f ratio

[austinstkong (Austin)]

I have started window shopping for a telescope and realised that I actually don't know what a f ratio is.

My research on the internet for some reason didnt seem quite helpful and all I was able to gather is that greater ratio results in fainter images and that it is calculated by focal length divided by aperture.

So further on from that, does it govern any other qualities of the image viewed and what type of ratios are best suited for doing what?

Thanks in advance.

Sorry if there has already been a similar post.

[Blue Skies (Jacquie)]

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.

[torana68 (Roger)]

F6 is a good compromise in a newtonian.

[astro744]

See http://www.televue.com/engine/TV3_Pa...n=Advice&id=86 for some info.

Faster telescopes, say f4 or 5 are far more demanding of the eyepiece that slower telescopes >f9. Slower telescopes also have a greater depth of focus than faster telescopes , ie. the focus is more sensitive on a faster system.

Coma is seen is a fast parabloidal mirror more so than in a slower one but it can be corrected with a device such as a Tele Vue Paracorr.

Fast telescopes perform better with highly corrected eyepieces such as that offered by Tele Vue (tested down to f4) whereas slower telescope perform OK with the poorer corrected eyepieces. Note a highly corrected eyepiece will still be better than a poorly corrected one particularly off-axis even on a slower telescope.

For visual use telescopes with equal aperture will show you objects with the same brightness regardless of speed. However, a faster telescope will record images more quickly than a slower telescope when doing astro-photography.

There are small differences between reflectors and refractors of the same size since a reflecter loses a little light due to the central obstruction of the secondary plus each reflective surface loses a little light upon each reflection.

[austinstkong (Austin)]

I see, great, thanks.