fast telescope?

Question

I know that the "f" stands for fast but what does that mean.My scope is f8.can some one shed some light on this for me...thanks

you see just when you think you know something you are proven wrong.I truely did think the "f" stood for fast,I thought I read it some where thanks for the info

Answer 1

As others have said, the f of f-ratios stands for the focal ratio. Saying that a telescope is 'fast' is actually a carry-over from photography. A lower f ratio allows for shorter time exposures to get the photos. This is why small f ratios are called 'fast'. Telescopes typically aren't called fast unless the f ratio is less than about 5, so yours would not be considered to be fast. If you do astro-photography, this can affect the length of time for your exposures.

Answer 2

Focal Ratio (f/stop)

The focal ratio, or f/stop, of any lens system (including telescopes), is computed by dividing the focal length by the clear aperture (usually expressed in millimeters). In other words, the focal ratio is the ratio of the focal length and clear aperture. Thus:

TELESCOPE FOCAL LENGTH / CLEAR APERTURE = FOCAL RATIO

For example, a telescope with a focal length of 1000mm and a 100mm (4") clear aperture has a focal ratio of f/10 (1000/100=10).


F/8 is not a fast telescope because it magnifies a narrow field of view and takes LONGER to find objects in the sky using visual references. F/4 is considered FAST because the same eyepieces used in F/8 now show a much wider field but with far less magnification and therefore are FAST to locate what you are looking for in the sky with a much greater amount of visual references.

The difference between fast scopes and others is important because often people will use a small 4.5" FAST F/4 scope to find objects in the sky and THEN move the giant beasty 14" F/8+ telescope to the exact same location, only now they know where it's at in the sky currently. You don't wanna move your 22" monster scope and find a patch of clouds is in your way or some other calamity like a planet is blocking your view.

To be clear a 4.5" can be F/4 or F/8 depending on the type of scope it is, one being FAST and the other being more narrow but better for magnifying. The F/4 will also track easier with a smaller scope due to the lens size whereas an F/8 with a 10" might track just as easily because of the larger mirror.

Answer 3

A fast telescope is one whose focal ratio is F/5 or less. Some professional telescopes have f/ratios as low as 1, but an amateur telescope usually won't have one faster than F/4 or F/3.5 due to the incredible difficulty of producing truly good optics. The f/ratio of a telescope is determined by diving the focal length by the diameter of the primary mirror or objective lens. I have a 6-inch F/8 Dobsonian, which has a focal length of 48 inches. For general purpose viewing, a F/6 or F/8 is much better than a faster telescope unless it's much bigger than 10 or 12-inches. Eyepieces perform better, collimation is much less demanding and it's much easier to make truly good mirrors and lenses. However, if you want a small wide-field portable instrument, plan to do astrophotography or want a 10-inch or larger Newtonian, a fast telescope is the only realistic option. Longer f/ratios make the telescope just too long to transport and set up, at least without assistance.

Answer 4

Hello extremly -

Actually the f stands for focal ratio, which represents the ratio between the focal length and the aperture. For example, a scope with a 10 in (250 mm) aperture and a 1250 mm focal length has a focal ratio of 1250 / 250 = 5 and would be designated f5. Most amateur scopes range from f4 to f16, with most falling in the lower half. A scope with a focal ratio of f6 or less is considered to be a "fast" scope. Focal ratios from f6 to f9 are considered "medium" while those above f9 are considered "slow." Fast, medium, and slow in this context has no relationship to the brightness of the image, but is a holdover from photographic nomenclature. Please see excellent source noted below for more good info - ref p. 42

Answer 5

The f stands for focal ratio (or f-ratio). The focal ratio is defined as the focal length f of an objective divided by its diameter D or by the diameter of an aperture stop in the system. The f-ratio tells you what sort of field of view a telescope has for a given aperture. A telescope with a low f-ratio has a wider field of view than another telescope with the same aperture but a higher f-ratio.

All info in this post has been shamelessly cribbed from wikipedia (see sources).

Answer 6

It's a somewhat fast telescope.

Fast telescopes generally sacrifice magnification power. But you can see lower magnitude objects with them.