# Echelle Grating Spectroscopic Technology for High-Resolution and Broadband Spectral Measurement
## 5 Applications
### Astronomical Instruments  
In the field of astronomy, spectral analysis is an extremely important way to observe celestial bodies. The spectral analysis determines the density, mass, movement, chemical composition, and distance from earth [82]. A high spectral resolution and broadband spectral range are the basic requirements for astronomical spectrometers. The light from deep space through a telescope is so weak that it usually takes more than one hour for an image. Spectrometers with common blazed gratings need multiple exposures for a broadband spectral range. During the longtime exposures, the state of celestial bodies may have changed. So, the capability of capturing the whole spectrum at one shot is another requisite for astronomical spectrometers. The echelle and the cross-dispersion are exactly applicable in astronomy. Early astronomical applications of the echelle spectrometer include the middle ultraviolet solar spectrometer from rockets [83] and the echelle spectrometer-spectrograph for the 91 cm telescope at Pine Bluff [84]. However, due to the limitation of the grating fabrication,the resolution is not high. With the improvement of the ruling technique, echelle spectrometers have been more widely used [85]. The high-resolution echelle spectrometer (HIRES) [86; 87] (Figure 9) is the most representative one, which was constructed for the Keck telescope. It uses an echelle as the dispersive element, and the focal length of the collimator is much larger than that of the spectroscopic imaging mirror to reduce the image. It is difficult for the optical design to image the information from a large aperture telescope to the detector of a limited area. Therefore, the spectrometers for astronomical observation have much more complicated optics than commercial instruments. To meet the extremely high-resolution requirements, they typically have a front optical path, a collimated optical path, a cross-dispersion optical path, an imaging optical path, and an additional correction optical path. In recent years, more and more observatories have begun to use echelle spectrometers for astronomical observations, as shown in Table 1. It can be seen that the spectral range of most spectrographs covers all of the visible spectrum and some of the near-infrared spectrum, such as EXPERT-III [88], CAFE [89], MIKE [90], and NRES [91; 92; 93]. There are also a few spectrometers whose spectral range only covers the near-infrared band, such as SPIRou [94; 95; 96]. Some spectrographs cover part of the mid-infrared spectrum, such as CRIRES+ [97; 98; 99; 100]. Many of these spectrographs follow a concept called \"white-pupil\", which was proposed by [PERSON] in 1972 [101]. In the white-pupil concept, the system pupil at the dispersive element is re-imaged on the entrance aperture of the camera. This pupil  
Figure 9: Top view and side view schematics of the HIRES (light from the telescope enters from the left) [86]. (Reprinted with permission from [86] © The Optical Society). image is fixed at a specific position, independent of the wavelength. For example, the optical design of NEID is based on a classic white pupil layout [102]. Generally speaking, the white-pupil design offers a higher overall throughput and better image quality than conventional designs for the same resolution. For higher performance, some spectrometers use a beam splitter to split the beam into two sections so as to optimize the image quality and optical efficiency of each section individually. Some spectrometers split the beam into red and blue arms, such as G-CLEF [103, 104, 105], ESPRESSO [106, 107, 108, 109], PEPSI [110, 111, 112, 113], and SALT-HRS [114], while the light in CARMENES is separated into the visible and near-IR channels by a dichroic beam splitter centered at 0.96 \(\upmu\)m [115, 116, 117]. In order to get a wider range of full spectrum coverage, some spectrographs employ several echelles with different incidence angles, such as CRIRES+. For higher spectral resolutions, we can use smaller slits, but the luminous flux is lower.