Systematic Evaluation of Thermal Seeing in AIMS Telescope: From Primary Mirror to Coude Path

The Accurate Infrared Magnetic Field Measurement of the Sun (

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telescope is a 1-meter off-axis Gregorian solar telescope designed for mid-infrared (12.3 μm) spectro-polarimetric observations. This paper presents a systematic, quantitative evaluation of three distinct thermal seeing sources in

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using a unified optical path difference (OPD) accumulation framework based on computational fluid dynamics (CFD) simulations: (1) the primary mirror, (2) the heat-stop at the primary focus, and (3) the vertical downlink path from mirror M6 to the Coudé laboratory, where buoyancy-driven "chimney effect" airflow develops. For the primary mirror, under the actual site conditions at Lenghu (average wind speed of 3.5 m/s), forced convection dominates. With a mirror-air temperature difference of approximately 4.5 K, the mirror seeing FWHM is estimated to be approximately 0.22 arcsec, with a variation range of 0.19–0.26 arcsec considering the possible temperature fluctuation. This contribution is more than one order of magnitude smaller than the median atmospheric seeing (3.74 arcsec at 632.8 nm) and is negligible compared with the diffraction limit at the operating wavelength.Under the current site conditions and observing requirements, the benefit of implementing active temperature control for the primary mirror is expected to be limited. For the heat-stop, maintaining a temperature difference ΔT≤5K relative to ambient keeps the full-optical-path RMS wavefront error below 25 nm, corresponding to a visible-band FWHM of 0.043 arcsec. The off-axis design of

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provides a significant advantage by allowing the beam to pass through the thermal plume only once. The chimney effect in the vertical Coude path is the dominant thermal seeing source. Seasonal variation degrades visible-band FWHM from 0.068 arcsec (summer, no heating) to 2.610 arcsec (winter, no heating). A counterintuitive non-monotonic behavior is discovered: in winter, moderate motor heating (100 W) slightly improves seeing to 2.333 arcsec, while any heating in summer degrades image quality. This suggests an optimal operating point exists for winter conditions where heating partially compensates for ambient temperature gradients. These results provide quantitative design guidelines for next-generation solar telescopes: primary mirror active cooling may be unnecessary for sites with moderate wind speeds; heat-stop temperature control should be adapted to specific operating conditions; and long vertical optical paths require careful thermal design with active compensation strategies for seasonal temperature variations.