Preamble

Astronomical Techniques and Instruments, Vol. 2, September 2025, 319–326 Article Open Access Stray light analysis for the AIMS Telescope Biyuan Gao , Dongguang Wang , Yingzi Sun , Yuliang Shen , Xiao Yang Junfeng Hou 1 State Key Laboratory of Solar Activity and Space Weather National Astronomical Observatories Chinese Academy of Sciences Beijing 100101, China *Correspondence:

INTRODUCTION

Stray light is unintended light propagation within an optical system that deviates from the nominal imaging path. These aberrant rays reach the detector through mecha- nisms such as scattering and reflection, generating para- sitic background noise independent of target signals . In astronomical observations, stray light significantly degrades image contrast and signal-to-noise ratio of observa- tional data. As the most luminous celestial body in the solar system, radiation from the Sun has an intensity orders of magnitude greater than other stars. For ground- based telescopes, the complexity of stray light becomes even more pronounced, directly impacting observational reli- ability and impairs the achievement of scientific objec- tives Stray light sources are classified into two categories: exogenous and endogenous. Exogenous sources primarily include atmospheric scattering and terrestrial environmen- tal reflections. During daytime observations, atmospheric scattering of sunlight produces intense background radia- tion from the sky. Endogenous sources originate from non-ideal characteristics of the optical system in a tele- scope, including surface micro-roughness of optical ele- ments, thermal emission from structural components, and ghost reflections caused baffles optical supports . In solar telescopes, high-energy-density solar irradiance accelerates the thermal deformation of optical ele- ments and the degradation of coatings, thereby exacerbat- ing stray light effects.

The AIMS telescope is installed at Lenghu in Qing- hai Province of China, at an altitude of 4 000 m above sea level . This solar telescope carries out solar spec- tral-polarization observations wavelength 12.32 μm, and imaging observations within the 8–10 μm band. Because of the overall stray light challenges, the AIMS telescope adopts an off-axis design to effectively mit- igate the influence of the intrinsic infrared radiation from the telescope on solar magnetic field measurements The impact of stray light on the AIMS telescope mani- fests in three dimensions: Spatially, it blurs fine solar fea- tures such as granulation boundaries and sunspot penum- bral fibrils, resulting in resolution degradation; spectrally, it contaminates specific emission lines, obscuring weak emission signals and distorting spectral line profiles and in magnetic field measurements, stray light-induced 2 School of Astronomy and Space Science University of Chinese Academy of Sciences Beijing 100049, China © 2025 Editorial Office of Astronomical Techniques and Instruments, Yunnan Observatories, Chinese Academy of Sciences. This is an open access article under the CC BY 4.0 license ( Citation: Gao, B. Y., Wang, D. G., Sun, Y. Z., et al. 2025. Stray light analysis for the AIMS Telescope.

Astronomical Techniques and Instruments (5): 319−326.

Abstract

Stray light has a significant effect on the overall performance of telescopes, particularly for infrared solar telescopes, in which internal thermal radiation causes additional stray light sources. We establish a 3-dimensional model using stray light theory and the real opto-mechanical structure of the Accurate Infrared Magnetic field measurements of the Sun (AIMS) telescope. We use the stray light ratio (i.e., the ratio of stray light energy reaching the detector to the light energy used for observations) to evaluate the imaging performance of the telescope. We analyze both thermal and non-thermal sources of stray light to comprehensively study the visible, 8–10 μm, and Fourier transform infrared systems of the telescope, finding stray light ratios of in the visible system, in the 8–10 μm system, and in the Fourier transform infrared system. This verifies that the opto-mechanical system design of the telescope can effectively suppress stray light, benefiting both imaging and magnetic field observations.

Keywords

Stray light analysis; Non-thermal radiation; Thermal radiation; Stray light ratio

polarization signal contamination substantially reduces the accuracy of vector magnetograms.

In this study, we perform a stray light analysis on the AIMS telescope. This paper is structured as follows: In Sec- tion 2, we introduce stray light theory and define the evalu- ation function of the system. Building on this, we con- struct a stray light model for the overall opto-mechanical system of the AIMS telescope in Section 3, and in Sec- tion 4, we carry out an analysis of the impacts of ther- mal and non-thermal radiation on the AIMS telescope.

Finally, our conclusions are presented in Section 5.

STRAY LIGHT THEORY For the stray light analysis, we implement the Monte Carlo method using the Advanced Systems Analysis Pro- gram (ASAP) software . Light rays are emitted from a source and pass through a three-dimensional model of the optical and structural components of the telescope. Every time a light ray intersects an object, additional scattered light rays are generated. The intensity of these scattered light rays is determined by weighting them proportion- ally to the intensity of the incident light rays and the Bidi- rectional Scattering Distribution Function (BSDF) of the surface. The scattered light rays are then collected by the detector to calculate the irradiance distribution.

Surface Properties The system has an open-structure design, with negligi- ble stray light generated by secondary scattering surfaces, so this study focuses exclusively on the contributions of pri- mary scattering. To ensure accurate simulation results, it is essential to precisely set the optical property parame- ters of the system components before conducting soft- ware simulations. When the root-mean-square (RMS) rough- ness of a mirror surface is much smaller than the wave- length of the incident light, the mirror scattering can be described by the modified third-order Harvey scattering model, as  

BSDF ( θ, θ 0 ) = b 0

where is the angle of incidence (the angle between the incident light ray and the normal of the mirror surface); is the scattering angle between the scattered light ray and the normal of the mirror surface; is the normalization coefficient (related to the surface roughness and the wave- length); is the characteristic length parameter, giving the spatial frequency characteristics of the surface rough- ness; and is the attenuation exponent, controlling the broadening degree of the scattering distribution (directly proportional to the concentration of scattering in the specu- lar direction) The Harvey model is configured for optical surfaces and high-reflectivity mechanical surfaces, in accordance with characteristics such as the surface reflectivity and

material properties of the mirror. Remaining surfaces are set as “black paint surfaces” to maximize the absorption of stray light. Here, we employ a Bidirectional Reflectance Distribution Function (BRDF) model, and its scattering model can be expressed as

where and are the outgoing directions of surface scat- tering and surface specular reflection, respectively, while , , and are fitting parameters Radiation Theory For thermal radiation, the focus is on the distribution of radiant energy from the source and the radiance reach- ing the detector. The radiance of a light source, , is given by

L = d 2 Φ d A cos θ d Ω , (3)

where represents the differential power emitted by the differential projected area of the light source into the differential solid angle . The units are ph s –1

, or in photometric units as cd m . The Planck blackbody equation can be used to calculate the spectral radiance of an extended source as

L ( λ, T ) = C 1

where , and is the temperature of the source in Kelvins After the extended source passes through the system, the irradiance at the position of the detector is given by

i = 1 L i ε i Ω i τ i , (5)

E SL =

where are the in-band blackbody radiance and emissivity of the -th surface, respectively; is the solid angle corresponding to the -th surface; and is the corre- sponding transmittance.

Evaluation Function To describe the stray light from thermal and non-ther- mal radiation and thereby evaluate the imaging perfor- mance of the system, we define the metric of Stray Light Ratio (SLR) as the ratio of stray light energy reaching the detector ( ) to the light energy reaching the detector required for observation ( ), as stray actual

SLR = E stray E actual . (6)

STRAY LIGHT MODELS OF THE AIMS TELESCOPE To establish the stray light model of the AIMS tele- scope, it is necessary to first determine its optical and mechanical structures, define surface scattering characteris- tics, and clarify the characteristics of internal and exter- nal light sources. For this, we use a Monte Carlo ray-trac- ing algorithm to construct a 3D model with the ASAP soft- ware, to simulate the propagation of light, and subse- quently analyze it.

The AIMS Telescope The AIMS telescope is a 1-m off-axis solar telescope.

Telescope system Folding axis system De-rotator system Collimation system Focal plane system As shown in , the AIMS telescope adopts an open truss structure with an alt-azimuth mount. A heat stop with a field of 6.4′ is integrated at the focal plane of the primary mirror. This configuration serves the twin functions of mitigating the thermal impact of out-of-field solar radiation on downstream optical paths, and reduc- ing the aperture requirements for subsequent optical compo- nents. The heat stop redirects extraneous sunlight outside the effective field of view, away from the main optical axis, while incorporating active cooling to suppress ther- mal noise. The detailed mechanical design of the heat stop is shown in To mitigate the impact of thermal radiation, the two infrared systems are cooled separately. The 8–10 μm sys- tem is cooled with an operating temperature of 100 K.

The FTIR system is locally cooled, maintaining the imag- The optical system of the AIMS telescope, shown in , consists of five distinct subsystems: the telescope system, the folding axis system, the de-rotator system, the collimation system, and the focal plane system. The focal plane system is further subdivided into five components.

In our stray light analysis, as presented in only the 550 nm visible system, the 8–10 µm infrared sys- tem, and the imaging optical path parts of the Fourier trans- form infrared (FTIR) spectrometer are taken into account.

These selected components play crucial roles in understand- ing and managing the stray light issues within the AIMS telescope, the system parameters of which are shown in Visible imaging system ing mirror at 80 K, the aperture stop at 62 K, and the field stop at 80 K.

AIMS 3D Model The 3D model of the AIMS telescope includes the dome and the opto-mechanical system, the latter of which is extremely complex. Many elements within it, such as minuscule components and intricate geometries, are not only time-consuming but also unnecessary to model for an effective analysis. Consequently, a simplified ASAP model is shown in composed of the dome, the opti- cal system, the mechanical structure (including the aper- ture, truss, pier, and mirror mount), and the detector. Due to the highly complex structure of the FTIR spectrometer, during the analysis, only the guiding optics and imaging sections within its optical system are taken into account.

The visible system 550 nm The 8–10 μm system 8–10 μm The FTIR system 12–12.5 μm

Inner outlet cooling and outer inlet cooling hoses Inner inlet cooling hoses

Adjustment mechanism Field stop

The stray light sources affecting the AIMS telescope include sunlight, sky background, and internal thermal radia- tion. We focus on thermal and non-thermal radiation. 2.684 m Ray Tracing With the AIMS telescope model established, we use ray tracing to determine the critical surfaces and illumi- nated surfaces. First-order stray light paths originate from surfaces that are both illuminated and critical . Identify- ing such surfaces is important. The critical surface, defined as the surface detectable by the detector, is obtained through reverse ray tracing. In the 3D model of the AIMS telescope, an extended light source is posi- tioned at the detector location. The reverse ray tracing parameters are approximately one hundred thousand rays with a light threshold of . The surfaces receiving the light flux are identified as the critical surfaces. The illumi- nated surface is obtained using forward ray tracing. A light source is placed at the front of the telescope, and approximately ten thousand rays are traced with a light Reflector Absorber Outer outlet cooling hoses threshold of . Multiple ray tracings are carried out, with each tracing corresponding to a different angle of the light source. The surface that receives the light flux is iden- tified as the illuminated surface. demonstrates the processes of forward and reverse ray tracing, and lists the illuminated surfaces and critical surfaces obtained.

STRAY LIGHT ANALYSIS Here, we analyze thermal and non-thermal radiation, where the thermal radiation consists of both external and internal thermal sources. External thermal radiation primar- ily comes from the Sun outside the field of view of the optical system, and internal thermal radiation comes from the telescope itself. It is worth noting that for the visible system, we only analyze only non-thermal radiation, while for the 8–10 μm and FTIR systems, both thermal and non-thermal radiation are taken into account. thermal Radiation According to the system model and parameter set- tings established in Section 3, following the ray tracing, the results of non-thermal radiation analyses for the visi- ble system, 8–10 μm system, and FTIR system are pre- sented in . Here, the analysis was extended up to 16'; the curve gradually levels off, so only data up to 6' is shown. presents the mean values of SLR both within and outside the fields of view of the three systems.

The SLR of the infrared system in our experiment is one order of magnitude higher than that of the visible sys- tem, which may be attributed to the coatings of the infrared optical components. When reflectivity and transmit- tance of the coatings in the infrared band are consistent with that in the visible light band, as shown by the dot- ted line in . After modifying the parameters of the coatings, the SLR decreased by 80% compared to the origi- nal value.

The detailed structure is shown in (B), that is, the detailed diagram showing the structure of the heat stop.

Forward ray tracing Reverse ray tracing Illuminated surfaces Critical surfaces 8–10 imaging lens barrel structure 8–10 imaging lens L4 8–10 dewar structure 8–10 lens barrel Cold stop Cold stop FTIR Spectrometer mounting structure FTIR L4 EDGE Heat stop FTIR sleeve and support bracket M7, 8, 9 and other multiple frame and support structures M1 etc.

Near mounting structure M1 frame Multiple optical surfaces M2 mounting structure and support Beam splitter frame and support structure M7, 8, 9, 10, 11 and other multiple frame and support structures Platform and support structure Platform Upper truss and dome of the platform Detector sleeve Decentering lens barrel structure, end face of intermediate focus sleeve End face of intermediate focus sleeve

8−10 FTIR Visible 8−10-simulation

Incident angle/(') for the 8–10 μm system, the FTIR spectrometer system, and the visible system, respectively. The black dotted line shows the situation after the parameters of the 8–10 μm system have been modified.

The external thermal radiation mainly originates from the Sun outside the field of view of the optical system. In our model, the temperature of the Sun is set at 5 778 K.

A telescope with a 1-m aperture receives 0.95 W of solar radiation in the 8–10 μm wavelength band and 0.006

77 W

in 12–12.5 μm wavelength band. shows the SLR of external thermal radiation varying with the incidence angle. The external thermal radiation remains effectively constant. The SLR of the 8–10 μm system is approxi- mately , and that of the FTIR spectrometer sys- tem is approximately Internal Thermal Radiation Internal thermal radiation is the thermal radiation emit- ted by the telescope itself, including its optical and mechan- ical components. The impacts of ambient temperature and material emissivity variations are also considered. In our telescope model (as described in Section 3), we place an extended light source at the detector, and obtain the lumi- nous flux on each critical surface by reverse tracing the light rays from this source. Using the formulae defined in Section 2.2, we calculate the irradiance at the detector posi- tion, with each critical surface serving as an extended

System Incidence angle/(') light source. presents the critical surfaces of the 8– 10 μm and FTIR spectrometer systems, and the irradi- ance of the stray light produced on these surfaces.

In the internal thermal radiation model, the critical sur- faces are defined as heat sources. We calculate their energy as blackbody radiation and incorporate these val- ues into the model, to determine the total energy reach- ing the detector. The total thermal radiation amounts detected by the 8–10 μm and FTIR systems under both cooled and uncooled conditions are shown in together with the SLR values of these two systems.

For the 8–10 μm system, cooling the system to a tem- perature of 100 K causes the internal thermal radiation to reduce by 2.3 orders of magnitude. For the FTIR spectrome- ter system, when the imaging mirror was cooled to 80 K, the aperture stop was cooled to 62 K, and the field stop was cooled to 80 K, causing its internal thermal radiation to also reduce by 2.5 orders of magnitude.

Projected solid angle* /(sr)

after the derotator are all located in the Coudé room, with relatively constant temperature, and the 8–10 μm and FTIR systems are both refrigerated. Consequently, the impact of ambient temperature is extremely small.

In summary, considering the non-thermal radiation, the external thermal radiation and the internal thermal radia- tion yields an SLR of in the visible system, in the 8–10 μm system, and the FTIR system.

Emissivity Temperature/K 300 K. Projected solid angle* /(sr) In this study, we systematically analyze the stray light in the AIMS telescope, focusing on the visible sys- tem, 8–10 μm infrared system, and FTIR spectrometer sys- tem. By establishing a 3D model with the ASAP software, using the Monte Carlo ray-tracing method, we evaluate the SLR considering both thermal and non-thermal radiation.

For the infrared system, with the same coating reflectiv- ity and transmittance as the visible system, the SLR is lower by 80%, meaning that the coating is very impor- tant in an infrared system to suppress non-thermal stray light. For the 8–10 μm system, the internal thermal radia- tion is reduced by 2.3 orders of magnitude when the sys- tem is cooled to 100 K. For the FTIR system, when the imaging mirror, the aperture stop, and field stop are cooled to 80 K, 62 K, and 80 K, respectively, the inter- nal thermal radiation reduces by 2.5 orders of magnitude.

Additionally, the influence of the ambient temperature on the stray light is limited. The SLR is 9.31 × 10 for the visible system, 2.83 × 10 for the 8–10 μm system, and 1.54 × 10 for the FTIR spectrometer system. Our results show that the opto-mechanical system design of the AIMS telescope can effectively suppress stray light, which is beneficial for both imaging and magnetic field observations.

Uncooled Cooled Uncooled Cooled

ACKNOWLEDGEMENTS This research was supported by National Natural Sci- ence Foundation of China (12473086 and 11427901), National Key Research and Development Program of China (2021YFA1600500), and Youth Innovation Promo- tion Association of the Chinese Academy of Sciences (2022057).

AI DISCLOSURE STATEMENT The authors utilized AI tools for both literature research and English writing/ revision, as detailed below:

Literature review: The AI tools Kimi and Deepseek wereused to assist in retrieving, organizing, and analyz- ing relevant scientific literature.

English language support: Deepseek was employed for language and grammar checks within the article. The authors carefully reviewed, edited, and revised the Deep- seek-generated texts to their own preferences, assuming ulti- mate responsibility for the content of the publication.

The AI tools functioned as auxiliary resources, and the authors retain full responsibility for all content, data, and conclusions presented in the manuscript.

AUTHOR CONTRIBUTIONS Biyuan Gao was responsible for conducting the research and writing the paper. Dongguang Wang and Yingzi Sun provided professional guidance throughout the research process. Junfeng Hou contributed the core ideas for the study. Yuliang Shen offered crucial technical sup- port in the field of optics with his expertise. Xiao Yang pro- vided guidance on paper writing. All authors read and approved the final manuscript.

DECLARATION OF INTERESTS The authors declare no competing interests.

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