Why bother going after long focal length?

[Villads]

As the title suggest, I'm wondering why you should be going for a long focal length, when you, presumably, just can buy lenses to help magnify your view? Does putting things like Barlow lenses on, worsen the image?

[AstroKeith]

To design and manufacture 'fast' optical systems (ie short focal lengths) requires complicated designs involving more components, higher tolerances and exotic materials = potentially much higher costs.

This is essentially because the aberrations that we dont want get worse on faster systems and more and more complexity is required to counteract them.

A f15 refractor can have a simple objective lens and with a long focal length eyepiece, which can also be simple, produce excellent images. The only drawback if the length of the scope. Centuries ago this is all they could do and focal lengths of 40 ft were not uncommon.

[The Lazy Astronomer]

You could add a barlow to a really small scope, but it dims the "image" (for lack of a better word) at the eyepiece. So higher f ratio = dimmer image. 

Extreme hypothetical situation: use barlows to make a 50mm f40 scope (2000mm focal length) vs a 400mm f5 scope (also 2000mm focal length) - I know which one I'd rather look through!

[Louis D]

Even with reflectors that don't suffer from increasing chromatic aberration at faster f-ratios like refractors do, figuring an f/3 paraboloid is much more demanding than figuring an f/8 paraboloid.  In fact, the f/8 barely differs from a spherical mirror, so very little parabolization is required.