We used a small F number in a design of lens unit (where F is the ratio of focal length to aperture) because an observed flux of diffuse light source is proportional to F~2. Moreover, it is preferable to cover the sky from the zenith to the horizon in the same frame in order to estimate the brightness of atmospheric diffuse light at the same time. The lens is developed by the Genesia Corporation (Figure 1). Its F value and a focal length / are 2.8 and 32.5 mm respectively. With the EEV CCD 42-80, it gives a FOV of 92°x46°. The optical filter is designed to avoid the prominent airglow emissions (see Figure 2). Both of the optical filter and shutter are set at pupil position. Due to an oblique path from off-optical axis, the wavelength of light transmitted through the interference filter is shifted toward a shorter wavelength (e.g. roughly 10 nm for the 50 degree field position). To avoid this effect, the filter is made of colored glass. The whole lens system is mounted on the thick honeycomb plate and supported by the shutter and filter box. The dewar is also mounted on the plate (see Figure 3). This honeycomb plate is fixed on an equatorial mount, which is useful to track the stars in the region between 48° N and 48° S terrestrial latitude.
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