This f/stop stuff is confusing me

[the lemming]

Seems like I'm getting confused with the focal length of telescopes and confusing the f/5 and f/11 figures with their photography counterparts, which took me quite a while to grasp to begin with.

Are these focal length numbers important to have one way or the other?

And do they have corresponding specialties, such as on end of the focal length is best for this and the other end best for that?

Cheers

[themos]

The f-ratios (like f/4 , f/8 f/10) are the same as in photography, I think.

yes, they do have specialities. Generally speaking, you need "fast" ratios for large, faint objects (deep sky objects) and "slow" ratios for small, bright objects (usually the planets and moon/sunfeatures).

[brantuk]

Yup - for imaging planets the slower scopes (f7 and above) are more appropriate, and the faster scopes (f6 and down) are better for photographing dso's (nebulas, clusters, galaxies etc).

But for observing you can use most any scope on all objects - you'll soon get a feel for the capabilities of your scope on different object.

Have a look in the Primers Section for how to calculate the f-ratio, and its relationship to focal length and aperture size, field of view, etc

http://stargazerslounge.com/primers-tutorials/63184-primer-understanding-choosing-eyepieces.html