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The Small Telescopes Performance in the Visual to Near Infrared Wavelengths

Optical system is important and optimized for highly spatial resolution in certain wavelength bandwidth. We tested three small refractor telescopes with different aperture (two telescopes with 80 mm in diameter and one with 66 mm diameter) and focal-length (544 mm, 400 mm, and 389 mm, respectively) in order to know the resolution from visual to near infra red regions. The images of sinusoidal bar test chart were recorded from CCD detector. The reference filter of Sloan Digital Sky Survey (SDSS) is also attached in front of detector. The filters have the range of G (401-550 nm), R (555-695 nm), I (690-820 nm), Z (>820 nm), Z_s (826-920 nm), and Y (950-1058 nm). The last filter is referred to the limit of quantum efficiency of the detector. The maximum frequency for each pixel from each Modulation Transfer Function (MTF) was performed. We found that smaller diameter telescope is better resolution in the visual wavelength than the wider diameter and longer focal-length. In the near infra red region, the opposite results were obtained. The coating lens quality may affect the wavelength dependences. This study has advantages of selecting low cost and high resolution optical system for different applications, e.g. very young crescent moon observation, etc.