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In achromatic models, shorter focal lengths produce more chromatic aberration than a longer focal length in any given aperture. Longer focal lengths may also produce sharper images due to reduced chromatic aberration; and because long focal length optics are easier to make, there's a reduced likelyhood of imperfections. For this reason Long focal lengths are better for lunar and planetary targets.
Short F/L achromatic refractors are more designed towards wide-field deep sky viewing and are not particularly suited to planets.
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