The solar upper atmosphere including the corona, transition region, and chromosphere is composed of hot and highly dynamic magnetized plasma, from which highly ionized ions emit abundant extreme ultraviolet (EUV) spectral lines. Existing EUV imaging spectrometers can only conduct imaging on one or two target regions of the solar upper atmosphere but cannot diagnose the whole region in a wide spectral and spatial scale using a single instrument. This severely restricts our understanding of the energy and material transport processes in solar eruptive activities. Therefore, we propose and design a solar EUV three-waveband imaging spectrometer with an elliptical varied line-space (EVLS) grating that operates at non-Rowland circle mounting. This innovative solar EUV imaging spectrometer boasts exceptional spectral imaging performance in an extremely large off-axis slit field of view (FOV) while maintaining a compact instrument package. Furthermore, it provides excellent grating aberration correction even at very high spectrograph magnifications and beam speeds. We hope that our spectral imaging strategy and instrument system design will be instrumental in the simultaneous observation of the solar corona, transition region, and chromosphere in the near future.

The instrument utilizes an EVLS grating as the diffraction spectroscopic element. To achieve simultaneous correction of aberrations and free-astigmatism in all three spectral bands, we analyze the grating for aberrations by employing the optical path function and Fermat's principle. The correction condition of off-axis aberrations for the grating is obtained by optimizing the elliptical base shape parameters, line-space parameters, and structure parameters of the EVLS grating, with the grating's spectral focusing formula and spatial focusing curve formula considered. The global optimal solution for the instrument is then obtained via the simulated annealing algorithm and computer-aided design method to build the optimal model of the solar EUV three-waveband imaging spectrometer. Finally, the Monte Carlo method is adopted to non-sequentially trace different spectral line pairs in the target spectral band to verify the spectral imaging performance of the designed system.

Figure 3 shows the final optimized optical system layout of the designed solar EUV three-waveband imaging spectrometer. The working wavelengths of 17-21 nm, 70-80 nm, and 95-105 nm are respectively utilized for observing the solar corona, transition region, and chromosphere. The detector for the 17-21 nm band adopts the charge-coupled device (CCD) structure of e2v technology with a pixel size of 13.5 μm, while the detectors for the 70-80 nm and 95-105 nm bands leverage an active pixel sensor (APS) structure with micro-channel plate (MCP) technology, and the pixel size is 20 μm. The entire instrument has an optical envelope volume of 1700 mm×370 mm×100 mm, and the slit has five different widths to adapt to different spatial and temporal scales of solar eruption activities. High-resolution spectral imaging of the two-dimensional solar disk with an FOV of 9.6′×5.0′ can be achieved by stepwise rotation of the off-axis primary mirror.

The instrument exhibits excellent imaging performance. The root mean square (RMS) radii at 17-21 nm are all less than 6 μm, while the RMS radii at 70-80 nm and 95-105 nm are mostly less than 10 μm. As the FOV increases, the radius of the diffractive spot RMS changes smoothly [Figs. 5(d)-5(f)], which indicates good correction of off-axis aberrations. At 19 nm, the modulation transfer function values at the Nyquist spatial frequency (37 lp/mm) are all greater than 0.6 [Fig. 6(a)], and the geometric encircled energy within a single pixel size (13.5 μm) is better than 82.5% [Fig. 7 (a)]. At 75 nm and 100 nm, the modulation transfer function values at the Nyquist spatial frequency (25 lp/mm) are both greater than 0.4 [Figs. 6(b)-6(c)], and the geometric encircled energy within a single pixel size is 80.5% and 85.9% [Figs. 7(b)-7(c)] for each. Generally, the spatial resolution of the system is better than 0.6″. The simulation results of non-sequential ray tracing show that the slit image length is 19.67 mm, which is consistent with the theoretically calculated value of 19.60 mm [Fig. 8 (b)]. The slit images of the spectral lines with their respective calculated ideal spectral resolution intervals at the three center wavelengths of 19, 75, and 100 nm are separated [Fig. 8 (a)]. Therefore, the spectral resolution of the imaging spectrometer system is better than 0.006 nm in the 17-21 nm band and better than 0.008 nm in the 70-80 nm and 95-105 nm bands.

We propose an innovative slit-scanning spectral imaging architecture that operates at 17-21 nm, 70-80 nm, and 95-105 nm. It can simultaneously diagnose and observe important plasma spectral lines in the solar corona, transition region, and chromosphere. Meanwhile, the theory of correcting the image aberrations caused by EVLS grating operating at non-Rowland circle mounting is studied. This structure can correct off-axis grating aberrations in a relatively compact design to achieve high-resolution spectroscopic imaging with broadband and large off-axis FOV. The ray tracing simulation experimental results reveal that the system's spatial resolution is better than 0.6″, and its spectral resolution is better than 0.006 nm at 17-21 nm, and better than 0.008 nm at 70-80 nm and 95-105 nm respectively. The advanced design research of this instrument has theoretical significance for the development and research of China's solar EUV imaging spectrometers in the near future and provides references for the model selection of China's future solar space exploration missions.