Is the focal ratio of a telescope a factor when determining image quality?

Question

How would a 6" reflector with a F/8 compare to a 8" reflector with a F/6 if they both have the same focal length.
I know the larger aperture has advantage,but what about focal ratio ?

Answer 1

Focal ratio does not determine image quality. Image quality is determined by the optics and mechanics of the specific manufacturer.
The smaller focal ratio will provide a larger field of view of the sky and less magnification.

Answer 2

Depends on the telescope. A higher focal ratio will reduced chromatic aberration - the focusing of different colours to different points - in non-achromatic and achromatic refractors, which will mean a sharper image. The former were the first kind of telescope and were often built VERY long to minimise the chromatic aberration inherent with a single objective lens. The achromatic refractor uses a two-element objective to bring red and blue light to the same focus, which cancels much of the chromatic aberration making the telescopes much shorter and more practical, but some CA still remains and it's worse with a lower focal ratio. Focal ratio will have little effect on the sharpness of an apochromatic refractor (bring red, green, and blue all to the same focus), and none on a reflector or catadioptric telescope. All modern professional telescopes are reflectors excepting a few very specialised ones (there's a refracting solar telescope in use).

Answer 3

The focal length of a telescope, for those who don't know, is determined by the curvature of the mirror or lens. It's the distance from the objective to the focal point where the light rays converge.

Focal ratio is calculated by dividing the focal length by the aperture, making sure they are in the same units.

So to your question...

Basically, the larger that number under the F, the "slower" the telescope is, and the smaller the number, the "faster" it is.

Slower scopes = narrower field of view, more magnification per eyepiece, less edge distortion, longer exposure times in astrophotography, easier to collimate, more depth of field, and perhaps more detail on photographs.

Faster scopes = wider field of view, less magnification per eyepiece, more edge distortion, shorter exposure times in astrophotography, more difficult to collimate, shallower depth of field, and people claim astrophotos look more washed out.

To figure out what type of magnification an eyepiece will give you, magnification = (telescope focal length)/(eyepiece focal length)

So both of those telescopes will give the same magnification but the image will be brighter in the larger aperture telescope.

Which is better depends on what you want out of the telescope. Very large telescopes are often made "fast" to keep the dimentions practical, also, it's difficult to grind the curve needed for very slow telescopes, especially with the larger mirrors.

Answer 4

The lower "F" number projects more light hence has more usable magnification capacity. Magnification is achieved by the ratio of objective to eye piece focal lengths.