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Pete Presland

focal length and ratio

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hi guys i own the following scopes

celestron c8n focal length 1000 and ratio f5

meade 125 pe focal length 1900 and ratio f15

i know that the higher ratio's are more forgiving to eyepieces from what i have read on other threads (very informative this place ;) )

but what are the pros and cons of each with there very different lengths and ratio's

also i know what the focal length is ,but what does ratio mean?

thanks pete

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The focal ratio is the focal length divided by the aperture.

For example, for the C8N:

focal length = 1000mm

aperture = 200mm (8 inches)

focal ratio = 1000/200 = f/5

HTH

Andrew

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Focal ratio refers the ratio between the aperture and the length of the scope, essentially, it tells us how quickly the light is brought to focus. A low ratio (F/4.5) means that the light cone is short, which in turn gives a short focal point onto which one then has to focus the EP, which is why it is tricky to get a 4mm EP to grab a sharp image, for instance.

Or that's how I see it any way.

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My thoughts on this (agreeing with Yeti Monster)

A scope with a small f number is called fast. Photographers know that a faster lens produces a brighter image. For a given focal length (magnification) you are letting in more light.

A fast scope requires more curved optics (mirrors or lenses). The incoming light has to be bent more (into a shorter, fatter cone). The field will be less flat and EPs have a harder job to do.

If you think the other way, in terms of a fixed aperture (how we often buy telescopes?), then a higher f number means more magnification and a lower one less magnification for a given EP.

If you get into imaging then are other issues to think about too.

Andrew

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another thing to add is the effect of focal length on magnification in case you were not aware. magnification = focal length divided by aperture.

celestron c8n focal length 1000 and ratio f5

meade 125 pe focal length 1900 and ratio f15

in your case the almost double FL of the meade means that any given eyepiece in that scope would give you double the magnification of the celestron (approx.). a barlow has this same effect.

I have created an excel sheet which compares various elements of my own scopes with eyepieces and includes field of view, magnification and the effect of a barlow. I have amended this to show your scopes' specs. hope it's useful. if anyone wants the spreadsheet just send me a PM.

lenses.pdf

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another thing to add is the effect of focal length on magnification in case you were not aware. magnification = focal length divided by aperture.

celestron c8n focal length 1000 and ratio f5

meade 125 pe focal length 1900 and ratio f15

in your case the almost double FL of the meade means that any given eyepiece in that scope would give you double the magnification of the celestron (approx.). a barlow has this same effect.

I have created an excel sheet which compares various elements of my own scopes with eyepieces and includes field of view, magnification and the effect of a barlow. I have amended this to show your scopes' specs. hope it's useful. if anyone wants the spreadsheet just send me a PM.

thanks very much shane for the excell sheet! i have printed it,very informative especially maximum mags will keep that in mind ;)

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Focal ratio refers the ratio between the aperture and the length of the scope, essentially, it tells us how quickly the light is brought to focus. A low ratio (F/4.5) means that the light cone is short, which in turn gives a short focal point onto which one then has to focus the EP, which is why it is tricky to get a 4mm EP to grab a sharp image, for instance.

Or that's how I see it any way.

that seems makes sense thanks

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My thoughts on this (agreeing with Yeti Monster)

A scope with a small f number is called fast. Photographers know that a faster lens produces a brighter image. For a given focal length (magnification) you are letting in more light.

A fast scope requires more curved optics (mirrors or lenses). The incoming light has to be bent more (into a shorter, fatter cone). The field will be less flat and EPs have a harder job to do.

If you think the other way, in terms of a fixed aperture (how we often buy telescopes?), then a higher f number means more magnification and a lower one less magnification for a given EP.

If you get into imaging then are other issues to think about too.

Andrew

certainly giving imaging a go,slow progress at the moment!

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