Which is better

[brascoe]

Im just checking out scopes again for what seems like an eternity i was wondering when it says on the specs f4, f5, f6, etc is it better when the numbers go up or down. Looking to buy my scope from the for sale section on here hopefully it wont be long.

[Peter Drew]

The shorter focal lengths, F4,F5 will give lower magnifications with the usual range of eyepieces, they will give wider fields of view and suitable for photographing star clusters, galaxies and nebulae. The longer focal lengths F6,F8 will give higher magnifications with smaller fields of view and will be ideal for closeups of the Moon and planets.

A F6 reflecting telescope is a good allrounder for a beginner.

[alowen]

Nicely put Peter, best way i've heard it described yet on here.

Al

[umadog]

As Peter says, scopes with "faster" focal ratios tend to have shorter focal lengths and are generally capable of producing wider fields of view. The only point at which I'd disagree is about high power viewing. I think it's a bit of a generalisation, perhaps even a myth, that slower scopes are better for high power viewing.

Why? For starters, the statement ignores objective size. A 12" f/5 is longer focal length than an 8" f/6; and 10" f/4.7 is the same focal length as an 8" f6. By the way, don't get confused between focal length and focal ratio: they're different things. In addition, you can always pop in a Barlow and double your effective focal ratio, should you want to do that. A fast scope can be made slow. Finally, there are plenty of good short-focal length eyepieces you can buy. You won't have any problem hitting the seeing limit at any focal ratio or (within reason) focal length.

Don't worry about the secondary mirror size, either. It's true it's proportionally larger in a faster scope, but Newtonians generally have a pretty small secondary obstruction. Obstructions below 20% by diameter are indistinguishable from unobstructed.

The influence of focal ratio is most felt in other ways: coma, eyepiece aberrations, and collimation. Coma is an off-axis aberration induced by the parabolic shape of the Newtonian's primary mirror. It's more pronounced in wider field and lower power views and much more pronounced in faster scopes. You will have prettier views at f/6 than f/4.7, for instance. In fact, at focal ratios below f/5 many people use a corrector lens to get rid of coma. Eyepieces can harbour aberrations such as astigmatism, but these aren't generally very noticeable in slower scopes. They do become prominent at speeds of f/5 and slower. So, to generalise, cheaper eyepieces work better in slower scopes. Again, slower scopes give better views. You also have to be more careful in collimating a faster scope. The tolerances are smaller so you need to be more accurate. There are power laws governing increasing aberrations and tighter collimation tolerances, so this stuff becomes critical quickly.

It might sound like there's a lot wrong with fast scopes. In fact, all these issues can be overcome (coma correctors, good eyepieces, good collimation tools and technique). The benefits are the wider field of view compared with slower scopes of the same aperture. Shorter tube lengths, so easier transport. Lower eyepiece heights. It's up to you what trade-offs you're prepared to make. As Peter says, a good 8" f/6 is a great compromise.