and secondary mirror displacements and deformations in a Zernike basis. Although the model is valid over a significant, useful range of deformation amplitude and field of view, it is particularly convenient for the design and investigation of active optics control systems operating close to the desired condition of an unperturbed telescope. The formalism of the linear model and comparison with the OPD results generated by ray-tracing are discussed.