太 阳 短 波 极 紫 外 双 波 段 成 像 光 谱 仪 设 计

Objective Spectroscopic imaging observations in the extreme ultraviolet (EUV) short- wavelength range (10 - 40 nm) provide rich information about eruptive solar activities in the upper solar atmosphere. Meanwhile, they encompass emission spectral lines from multi-charged ions (e. g., high-charge iron ions, helium ions, and magnesium ions) with electron excitation temperatures ranging from 10⁴-10⁷ K. Such observations play a crucial role in diagnosing plasma temperature, density, and velocity in the solar corona, making the He II 30.4 nm emission spectral line particularly significant for diagnosing small-scale solar eruptive events and conducting global helium abundance measurements. However, regardless of whether launched in the past or currently in orbit, existing imaging spectrometers worldwide face limitations in performing high-spatial and high-spectral resolution diagnostic observations in the EUV short-wavelength range encompassing the He II line. Therefore, we propose and design a solar EUV imaging spectrometer capable of simultaneously operating in the 17-21 nm and 28-32 nm wavebands, which features a large off-axis slit for wide field-ofview (FOV) imaging. The instrument utilizes a non-Roland grating structure and a toroidal varied-line-space (TVLS) grating design, enabling simultaneous acquisition of high-spatial and high-spectral resolution, and large instantaneous slit FOV imaging without the need for spectral scanning or detector displacement. Methods The slit scanning solar EUV imaging spectrometer utilizes a narrow slit to restrict the FOV and employs a combination of slit scanning, concave grating, and a two-dimensional flat field detector to achieve high spatial and spectral resolution imaging over a two-dimensional area. Solar EUV radiation passes through a preceding off-axis telescope primary mirror, forming a real image at the telescope focal plane. A narrow slit positioned at the telescope focal plane captures a portion of the image by the instantaneous FOV (IFOV). The light passing through the slit undergoes TVLS grating dispersion and is ultimately directed to two detectors, corresponding to the two wavebands of interest. The TVLS grating as the core dispersive element is analyzed for aberrations in our study. By considering the TVLS grating’s toroidal base parameters, grating groove density function, and imaging structure parameters along with the instrument’s optical path function, we perform aberration analysis. Based on the desired properties of anti-dispersion and off-axis aberration correction, we derive constraints for optimizing the TVLS grating parameters. Subsequently, we employ system resolution as an optimization constraint to further refine the instrument’s design, obtaining the initial structural parameters of the imaging spectrometer. To achieve optimal system performance, we utilize ZEMAX software for optimization via the initial parameters and aberration optimization functions. Meanwhile, the narrow slit imaging of different spectral lines in the target wavebands by non-sequential ray tracing is simulated to validate the spectral imaging performance of the designed system. Results and Discussions The final optimized optical layout of our solar short EUV dual-waveband imaging spectrometer is shown in Fig. 4. It operates in the wavelength range of 17-21 nm and 28-32 nm, employing e2v CCD detectors with a pixel size of 16 µm. The entire instrument’s optical volume measures 2000 mm×280 mm×115 mm. For different spatial scales of solar eruptive targets, three slit widths of 1″, 2″, and 20″are available. By performing stepwise rotation of the primary mirror, high-resolution spectral imaging of a 10′×12′two-dimensional solar disc can be realized. The instrument exhibits excellent imaging performance, with the root mean square (RMS) spot size in both spatial and spectral directions being less than 6 µm in the 17 - 21 nm and 28 - 32 nm bands (Fig. 6). The RMS spot size changes smoothly with wavelength and gradually increases with larger off-axis FOV (0-6′). At the Nyquist spatial frequency (31.25 lp/mm), the modulation transfer function (MTF) values for both meridional and sagittal directions at the four edge wavelengths (17, 21, 28, 32 nm) are all greater than 0.5 (Fig. 7). The encircled energy within a single pixel for the four edge wavelengths is 90.4%, 91.6%, 88.8%, and 81.0% respectively (Fig. 8), all exceeding 80%. In the non-sequential mode, the slit image lengths for different spectral lines in the two bands are 23.28 mm, which is in close agreement with the theoretical value (23.04 mm), with all exhibiting clear peaks (Fig. 9). These results demonstrate that the instrument possesses excellent spectral imaging performance, with spatial resolution better than 1″and spectral resolution better than 0.0055 nm. The instrument has flexible tolerance capabilities. By adopting the given tolerance values (Table 5), we perform a Monte Carlo analysis in ZEMAX software using sensitivity mode for wavebands at 19 nm and 30 nm. The results indicate that the most significant effects on spectral imaging performance are exerted by tilt tolerances of the primary mirror elements, the secondary mirror’s quadratic coefficients, and the grating’s element tilts. The RMS spot size at the image plane changes within 0.5 pixel size with a probability of 98%. Under these tolerance limits, the image quality degradation of the imaging spectrometer remains within a controllable range. Conclusions We propose and design a high-resolution spectroscopic imaging architecture capable of simultaneous operation in the 17-21 nm and 28-32 nm wavelength ranges. The instrument employs a TVLS grating as the dispersive element. By analyzing the TVLS grating aberrations under the non-Roland structure using the optical path function and Fermat’s principle, correction conditions for off-axis grating aberrations and anti-dispersion spectroscopic imaging are derived. Ray-tracing simulation results demonstrate that the off-axis grating aberrations and image dispersion of the imaging spectrometer are well corrected for enabling the system to yield spectral imaging performance close to the diffraction limit. The system exhibits a spatial resolution of better than 1″and a spectral resolution of 0.0055 nm. Sensitivity-based tolerance analysis indicates that the designed solar EUV imaging spectrometer possesses flexible tolerance capabilities. The advanced design of the proposed spectrometer provides a theoretical basis for achieving high spatial resolution, high spectral resolution, and wide temperature diagnostics for solar coronal eruptive activities within a two-dimensional solar disc FOV. Additionally, it holds theoretical and practical significance for the development and construction of future EUV spectroscopic instruments in China.