The Advanced Technology Solar Telescope (ATST) will be the most powerful solar telescope and the world's leading resource for studying solar magnetism that controls the solar wind, flares, coronal mass ejections and variability in the Sun's output. Development of this four-meter off-axis solar telescope has presented many optical design challenges including:
• support of both Nasmyth and flexible coude lab instrumentation,
• incorporation of an integrated adaptive optics system,
• thermal control of optics, and
• optical alignment of multiple off-axis conics.
This paper gives an overview of the optical design, error budgeting, and the performance modeling done to ensure the telescope will satisfy its optical performance requirements.