# Echelle Grating Spectroscopic Technology for High-Resolution and Broadband Spectral Measurement
## 6 Discussions and Outlook  
In this review, we provided a general overview of the echelle spectroscopic technology. We discussed the characteristics of the echelle, the optical layout strategies, the key parameters of detection, the calibration methods, and the applications in commercial and astronomical instruments. Compared with the scanning-based spectrometers using common blazed gratings, echelle spectrometers have remarkable advantages. They have high spectral resolution and large optical throughput within the full spectral range. Such characteristics ensure the compact optical size and high-speed spectral analysis. These capabilities make the echelle spectrometer an appealing tool for identifying fine chemical compositions in the civilian field and studying celestial bodies in astronomy. Commercial instruments pursue high performance at a low cost. The products need to meet the requirements of applications at an available price that most users can afford. The spectral resolution of a few picometers from ultraviolet to near infrared light is propitious for identifying metals and their isotopes. Higher spectral resolutions have fewer applications but higher costs. The current echelle spectrometers are still expensive due to the optics and the area array detector. The improvement of the optical fabrication technique for the echelle itself and the non-spherical mirrors is the key to reducing the price of optics. As for the detector, large-area, highly sensitive, and broadband CCDs or CMOSs are rapidly \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Observatory** & **Spectrometer Model** & **Spectral Range (nm)** & **Resolution** \\ \hline \multirow{2}{*}{Cala Alto Observatory} & CAFE & 365–980 & 70,000 \\  & CARMENES & 520–1710 & 93,400, 82,000 \\ \hline \multirow{2}{*}{Cerro Tololo Inter-American Observatory} & CHIRON [119] & 410–880 & 80,000 \\ \cline{2-4}  & CRIRES+ & 950–5300 & 50,000, 100,000 \\  & ESPRESSO & 380–780 & 59,000, 134,000, \(\approx\)200,000 \\ \hline \multirow{2}{*}{Kitt Peak National Observatory} & EXPERT-III & 380–900 & 56,000, 110,000 \\  & NEID & 380–930 & 100,000 \\ \hline \multirow{2}{*}{Las Campanas Observatory} & G-CLEF & 350–950 & 19,000, 108,000 \\  & MIKE & 320–1000 & 19,000, 25,000 \\  & NRES & 390–860 & 53,000 \\  & PFS [120] & 388–668 & 38,000 \\ \hline \multirow{2}{*}{Mauna Kea Observatory} & IRD & 970–1750 & 70,000 \\  & SPIRou & 980–2440 & 70,000 \\ \hline Mt. Graham International Observatory & PEPSI & 384–913 & 50,000, 130,000, 250,000 \\ \hline South African Astronomical Observatory & SALT-HRS & 370–890 & 15,000, 40,000, 65,000 \\ \hline Lijiang Observatory in China & CES [30] & 570–1030 & 37,000 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Echelle spectrometers at observatories. developing, but it is hard for the cost to drop in a short time. Therefore, the echelle cross dispersive technique is still only available in advanced equipment. On the other hand, the volume of spectrometers with echelles is too large to be portable. The development of a low-cost and portable device is the direction of the spectrometer of echelles. For astronomical spectrometers, an extremely high performance is required, and the budget is usually adequate. So, the optics are much more complicated than those of commercial spectrometers, and the optical system consists of many lenses instead of a few mirrors. The highest obtainable spectral resolution is proportional to the grating size and even the instrument volume. Although the cost of optics is not a main limitation for the perfect optical performance, the volume influences the total budget and building difficulty greatly in the thermal IR because the price of the cooling vacuum container holding the spectrometer rises sharply as the volume increases [121]. An immersed echelle is a solution to reduce the size. It is an echelle in which the optical pass is filled up with high-refractive-index (n) material.