MODS: Optical design for a multi-object dual spectrograph

The paper describes the optical design for the Multi Object Dual Spectrograph (MODS) for the Large Binocular Telescope (LBT). MODS is designed to cover the entire spectrum accessible to silicon CCDs from a ground-based telescopes with the highest possible throughput. Multi-object capability is available using 0.6 are-second slit masks covering a high quality field of 4 are-minutes in diameter with an extended field of up to 6 are-minutes in diameter with reduced image quality. Under the very best seeing conditions and with the LET adaptive optics in operation, slit widths of 0.3 are-seconds can be used to enhance the resolving power and/or reduce the background. The optical path is divided into blue and red channels by a dichroic beam splitter following the slit or slit masks. The blue channel covers a wavelength range from the atmospheric cut-off at similar to 320 nm to similar to 550 nm while the red channel covers the range from similar to 550 nm to the limit of useful sensitivity of silicon CCDs (similar to 1000 nm). This approach allows the optimization of transmissive and reflective coatings to provide the very highest throughput for each channel. The design is conventional in the use of reflective parabolic collimators. However, the cameras are designed as decentered Schmidt/Maksutovs with large aspheric coefficients for the inner surfaces of the correctors. This approach enables the field flattener and detector to be positioned outside the beam entering the camera where it will not vignette. Figures are presented showing image quality for imaging and spectrographic modes.