Solar activity is the source of space weather, and active bodies such as solar flares and coronal mass ejections are large-scale magnetic energy-releasing processes in the solar atmosphere. However, their energy accumulation and triggering are rooted in the chromosphere and photosphere and have smaller scales (Low, 2015; Wang et al., 2015; Choudhuri, 2017). Ground-based, large-aperture solar telescopes are still the best choice for studying the ultra-fine structure of solar active regions. Adaptive optical technology is also increasingly mature, which ensures the high-resolution observation of ground-based telescopes near the diffraction limit (Cao et al., 2010; Rimmele and Marino, 2011).

For solar observation, we not only need larger aperture telescopes to achieve higher spatial resolution and collect more photons, but we also need higher imaging contrast to achieve a higher detection signal-to-noise ratio and photometric accuracy. However, a prominent drawback of solar telescopes is the high scattered light, which makes the imaging contrast far from the requirements.

The large-aperture ground-based solar telescopes that have been built internationally, such as the 1 m Swedish 1-m Solar Telescope (SST) (Scharmer et al., 2002), 1.5 m GREGOR (Schmidt et al., 2012; Schmidt et al., 2016; Kleint et al., 2020), 1.6 m Goode Solar Telescope (GST) (Rimmele et al., 2004; Cao et al., 2010; Shumko et al., 2014; Schmidt et al., 2017), 1 m New Vacuum Solar Telescope (NVST) (Liu et al., 2014), 1.8 m Chinese Large Solar Telescope (CLST) (Rao et al., 2020), 4 m Daniel K. Inouye Solar Telescope (DKIST) (Rimmele et al., 2020; Rast et al., 2021), and 2.5 m wide-field and high-resolution solar telescope (WeHoST) (Fang et al., 2019) under construction, are equipped with solar adaptive optical systems between the telescope and scientific terminal to correct atmospheric turbulence and the tracking and wavefront errors of the telescope itself. The optical path of this structural form is very complex, and there are also many optical mirrors for light passing through (taking the 1-meter new vacuum solar telescope as an example, a total of 21 optical mirrors are employed from the telescope to the back-end scientific instrument through an adaptive optics system). As a result, the spatial resolution of the obtained images can approach the optical diffraction limit, but the image contrast is severely limited.

In order to achieve high-contrast imaging of solar targets, it is necessary to strictly control the stray light of the system, as well as the wavefront residual error (especially high-frequency error) and non-common path aberration of the system, to achieve a high Strehl ratio (SR) of imaging. This article proposes using the deformable secondary mirror as an adaptive optical correction device, tightly coupling the telescope and adaptive optics, which can greatly simplify the optical path and significantly reduce the number of optical mirrors in the system, thereby reducing both the effects of stray light and the high-frequency wavefront aberration of the system. It is the best choice to achieve high-contrast solar target imaging. At the same time, the new solar adaptive telescope system has very high optical efficiency, which can reduce the exposure integration time of the imaging camera (especially for magnetic field measurement and limited imaging of weak light, such as the narrowband of the chromosphere), greatly improving the temporal resolution of telescope imaging. Therefore, the large-aperture solar adaptive telescope using a deformable secondary mirror can achieve continuous observation of solar targets with high contrast, high spatial resolution, and high temporal resolution simultaneously.

As good night vision and imaging occur in the infrared band, the deformable secondary mirror in a night astronomical telescope has low requirements for driver density. Due to the relatively poor daytime vision and imaging in the visible band, the deformable mirror requires a much higher driver density. Most of the night astronomical deformable secondary mirrors using voice-coil motors (Wildi et al., 2003; Esposito et al., 2011; Briguglio et al., 2013; Morzinski et al., 2014; Briguglio et al., 2018) have the disadvantage of large actuator spacing, leading to complex thermal control, and are not suitable for daytime solar applications. The piezoelectric deformable secondary mirror that we proposed can achieve high density and has low thermal radiation, making it especially suitable for high-contrast solar imaging applications. This deformable mirror with high driver density has been successfully tested on a night astronomical telescope and achieved high-contrast imaging with SR close to 0.5 in the 0.64 um wavelength band (Guo et al., 2016a; Guo et al., 2016b; Guo et al., 2023). In the field of solar observation, the 4 m European Solar Telescope (EST) (Soltau et al., 2010) is considered to use the deformable secondary mirror in multi-conjugate adaptive optics (MCAO) to carry out ground-layer corrections.

The solar adaptive telescope we proposed adopts our self-developed small-spacing and high-density piezoelectric deformable secondary mirror (PDSM). The optical components of the telescope and the adaptive optical system only consist of one primary mirror and one secondary mirror, making the optical path extremely simplified. It is particularly suitable for high-order correction of the wavefront disturbance induced by atmospheric turbulence, and it is very conducive to high-contrast imaging. Moreover, the optical path of the magnetic field measurement channel is completely symmetrical, and this will not introduce additional calibrations for polarization instruments, which can effectively improve the accuracy of the polarization measurement.

In this paper, the preliminary design of PASAT is introduced in Section 2. In Section 3, the key components are described, including the deformable secondary mirror, correlating Hartmann–Shack (HS) wavefront sensor, high-resolution photometric magnetism measurement, and high-resolution imaging systems. In Section 4, the system’s performance is evaluated. Finally, we conclude the work in Section 5.

The optical configuration of PASAT, as shown in Figure 1, is composed of a 60 cm aperture primary mirror, a 163-element deformable secondary mirror, a correlating Hartmann–Shack wavefront sensor, a high-resolution photometric magnetism measurement at 617.3 nm, and two-channel high-resolution imaging systems at the Hα (656.28 nm) and TiO (705.8 nm) bands. The overall configuration of PASAT is shown in Figure 2. The vacuum tube for the primary optical system and the polar axis equatorial mount are used, and a thermal field stop and its active thermal control system are installed at the main focus F1 between the primary and secondary mirrors, which can reflect sunlight outside the imaging field of view from the optical system and effectively reduce the solar radiation power in the secondary mirror, relay optical components, and observation equipment. This technical solution has been successfully applied for the astronomical observation equipment of the Shanghai Astronomical Museum (Shanghai Science and Technology Museum Branch)—the Educational Adaptive Optics Solar Telescope (EAST) (Rao et al., 2022), which can ensure the stability of the optics of the instruments. The wavefront sensor and high-contrast imaging systems are fixed in the instrument box. The parameters of the primary mirror are clear aperture, 600 mm; vertex curvature radius, 1,800 mm; conic constant, K = 1.000; and central obscuration, 180 mm.

The main technical parameters of the PASAT are shown in Table 1.

The closed-loop residual variance of the AO system is mainly composed of the measurement error of the HS sensors and the fitting error of PDSM (Hardy and Thompson, 2000), and we estimated the system performance based on the parameters of these two parts. The Strehl ratio of the 617 nm imaging system with the shortest wavelength is also better than 0.68 when r0 is only 5 cm, which ensures the high-contrast imaging of the solar target by this instrument. Table 6 shows the analysis of the aberration correction results using the layout matching of the deformable secondary mirror under different atmospheric coherence length (r0) conditions.

For the optical imaging system, the contrast of the imaging results is closely related to the SR of the point spread function of the imaging system. The higher the SR of the imaging system is, the better the quality of the imaging results, the higher the contrast, and the clearer the discernible image details will be. Since image degradation is closely related to the residual aberration of the system, the simulation of system imaging is also carried out according to the real imaging process. In order to obtain the degenerative effects of different regions of a typical solar photosphere image with different correction capabilities, we selected a simulated solar image containing sunspot umbra, penumbra, and granulations, as shown in Figure 5. This image corresponds to the sunspot run presented in Rempel (2012), which was computed with the MURaM code (Vögler et al., 2005), and the simulation run is also publicly available (https://download.hao.ucar.edu/pub/rempel/sunspot_models/). The original simulated image has a total of 4,000 × 4,000 pixels, with a pixel resolution of 0.0166″, corresponding to a pixel size of 12 km. The simulated atmospheric turbulence distribution follows the Kolmogorov spectrum, where the atmospheric turbulence condition is D/r0 = 10, and in order to obtain the imaging results of different Strehl ratios, a variety of residual aberration conditions are obtained by setting different AO system correction capabilities. The residual aberration of the corrected system is used to calculate the corresponding PSF, and the degraded image is obtained by the convolution of the PSF and the original target image (Wöger, 2010). After that, the convolution of the point spread function and the diffraction-limited image of the 60 cm telescope were used to obtain the degraded image with different correction capabilities. The contrast of different regions of the degraded image was calculated to show the influence of the correction ability on the imaging system.

By simulating the situation of different atmospheric turbulence disturbances and different correction capabilities of the adaptive optics systems, the contrast of solar granulation and sunspots can be directly improved by enhancing the SR of the imaging system. Figure 6 shows the overall differential comparison of the different image qualities for the same solar target when the SR is gradually increased from 0.03 to 0.9. Figure 7 illustrates the differential comparison of the local details of solar granulation and sunspots under different SR conditions.

According to the simulation results, to obtain imaging results with high contrast and rich details, the solar telescope needs to be equipped with an advanced adaptive optics correction system. Only if the imaging system SR ≥ 0.5, the details between the solar granulation, especially between the sunspot internal, can be clearly identified. This also presents higher demands for the development of instrumental equipment. Achieving near-diffraction-limited imaging resolution in solar observation images while maintaining high SR at the same time is crucial for ensuring high measurement accuracy of the instrument, and this will provide more accurate measurement data for cutting-edge solar physics research.

Based on simulation experiments, the correction field of this instrument for high-order CAO is approximately 10″-20″ and that for large-filed GLAO imaging is around 2'. To achieve high-resolution imaging with an 8′full FoV, post-processing techniques for image reconstruction must be employed. On the other hand, one important scientific objective of this equipment is to obtain high photometric accuracy and high-resolution images, particularly for the post-processing of Fe I magnetic field channel measurement data. This not only requires reconstruction algorithms to effectively improve image resolution but also requires the physical reality and light intensity reality of the reconstructed images. Phase diversity (PD) image reconstruction is to simultaneously collect one or more sets of images of the same target through different optical channels and reconstruct a clear target based on the measurement of the wavefront aberration that causes image distortion (Gonsalves, 1982; Löfdahl and Scharmer, 1994; Thelen et al., 2009; Löfdahl and Hillberg, 2022). Therefore, the PD image reconstruction method is a type of image reconstruction based on optical measurement, and its algorithm principle can ensure high photometric accuracy of the reconstructed results. We use a camera exposure external trigger control circuit to simultaneously capture the focus and defocus images and reconstruct a clear image from a degraded target based on accurate wavefront detection. On this basis, we can ensure image restoration with a large FoV and high photometric accuracy. Further research and experimental verification will be conducted in the future.

This paper reports a novel solar adaptive telescope, PASAT, and the systematic design and performance evaluation are also presented. The PASAT is planned to be put into operation in 2026 and built at the Wumingshan Observation Station in Daocheng, Sichuan, China, which has good meteorological conditions. After years of site selection, the Daocheng Wuming Mountain at an altitude of 4,800 m was chosen because it has natural advantages such as low atmospheric scattered light, an average daytime r0 value of 7.65 cm, and approximately 250 available sunny days throughout the year (Song et al., 2020). Many telescopes, such as the 2.5 m WeHoST, are planned to be placed at this station. After the completion of PASAT, it will become the first high-resolution magnetic field measurement solar-adaptive telescope without calibration units in the world.

The biggest technological innovation of this instrument is that it uses the simplest optical path to obtain richer deep-level image information while ensuring a certain degree of high spatial resolution and pursuing the ultimate image signal–noise ratio (SNR) and photometric accuracy. It can provide high-quality data support for space weather forecasting and early warning applications.

More importantly, PASAT provides an exploration of a new large solar telescope with the capability of high-contrast imaging as the pathfinder. Its accumulated relevant techniques can be applied to the construction of similar, larger solar telescopes in the future.