Thread: f ratio
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Old 19-07-2010, 04:10 PM
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Blue Skies (Jacquie)
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Join Date: Nov 2005
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Well you've got the definition right - focal length divided by aperture.

It will affect your field of view - a short or 'fast' f ratio will give you a wider field of view - traditionally useful if you do variable stars, hunt for comets or do photography. It also means a lesser range of useful magnification to you. It you think about it, magnification is the focal length divided by the focal length of the eyepiece (eg 12mm, 15mm, 20mm etc) so less focal length means less times you can divide the eyepiece into it. Big dobsonians may have a very short focal length deliberately to reduce their height or size.

A long focal length is traditionally good for observing the plants and the moon, as you get a narrower field of view and higher magnification. The planets only take up a small area of sky so some people would say why waste it? Also long focal length is generally not good for photography (unless you're trying to image the planets, that is!) as the small field of view means its much harder to track and/or hide tracking errors. Schmidt-Cassegrains have a long focal length but this is just part of their design. There are ways to mitigate it if needed.

If you're into deep sky observing only, somewhere in between usually suits nicely - you're more interested in aperture there but you also need a good field of view, not too narrow, to find your objects.

These are very general rules, there are plenty of exceptions and reasons for why you might see different f ratios that seem extreme, so its best to ask why the scope was designed that way before you dismiss it offhand as being too long or too short.
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