Vixen's standard "photo-visual" SD apochromatic refractor with the focal length of 625mm. Vixen SD apochromatic refractors feature "Super extra-low Dispersion" optical glass in its objective lens. With the use of a SD reducer HD kit or SD Flattener HD kit, the image circle of 44 mm in diameter is obtained to be used for photography with DSLR cameras with full-frame imaging sensor. A lens of refracting telescopes has chromatic aberration which is caused by different wavelength of visible light (colors) focusing at different distances. The Vixen SD lenses, made of FPL53 super extra-low dispersion glass, allow for correcting the chromatic aberration over a wide spectrum of light. The SD lenses focus visible rays of light from the Cray(red), d-ray(yellow), e-ray(green), F-ray(blue) to g-ray(violet) at nearly the very same position. As shown in the diagrams of spherical aberration, it verifies that the chromatic aberration is highly corrected over a wide spectrum of light with the SD lenses. Especially the g-ray, which affects image contrast, is suppressed excellently. The SD81S delivers outstanding sharp images with high contrast for both visual and photographic applications. Brighter F7.7 images will satisfy the most demanding visual astronomer or astrophotographer