The Adaptative Optics Test Camera (AOTC) is designed to be an engineering instrument to be used during the Giant Magellan Telescope (GMT) assembly, integration, verification, and commissioning to verify the Adaptive Optics (AO) system-level diffraction-limited image quality performance. The AOTC will functionally replicate the external interfaces of the GMT AO instruments, including the Natural Guide Star Wavefront Sensor (NGWS) and Laser Tomography Wavefront Sensor (LTWS) as external AO sensors, as well as a prototype On-Instrument Wavefront Sensors (OIWFS) to measure non-common-path wavefront errors not sensed by the external sensors. We present the initial optical design of this instrument, which passed a conceptual design review in 2023, as well as a preliminary performance assessment of the OIWFS. The AOTC consists of four cameras. The first is a large field of view camera to aid in the instrument target acquisition. A Shack-Hartmann sensor measures focus and low order wavefront errors in the non-common path between the external sensors and science focal plane. A high-speed imager measures the target motion and telescope segment piston error. Finally, a scoring camera, representing the science camera, evaluates the AO image quality. We describe the optical parameters and the strategies used to achieve each sensor function and attend their performance requirements, along with the initial choices for commercial detectors for each function. We also present the preliminary performance budget for assessing system performance and determining tolerances for mechanical manufacturing and alignment.
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