The ultraviolet (UV) transmittance of the entire atmosphere is a key parameter for understanding the transmission of UV radiation through the atmosphere. This parameter is crucial for advancing ground-based UV astronomical observations and developing accurate atmospheric transmission models. In astronomy, stellar UV radiation serves as an essential source of information. By studying UV emissions, we can not only gain insights into evolutionary processes of stars, but also obtain critical data on chemical abundances of the universe and the elemental evolution of stars. Currently, stellar UV radiation is primarily observed from space. However, space-based observations are costly and pose significant technical challenges. Therefore, exploring the feasibility of ground-based UV observations is necessary. Due to atmospheric absorption and scattering, only a limited portion of the UV spectrum (280?400 nm) can be observed from the ground. Nevertheless, this accessible wavelength range still provides valuable astronomical information, making ground-based UV observations an important avenue for further research. A fundamental requirement for conducting ground-based celestial UV observations is the identification of an observatory site with optimal conditions for detecting UV radiation. Once such a site is selected, large-scale UV observation equipment can be deployed to measure stellar UV emissions. Typically, an ideal observatory site should exhibit high atmospheric UV transmittance, which necessitates precise measurements of UV radiation reaching the ground. These measurements are essential for evaluating the suitability of a site for UV observations. Investigations reveal that most ground-based UV radiation measurements, both domestically and internationally, focus primarily on daytime solar observations. However, these measurements are generally limited to the Sun as the sole target. There is currently a lack of equipment specifically designed for observing the UV radiation of stars at night. However, nighttime measurements offer significant advantages, including access to a greater number of observation targets and the ability to survey multiple regions of the sky. These measurements are crucial for constructing atmospheric transmission models and advancing astronomical observations. Therefore, developing an instrument capable of accurately measuring stellar UV radiation at night is essential for determining the atmospheric UV transmittance under nighttime conditions.

Due to atmospheric absorption and scattering, the UV radiation reaching the ground is relatively weak. To address this challenge, we develop a specialized instrument for measuring faint UV radiation at night. First, considering characteristics of stellar UV radiation that reaches the ground, a photomultiplier tube (PMT) is employed for detection, while guide star technology is integrated to enable long-term tracking of the instrument. Since PMT cannot be used for imaging, and guide star technology requires target star imaging to correct tracking errors, we propose a novel stellar radiometer design. The proposed radiometer employs a dichroic mirror for spectral separation and is divided into two channels: an imaging channel and a detection channel. The imaging channel captures the star’s visible light, facilitating target acquisition and tracking through guide star technology, thereby compensating for the limitations of PMT. The detection channel focuses the UV radiation onto the cathode surface of the PMT while ensuring that the exit pupil of the optical system is well-matched to the PMT cathode. This design minimizes measurement errors introduced during the guiding process and enables precise UV radiation measurements. Additionally, a field diaphragm is positioned at the primary image plane to adjust the field of view and reduce the influence of sky background noise. In the instrument design process, spectral characteristics of the dichroic mirror are analyzed, confirming the feasibility of the proposed system. Based on the available atmospheric UV transmission window, stellar UV radiation will be measured in the 288–400 nm range, divided into sub-bands. By measuring stellar radiation at various zenith angles, the atmospheric UV transmittance for each band will be determined using the Beer-Lambert law and the Langley-plot calibration method.

Through analysis and calculation, we design a stellar UV radiometer suitable for nighttime observations (Fig.5). The root-mean-square (RMS) radius of the imaging channel within the field of view is smaller than the Airy disk (Fig.6), meeting the requirements for guide star tracking. The exit pupil diameter of the detection channel is 6 mm (Fig.7), which is smaller than the 8-mm cathode surface of the PMT, effectively minimizing the influence of the guide star and ensuring accurate UV radiation measurements. Following the development of the instrument, field tests are conducted (Fig.8). During testing, the field diaphragm has a radius of 0.25 mm, corresponding to a field of view of 28 arcsec. The stellar UV radiometer is used to observe the star HR153 by controlling the equatorial mount. Results demonstrate that the imaging channel successfully control the equatorial mount, enabling long-term star tracking with an accuracy of ±2″ (Fig.9). Subsequently, the measured data from the detection subchannels u1, u2, u3, and u4 are subjected to linear fitting (Fig.10), and Langley calibration is performed to derive the atmospheric UV transmittance in Changchun (Fig.11). A comparison between the measured data and software-simulated results shows strong agreement (Fig.12), confirming the reliability of the instrument.

In this study, we develop a device capable of measuring the UV radiation of stars at night, with design results meeting practical application requirements. After the development phase, field tests demonstrate that the device achieves its intended performance, with the star offset during tracking remaining within ±2″. Additionally, UV radiation in the 288?400 nm range is measured across different sub-bands, and the atmospheric UV transmittance for each channel is obtained. These results indicate that the device enables long-term tracking of target stars and high-sensitivity UV detection, confirming its reliability. Furthermore, the proposed portable stellar radiometer has the potential to aid in the selection of observatory sites. In future developments, a smaller field stop will be implemented to further reduce sky background interference and enhance the instrument’s limiting magnitude. Additionally, narrower bandwidth filters will be used to improve the precision of stellar UV radiation measurements. These advancements will contribute to the establishment of a domestic standard atmospheric UV model and support the development of ground-based UV astronomical observations.