[Wavytone]

If you are trying to decide on f/ratio, bear in mind there are two limits:

- the lowest useful magnification (and longest useful eyepiece focal length), which is set by the focal ratio of the scope, when expressed in terms of eyepiece focal length this rule equates to 6mm x the focal ratio ; and

- the highest useful magnification, which is set by its aperture (x1 to x1.5 per mm of aperture).

Now look at eyepieces available on the market... Lots to choose from in the range 7 - 30mm, a few above 30mm to to 55mm in 2" barrels and a few below down to 2.5mm.

If you choose say an f/4, an f/7 and say an f/12 scope for comparison, then work out the eyepiece ranges, you will find that an f/7 scope allows a 10:1 range between highest (4mm eyepiece) to lowest magnification (42mm eyepiece) with readily available eyepieces. With f/4, it's fine for low powers but forget high power. With f/12, high power but forget really low powers.

So... questions:

1. Why do people opt for Newtonians faster than f/7 ? Size, and the difficulty of making a stable mount for it. It's a lot easier if the scope is f/4 to f/5.

2. Why do people buy SCT's (f/10) from Meade/Celestron ? Because they are very compact, and at f/10 give good views of the planets.

3. Why doesn't anyone build an f/7 Newtonian ?
Well... I don't know. 30 years ago they did, and they work spectacularly well from personal experience, BUT they are long.

Ultimately the choice comes down to what you want to observe - if its planets - go for f/10 to f/15.
If its widefield views of the Milky Way, f/4.
If you want an all-rounder that can do both well, choose f/7. For this reason a lot of the f/7 refractors are probably Nirvana, from the perspective of a visual observer.

Note also you should also choose eyepieces to match the scope.