Telescope speeds

[142mel]

Can anyone tell me what the difference is between fast telescopes, say F6 or less and slower scopes, F7 and higher. I know the cheaper eyepieces don't work well with fast scopes so would it be better to have a slower scope?

Awaiting my first scope.

[LukeSkywatcher]

I MAY be wrong here but i dont think i am:

Its ALL to do with focal length (the length of the scope tube).

Faster scopes (F6) are generally shorter in length then slower scopes (F7+) which means it takes a lot less time and distance for light to travel down the scope to the EP. This inturn means that the light in faster scopes is more concentrated and focused because it has not been diffused by a longer distance/time before reaching your eyes. Hence the images in scopes with a lower (faster f ratio) are sharper and better then in a scope with a higher (slower f ratio).

At least thats how i THINK it works.

I would imagine there is very little difference between an f6 and an f7 scope.

[Moonshane]

Paul's some of the way there. the speed of a telescope is the relationship between aperture and focal length. a scope which has a 200mm aperture (width of the large hole the light comes in through) and a 1000mm focal length (distance from mirror / objective lens to focus point) has a speed of f5 i.e. 1000/200. if you have a different scope with an aperture of 100mm you have a slower scope at 1000/100 = f10.

slower scopes generally have less chromatic aberration (refractors) and are more forgiving re eyepieces as you say. they are usually better for imaging/viewing planets and the moon and possibly double stars. long tube refractors and SCTs are generally slow scopes.

faster scopes (most newtonian reflectors and short tube refractors) generally gather more light more quickly so are better for imaging/viewing galaxies etc but also allow for a shorter tube for the aperture and are therefore more portable; albeit you often need better quality eyepieces to use them to their best advantage - but not always depending on how 'fussy' you are.

in a way it's similar to cameras, where eg f2.8 will give faster shutter speeds but less depth of field and vice versa for f16.

I'd say a fast scope of f6 down and a slow scope f6 and up but there's some overlap of course and most scopes can be used to good effect on lots of targets which they are not 'best suited' for.

hope this helps

[142mel]

Thanks guys, very helpful. Love this site, so much help available.

Mel

[Capricorn]

The term fast and slow come from the photographic field. In this the light collected by the lens is imaged directly on to the sensor, digital or flim.

An objective of say 200mm collects the same amount of light immaterial if it is fast or slow.

What alters is the primary image size. As per a camera lens on to the sensor/film.

A fast scope, for a constant diameter, has a shorter focal length then a fast scope.

The image (primary) is very closely proportional to the focal length. So a short focal length produces a smaller primary image then a long focal length scope.

Since both scopes, being the same diameter, collect the same quantity of light then the smaller image will be brighter (but smaller) then the primary image created by the longer focal length scope.

As the image is brighter in camera terms you can use a faster shutter speed to capture the image.

Now the hiccup, if you have a 200mm f/4 scope and a 200mm F/8 scope and you magnify the image from both to say 100x then both images are equally bright to the eye.

The advantage of a fast scope is that if you are looking for a dim DSO then the scope collects X amount of light and brings it to a small but bright(ish) image that you may see and then centre your sope on. Then you try other eyepieces to get the optimal view.

[brianb]

Visually - at the same magnification - it theoretically makes NO DIFFERENCE AT ALL what the focal ratio is. What you ACTUALLY get with a long focus is easier access to high magnifications, easier to figure / more accurate optics, far less in the way of collimation issues and reasonable performance from budget eyepieces.

Short focus scopes are often not as good optically because it's hard to make the deeper curves accurately; they're harder to collimate and they don't work well with cheaper eyepieces. You get a brighter image at the same focal length which gives them an advantage for imaging. You get a wider field at the same aperture ... but only because it's easier to get really low powers.

Somewhere around f/8 is IMHO ideal for visual work in small to moderate apertures. The usual f/5 is simply too short for general visual work ... OK at low power but compromised for high power work.

[ollypenrice]

A crisp answer fom Brian.

Olly