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High f numbers are also less demanding on the eyepieces, the collimation (newtonian reflectors) and the objective lens (refractors) - all other things being equal, they give a cleaner image than shorter f ratios, and/or do so more easily. The main trade offs are their narrow field of view, the longer exposures needed in imaging, the bigger mounts required, and the fact that they generally get pretty unwieldy from medium size apertures up. Note the above doesn't necessarily apply to folded designs like Cassegrains, which also have high f numbers. They have other challenges though.
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