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Hi All

What the F in a scope ?

In my photography days a aperture of say F2.8 gave a very narrow depth of focus against a aperture of F16 or F22 where everything is in focus

How does this work on scopes as they are all fixed focal length ( as far as I am aware )

I have seen scopes F4 and scopes F16 and above for sale

Is it a case of

Say F4 on a scope will give good image quality on only the image found and the foreground and background will be out of focus

And with a F16 scope most everything foreground middle and background detail will all be in focus ?

This is a point I need to understand before buying my first scope

Thanks :D

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F is the focal ratio, which is the focal length divided by the aperture.

So, my 12" newtonian has a focal length of 1200mm, divided by 300mm gives f/4.

In telescope terms, it means speed. An f/4 will collect more light for a given time that an f/8.

The concept of depth of field is irrelevant because of the distances involved.

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Forget focal depth when it comes to astronomy. Everything is effectively at infinity. The 'F' in telescope terms is the ratio of aperture to focal length.

For visual astronomy, short focal length scopes ~f/5 give bright images at a low magnification for any given eyepiece. These are particularly suited to deep sky observations, nebulae, galaxies on such. Longer focal lengths, ~f/10+ give relatively narrower fields of view and higher magnification for a given eyepiece. These are more suited to lunar and planetary observation. Though there is considerable cross-over.

In imaging, the 'F' means the same thing, but is really more relevant for the length of exposure required. Fast f/5 means short exposures, slow f/10 means long exposures. (to capture the same level of detail in a given field of view.

Others will be along to explain it in clearer more eloquent terms, but that is my basic understanding.

Edit: need to work on my typing speed.

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for visual observing there is no relevance in the f number (speed) of a telescope. a 250mm aperture is a 250mm aperture no matter what the focal length. as someone else said, the photons have no idea what shape the mirror is and this is the factor that affects focal ratio.

by definition a longer focal length will provide more magnification and less field with any given eyepiece than a shorter focal length but this can be addressed by using a different eyepiece:

two scopes as follows:

250mm aperture f5 = focal length of 1250mm

250mm aperture f10 = focal length of 2500mm

in the above scopes a 20mm eyepiece would provide 62.5x magnification in the first scope and 125x magnification and half the field in the second scope. if you want 125x magnification and the smaller field in the first scope, use a 10mm eyepiece (or a 2x barlow) if you want 62.5x magnification and the wider field in the second scope then use a 40mm eyepiece.

the only exception to the first statement above is when the exit pupil (eyepiece focal length divided by focal ratio) gets larger than your eye.

e.g. a 40mm eyepiece in the f5 would equal an exit pupil of 8mm and in the f10 of 4mm. The latter would be OK but the former would be too big as if your pupil only dilates to 5mm then some of the light hitting your eye would not get through and therefore the image would be washed out and no darker than the sky so fainter objects would be harder to see.

as Psychobilly says though, a faster focal ratio (faster than f5) means that to get the best from the scope you need to accurately collimate and buy better quality eyepieces (and be restricted to a focal length of about 25mm when it comes to eyepieces).

this is one of the reasons most larger newts are f5. convenience means that it's beneficial to make very large scopes quite fast (e.g. f4) and with these it's recommended to use good quality eyepieces and even a coma corrector.

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Instead of thinking about depth of field - think in terms of depth of focus.....

The focus tolerance is only = 4 x lamda x F^2, so for green light (550nm) with an f10 scope it would be 0.220mm and only 0.055mm with an f5 system


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