A new concept and a preliminary design for a high resolution (HR) and very-high resolution (VHR) spectrograph for the LBT

A way to fully exploit the large collecting area of modern 8-10m class telescopes is high resolution spectroscopy. Many astrophysical problems from planetary science to cosmology benefit from spectroscopic observations at the highest resolution currently achievable and would benefit from even higher resolutions. Indeed in the era of 8-10m class telescopes no longer the telescope collecting area but the size of the beam - which is related to the maximum size in which reflection gratings are manufactured - is what mainly limits the resolution. A resolution-slit product Rphi ~ 40,000 is the maximum currently provided by a beam of 20 cm illuminating the largest grating mosaics. We present a conceptual design for a spectrograph with Rphi ~ 80,000, i.e. twice as large as that of existing instruments. Examples of the possible exploitation of such a high Rphi value, including spectropolarimetry and very high resolution (R ~ 300,000), are discussed in detail. The new concept is illustrated through the specific case of a high resolution spectropolarimeter for the Large Binocular Telescope.