# The Reproduction and Evaluation of Star Fields with the Milky Way in a Planetarium
## 1 Introduction  
Ever since the development of lighting technology, aimed at achieving a comfortable living environment at night, a sky full of stars has become a precious and rare sigh. Bright nights cause light pollution [1; 2], limiting the visible stars in the night sky to bright stars such as stars with small apparent magnitude. In this study, we focus on the Milky Way. The Milky Way is a mass of innumerable stars. The light we receive from most stars is weaker than that of the bright stars. However, this light can still be seen as a thin cloud in the night sky because the light of all the stars is combined. In particular, the stars in the area around the Scorpius and Sagittarius constellations are more evident than those in other areas. Therefore, under suitable conditions (a very dark night without light pollution and moonlight, no clouds, etc. ), it is easy to see the Milky Way in the summer season. However, in urban areas, the opportunity to observe the Milky Way is rare because of the bright night sky. To address this issue, planetariums worldwide have been working to artificially reproduce a starry sky using image reproduction systems [3]. In fields such as astronomy education and entertainment, planetariums have played a crucial role in communicating the majesty of the universe to the general public. Some studies have reported methods for observing and acquiring starlit sky images [4]. There have also been reports on the use of computer-graphics (CG) reproduction methods to generate displays [5; 6]. However, there has been insufficient discussion of the appropriate methods for reproducing astral images and the use of such methods in a planetarium that seeks to reproduce the starry sky. Among those that do address this subject, previous studies have investigated the relationship between the physical representation factorsin a planetarium as well as the perceptual assessment of faithfulness and preference for the represented stars by conducting psychometric experiments in a real planetarium dome [7,8]. However, the target stars in previous studies were restricted to bright stars of small magnitude (from zero to seven), without considering fainter stars and objects (such as the Milky Way and nebulas), to investigate the effects of color, brightness, and size reproduction. In psychometric experiments [7,8], bright target stars were projected in a completely planar dark sky, which is an unlikely condition in the actual star field. The star field we observe in reality has nonuniform luminance owing to the presence of fainter stars, clouds, zodiacal light, and light pollution. In the planetarium industry, it is well known that adding natural light is essential for faithfully reproducing a star field that conveys a sense of the depth of the universe; however, the best reproduction methods have yet to be determined. Accurate reproduction of the Milky Way is particularly important because it has a fixed position in the sky and attracts attention as a beautiful element of the starry sky. From a scientific perspective, even though the individual stars of the Milky Way are dim, they can be perceived as a \"visible mass\", forming a \"dense cluster\". Although several studies have reported contrast perception [9] and density perception in relation to texture [10], we found no relevant studies regarding the perception of a group of illuminated point lights in scotopic vision. In a planetarium, people observe projected point light sources on a dome screen that mimics a star in scotopic vision. Because the physically same reproduction of stars is impossible, it is important to perceptually reproduce a starfield properly and not physically. In addition, there is no research on the best method to faithfully reproduce the Milky Way. Therefore, how to mimic the star field and represent a starry sky more faithfully, including fainter stars in the Milky Way, is a prevailing problem in planetarium research.