## 4 Experiment Results
### Test of Spectrometers  
We take the test results from B1, B3, and B5 spectrometers as examples. The device for testing the spectral performance of the B1 spectrometer is shown in Figure 17a,b. A Hg-Cd lamp is used to illuminate the slit, and a small-pixel CCD detector with a pixel size of 5.5 \(\upmu\)m is used to acquire the spectral lines of the lamp. The measured spectral lines are shown in Figure 17c. There are abundant spectral lines of 546.1 nm, 508.5 nm, 479.9 nm, 467.8 nm, 435.8 nm, 404.6 nm, 365.0 nm, 346.3 nm, and 312.9 nm within the B1 wavelength range (300-560 nm). By calculating the centroid coordinates of these spectral lines in spectral dimension, it can be obtained that the dispersion power of the B1 spectrometer is 44.1 nm/mm. Pixel size of the detector used in the B1 spectrometer is 15 \(\upmu\)m, and the spectral sampling distance with 6\(\times\) pixels binning is 3.97 nm, consistent with the design. SRFs of the spectrometer are shown in Figure 17d, the \(SRF_{opt}\) is fitted from raw data, and the \(SRF_{ins}\) is the SRF after 6\(\times\) pixels binning. The spectral resolution is the full width at half maximum (FWHM) of \(SRF_{ins}\). Table 5 gives the spectral resolution at different fields and wavelengths of the B1 spectrometer. It can be seen that the spectral resolution in the whole image plane is uniform. Figure 16: Spectrometers of five sub-bands. (**a–e**) for spectrometers of B1–B5. Figure 15: Slit-to-grating rotation affects the spatial uniformity of spectral image. The slit is parallel to the grating groove in (**a**), and the dispersion direction is perpendicular to the monochromatic slit images. The slit rotates an angle relative to the grating groove in (**b**), and the dispersion direction is not perpendicular to the monochromatic slit images. Smile distortion in this paper is defined as the deviation of monochromatic slit image from a straight line, and it is one of the factors that affect the spectral uniformity. We use a small-pixel detector to measure the smile in order to improve the accuracy. In fact, rotation of the detector with respect to the slit will affect the test results, for that it will cause the linear shift in wavelength at different spatial channels in a detector row. However, it should be clear that the detector-to-slit rotation is not an optical characteristic, and it is caused by the installation of the detector. Therefore, when we test the spectrometer's smile with a detector that is not used for practical work, we need to exclude the detector rotation in the test process. Figure 17e shows the scatter diagram of 479.9 nm spectral line. It is obviously not parallel to the spatial coordinate axis, and has a tilt of about 2.5 pixels (pixel size 5.5 \(\upmu\)m) in the spectral direction. This slope is caused by the incomplete alignment of the spectral lines and the detector array during the test. That means the detector-to-slit rotation and the slope cannot be directly accounted for as the spectrometer's smile. By fitting a spectral line to a straight line and a curve, as shown in Figure 17e, the smile distortion can be calculated as the peak-valley value of the distance between the fitting curve and the fitting straight line. The smile at 479.9 nm is the largest, accounting for 1.6% of a pixel (pixel size 15 \(\upmu\)m). This value is the test result after excluding the detector-to-slit rotation. It  
Figure 17: **(a,b)** Test device and **(c–e)** test results from the B1 spectrometer. **(c)** Spectral lines of the Hg–Cd lamp, **(d)** SRFs from the 479.9 nm spectral line; the \(SRF_{opt}\) is fitted from raw data, and the \(SRF_{ins}\) is the SRF after 6\(\times\) pixels binning. **(e)** Scatter diagram of the 479.9 nm spectral line and its fitting curve.