The telescope's f/number

[lknowlen]

In my own opinion the f/number (focal ratio) of any telescope tells two things about the telescope and that is it's intended purpose and it's photographic performance. The brightness of a star depends on the telescope's aperture not the telescope's f/number. All telescopes of the same aperture at any magnification will show the same visual brightness. There are many that insist their long focal ratio telescope gets higher contrast, this is not correct. A refracting telescope does have more contrast than any other because of it's optical system not because of the f/ratio. You can see that when you are comparing very well made and very well corrected refractors, you will see there is no gain in contrast regardless of the f/ratio of each telescope. Even if a reflecting telescope is well made and has the same size secondary mirror obstruction as another, it will have the same contrast regardless of the f/number of each telescope. All of the confusion and there is much on this issue, is because of the photographic use of the f/number. A faster f/ratio does mean brighter images on film but not in a telescope. Aperture not f/ratio is the important factor with a telescope. Some photographers have a great deal of trouble with this concept. The f/number of any objective lens or a mirror of a telescope has nothing to do with the visual brightness of an image, the bigger the aperture the better. Just my thoughts :)

[aphelion]

I still don't understand exactly what the f number on a telescope does mean then... and what determines it.. in a camera the f stop is how open the aperture is, but in a telescope it seems to be fixed... yet a smaller f number seems to be desirable..

[dannat]

telescopes have fixed f/l, due to the fact the lens present inside them are fixed - also the aperture is fixed - there are no moving parts inside a telescope to control the aperture, we could say they are wide open all the time.

As for the desirability of the number, smaller f numbers like that on a camera will let in more light cf. longer f/stops- so for photography the lower the f stop the more desirable because the exposure time will be shorter, which is good as less you have to worry about the mount & tracking.

For viewing longer f/stops aren't a big problem, good planetary views can be had with a telescope of f10, or f15 like some of the SCT's. In fact some of the smaller f/stop like f5 reflectors, can suffer becuase the instrument isn't quite up to gathering the light & a good image across the field of view is difficult to obtain. You wn't experience coma with a f10 instrument.
hope this helps - but I think i'm waffling so will post & re-read

[programmer]

Same as a camera, focal length / aperture, with aperture normally being fixed in a telescope. Short F ratio creates some undesirable effects due to greater light cone angle, such as coma. With Google, there is nothing you don't know :)
http://en.wikipedia.org/wiki/Optical_telescope#Focal_length_and_ f-ratio

[a-l-e-x]

My question is, if it has nothing to do with the brightness of an image visually, why then do you need longer exposures when you hook a camera up to the scope with instruments that have a longer f ratio? Does this also include point and shoot digicams afocally held right in front of the eyepiece or just dslrs?

[Zuts]

Hi

The way I look at it and its probably wrong is as follows.

A longer FL instrument has a smaller field of view (appears more magnified for a given ep/camera fl) therefore while the actual image you are looking at appears as bright as in a shorter FL instrument (the portion actually visible in the ep/camera) you are actually gathering light from a smaller portion of the sky and so overall less light per unit area of the CCD and so the image takes longer to form on the CCD for a longer FL instrument.

Paul

[dannat]

Alex, like the f stop on a camera the smaller the f/stop, the brighter your pic will be, the aperture is open further & lets in mor elight than if your aperture was closed. Just like the human eye opens it's pupil in dark condirions to let in more light - the faster scopes (low f/ no.) let in more light - so you can expose for less time.

This relates to prime focus phoography, where the telescope is acting as the camera lens. As for afocal pics, this will be determined by the camera itself & does not relate to the telescope, so it will not matter whether you use fast sloe scope for afocal, it will depend on what eyepiece you have in place.

[Terry B]

The f ratio for photography through a scope is only relevant for extended sources. i.e. anything that isn't a star. It makes essentially no difference to the brightness of stars. It does make a difference to the size of the field on the camera.
To image very dim stars the aperture of the scope is the most important thing and not the f ratio.

[Ian Robinson]

f/4.66 on my 10" newtonian.

If I upgrade sometime in the future , f/3.5 - f/4 will be my preference.



I don't fancy having to climb a wobbly tall unstable ladder in the dark while alone to look through the eyepiece and shorter "tubes" are stiffer than long ones.

I also like a nice wide fov and the gain in photograhic speed (same criterion as you might use if trying to shoot under dull lighting (a fast , low f no lens will all you to record the image faster on film or the chip , whereas a slow (higher f number say f/22) requires a much longer exposure to capture the image on the same speed film, though it will give a deeper in focus field).