![]() The Fourier-transform spectrometer, an interferometer working in the time domain, can also fulfill a very high spectral resolution. Both of them have their own drawbacks: structural complexity and high costs for the former, extra telescope time for the latter. 2012), and (2) spectrographs with externally dispersed interferometry (e.g., Erskine et al. Two categories of modern spectrographs may offer an ultra-high spectral resolution of : (1) regular astronomical high-resolution spectrographs with échelle gratings as the main dispersion elements (e.g., Szentgyorgyi et al. In addition, UHRSs with are also essential to retrieve the temperature and molecular abundance of the planetary atmosphere to clarify the interactions between the solar wind and the stratosphere of a planet, for example, Jupiter (Kostiuk et al. 2017), as well as fine structures and magnetoconvections (e.g., Watanabe et al. In solar physics research, UHRSs (usually ) with a high spatial-resolution image system have been intensively employed to investigate the dynamic processes of the solar outer atmosphere and chromosphere (e.g., Morita et al. Several proposals about such UHRSs have been put forward (López-Morales et al. ( 2019) suggested that the appropriate spectral resolution for O 2 detection in the atmosphere of Earth-like planets should be. ( 2013), which can feasibly detect O 2 on next-generation ground-based Extremely Large Telescopes (ELTs). So far, a series of simulation studies based on both photometry and spectroscopy has focused on the detectability of gaseous O 2, a critical indicator of biosignatures in Earth analogs (e.g., Schneider 1994 Webb & Wormleaton 2001 Kaltenegger & Traub 2009), among which are UHRSs, combined with the cross-correlation technique and Doppler-shift analysis proposed by Snellen et al. 2010) at the ESO Very Large Telescope, and some are in construction. 2003) at the La Silla Observatory ESO 3.6 m telescope and the Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (Pepe et al. The UHRSs equipped with very high-precision and long-term stable calibration sources are at work, for instance, the High Accuracy Radial velocity Planet Searcher (HARPS Mayor et al. 2015) and is expected to play a central role in searching for habitable Earth-like planets around Sun-like stars (e.g., Fischer et al. 2011) and even Earth-mass planets (e.g., Pepe et al. It has been successfully applied in finding super-Earth planets (e.g., Mayor et al. Thanks to the long-term stability of the accurate wavelength calibration and stable point spread functions (PSFs), the UHRS is also a powerful facility for detecting periodic Doppler variations induced by substellar companions. In particular, UHRSs with under seeing-limited conditions have been used in studies of interstellar and circumstellar matter (e.g., Crawford 1995 Price et al. It has become an indispensable tool in understanding the physics of stellar atmospheres, turbulence of the interstellar medium, and the structures of circumstellar disks and stellar winds. 2000), all of which are rather significant in tracking the Galactic evolution. 2003), and abundances of molecules in interstellar medium (e.g., Lauroesch et al. 2013), isotopic ratios in stellar atmospheres (e.g., Knauth et al. Ultra-high-resolution spectrographs (UHRSs) with a spectral resolution have been extensively utilized in modern astronomical research, such as the measurements of chemical abundances of the old solar twins (e.g., Monroe et al. Ultimately, feasible optimizations for night-sky observations under seeing limited conditions are discussed. The VIPA spectrograph has promising applications in various astronomical observations in which ultra-high resolution and calibration precision are imperative, such as solar physics research, exoplanet searching with the radial velocity method, and O 2 detection in the atmosphere of Earth-like planets. In laboratory tests, the VIPA-based instrument with a homemade Yb:fiber ring laser frequency comb demonstrates a mode-to-mode tracking stability of 41 cm s −1 over a period of 6 hr. The single-mode, fiber-fed spectrograph with simultaneous wavelength calibration possesses a spectral resolution well in excess of while the size of the VIPA is several orders of magnitude smaller than that of a conventional échelle with comparable resolution. A prototype spectrograph using a Virtually Imaged Phased Array (VIPA) as the main dispersion element is presented, and its performance is fully examined in our laboratory.
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