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| The `speed’ of a telescope’s optics, found by dividing the focal length by the aperture. The smaller the f/number, the lower the magnification, the wider the field, and the brighter the image with any given eyepiece or camera. Fast f/4 to f/5 focal ratios are generally best for wide field observing and deep space photography. Slow f/11 to f/15 focal ratios are usually better suited to lunar, planetary, and binary star observing and high power photography. Medium f/6 to f/10 focal ratios work well with either. An f/5 system can photograph a nebula or other faint extended deep space object in one-fourth the time of an f/10 system, but the image will be only one-half as large. Point sources, such as stars, are recorded based on the aperture, however, rather than the focal ratio - so that the larger the aperture, the fainter the star you can see or photograph, no matter what the focal ratio. |
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