Lens Diameter and Focal length

[Ganymede12]

Does a refractor with a 70mm objective lens and a 700mm focal length perform the same as a refractor with a 60mm objective lens and an 800mm focal length?

[JamesF]

I think you really need to define what you mean by "perform", but as they have different focal ratios I think the answer has to be "no".

James

[E621Keith]

No, objective diameter affects image brightness, and focal length affects magnification. Given the same eyepiece, the 60/800mm image will be dimmer but more magnified than a 70/700. However, the 70/700 can change to a shorter eyepiece that will give the same magnification as the 60/800, but there is nothing the 60/800 can go to compensate for the lost of light caused by the smaller aperture.

[Moonshane]

agree with the comments above. focal ratio technically has no influence on visual observing and in theory a f11 scope and a f5 scope of the same aperture and magnification should perform the same. however, in practice (using my newtonians) I find that for the same aperture slower scopes perform far more effectively on contrasty bright images like planets than faster scopes. this might be (and probably is) related to the smaller secondary in my slower scopes. for other objects they are pretty identical (with the same assumptions).

[Macavity]

For achromatic (doublet) refractors, chromatic aberration (false colour) increases "rapidly" with diameter. In simpler terms an f/9 three inch refractor would be relatively colour free, but a six inch refractor would have to be f/18 to achieve similar results. In short (no pun intended) they get BIG very quickly. Or become (shorter) APOs or triplets and get very expensive instead?

See table in : Achromatic Refractors

[E621Keith]

That's true, fast scopes are more demanding on eyepiece too. While you can get away with cheaper eyepiece on a slow scope, you need expensive glass to cope with the steep light cone in a fast scope. CA also becomes a problem for refractors.

Both scopes the OP mentioned are f10 or more so this is not a problem. The view quality will depends on how well the lens were manufactured.

[Ganymede12]

OK, thanks. Next time I'll remember the f/ratio!

[E621Keith]

Remember the most important variable in a telescope is its aperture. Aperture is a fixed number on any scope. Focal ratio can be changed with barlow or reducer. Aberrations can be compensated by correction optics such as field flattener, coma corrector, chromacor etc... However, aperture cannot be changed.