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Mid-IR Spectroscopy at 30 m aperture

Mid-IR Spectroscopy at 30 m aperture. “Trade-off” equation: W col = SDR/400000 tan q S slit width (arcseconds) D telescope diameter R spectral resolution q grating angle W col diameter of collimated beam. A large spectrograph.

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Mid-IR Spectroscopy at 30 m aperture

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  1. Mid-IR Spectroscopy at 30 m aperture • “Trade-off” equation: • Wcol = SDR/400000 tan q S slit width (arcseconds) D telescope diameter R spectral resolution q grating angle Wcol diameter of collimated beam

  2. A large spectrograph • Slit width = 2 x diffraction limit (~0.28 arcsec at 20 mm) • Telescope diameter 30 m • Resolution 100000 • Wcol ~ 2/tan q (meters)!

  3. The grating • Free Spectral Range/Groove Spacing • 20mm = 500 cm-1, 1024 pixels at R=100000 • => 2.5 cm-1 covers array • FSR = 1/(2a sin q ) (a = groove spacing) • 1 line/mm corresponds to 5 cm-1 FSR

  4. Mid-IR Solutions • Maximize q to reduce beam size • Increasing q has minimal effect on groove spacing • Limitations—must be possible to cut the grating steep enough.

  5. A possible solution • R10 echelle, 0.2 x 2 meters, 7 mm/groove, work in order ~760 • Cross disperser 45 lines/mm, 26.5 degree blaze angle, 11 pixel minimum order separation, cover ~15 to 20 mm

  6. Design Challenges • Reflective, 4 mirror camera • Instrument size: ~4x2x1 meter • Cooling to 4 K: instrument weight at standard 0.2 gm/cm3 =1.5 tons • Grating fabrication

  7. State-of-the-art IR Spectroscopy circa 1975(JPL/Beer FTS at McDonald 2.7 m)

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