# High-Precision Manufacture and Alignment of Image Slicer Based on Thin Glass Bonding
## 1 Introduction  
The integral field spectrometer (IFS) is one of the most important scientific instruments in astronomical research [1; 2]. Currently, the main large-aperture astronomical telescopes, such as TMT-IRIS and VLT-MUSE, are equipped with IFSs [3; 4; 5; 6; 7], which simultaneously collect both image and spectral information and investigate the physical properties and kinematics within the field of high-resolution two-dimensional (2D) space [8]. The observation time is reduced, and the stability and consistency are improved compared with the conventional 2D imaging spectrometer. The imaging principle of the IFS is shown in Figure 1. The fore-optical system collects the target optical information and images it at the focal plane. An integrated field unit located on the focal plane divides the optical information of the obtained image into several units and rearranges them in the 2D field of view. Then, the light illuminates the slit and enters several dispersive devices in the spectroscope. The required three-dimensional data are obtained through image processing using the spectral information received by the detectors. As shown in Figure 2, three main techniques are used for the IFS: the micro-lens, micro-lens combined with optical fibers, and image slicer [9; 10]. Compared with the IFS using a micro-lens array to segment images, the principle of the image slicer-based IFS is simpler, and the image processing is easier. The utilization ratio of the detector is higher such that almost every pixel of the detector is used. The 2D spatial direction sampling and dispersion direction do not interfere with each other [11]. However, the image slicer is not typically used in IFSs because of its complex structure, large volume, and high cost [12]. Machining and traditional stacking are the two main methods for fabricating image slicers. Machining consists generally in fabricating multi-channel image slicers on a single piece of metal by single-point diamond turning [13; 14]. The channel on an image slicer cannot be made too thin because of the limitation of the applicable tools in fabricating individual channels. Moreover, machine manufacturing cannot meet the requirements on surface roughness and shape in the visible light region [15]. This fabrication method is usually applied for large-volume image slicers, such as ground-based telescope infrared integral field spectroscopy slicers, but it is not suitable for small-volume applications. Compared with machining, traditional stacking avoids the difficulty in the fabrication of thinner channels on image slicers. There are two methods for fabricating image slicers by traditional stacking. The first method consists in extracting unique slices from different blank mirrors and stacking all slices together. The second method consists in stacking together multiple eccentricity channels by fabricating all the channels of the image slicer in a single processing. Compared with the former method, the latter is low cost and higher in efficiency [16; 17]. The surface accommodates many different fabrication methods to cope with different spectrometer bands. This method can be applied to all types of IFS in different optical systems. The IFS prototype of the China Space Station Telescope (CCST) specific optical path is shown in Figure 3. Light passes through the fore-optical system and then enters the image slicer at the focal plane. The light is refocused, reflected through pupil mirrors, and enters the spectrometer through a pseudo slit. By folding the optical path, the spectrometer volume  
Figure 1: Principle diagram of integral field spectrometer. Figure 2: A summary of three main techniques for integral field spectrograph [10]. is reduced, thereby optimizing the space utilization of the rear end of the telescope. The working wavelength of the spectrometer is 350-1200 nm. The image slicer is manufactured using the second method of traditional stacking. However, this method uses molecular assembling to stack thin glass during the image slicer fabrication.