A wide-field IR spectrograph for the Giant Magellan Telescope

The Giant Magellan Telescope, with seven 8.4 meter primary mirrors, is taking shape as one of the most powerful telescopes of the next generation. We describe a conceptual design for a powerful 0.85 to 2.50 μm imaging spectrograph that addresses a 7′ by 7′ field of view for imaging and a 5′ by 7′ field of view for spectroscopy at the GMTs f/8 Gregorian focus. The all-refractive optical design presses the limits of available lens blank diameters, but delivers excellent images (∼0.15Prime; 80% encircled energy) with just four collimator elements and five camera elements. The collimated beam diameter is 300 mm, and the detector is a 6K by 10K array. The spectrograph will use interchangeable slit masks, and an assortment of VPH and conventional surface relief gratings. Each of the entire J, H, or K bands can be observed with a resolution of 3000. The scientific potential of ground layer adaptive optics (GLAO) using a constellation of sodium laser guide stars appears to be very high in the near infrared. Simulations suggest that 0.2″ FWHM images may be achieved across the entire 7′ by 7′ field of view of the spectrograph. We describe the design of the GLAO system with a versatile opto-mechanical design that allows rapid changeover between GLAO and seeing-limited observations.