MegaMapper: concept and optical design for a 6.5m aperture massively multiplexed spectroscopic facility

MegaMapper is a 6.5m Magellan-like telescope fitted with a wide-field-corrector (WFC) and atmospheric-dispersion-corrector (ADC) that delivers a 3. diameter corrected field-of-view. The telescope's focal surface is populated by similar to 25, 000 robotic fiber-positioners feeding a cluster of 36 DESI-like medium resolution spectrographs. We present the facility concept for MegaMapper including: conceptual optical and opto-mechanical designs for the telescope and WFC/ADC that deliver less than or similar to 0.4 '' image quality over the full FOV for zenith distances <= 50 degrees; the development of a new and modular robotic fiber-positioner focal plane design that can populate the focal surface at high densities (6.2 mm pitch or similar to 1 per arcmin(2)); and concepts for hosting the MegaMapper spectrograph cluster under environmentally controlled conditions inside the telescope enclosure. Building on existing and proven designs and technologies, MegaMapper aims to minimize the project's technical risk and cost while delivering a competitive next-generation massively multiplexed spectroscopic facility. MegaMapper will lead the study of inflation, dark energy, dark matter, and time-domain astronomy over the next decades by carrying out wide-field cosmological galaxy-redshift surveys, massive spectroscopic surveys of stars in the Milky Way halo and satellites, and by providing a spectroscopic follow-up counterpart to wide field imaging facilities like the Vera C. Rubin Observatory and the Nancy Grace Roman space telescope.