If the aperture is the most important spec of a telescope, the focal length is probably the second most important one.  The focal length of the telescope is the distance between the main, light-gathering lens or mirror and the point where the image comes into focus.

But just like aperture, focal length on its own does not mean much.  Focal length does drive 2 major characteristics of a telescope though: f/ratio and magnification.  More on those some other time.

The focal length of a telescope of course has a big influence on the size of a telescope.  The longer the focal length, the longer a telescope has to be … unless you cheat.

Trevor from AstroBackyard has a great YouTube video on choosing a telescope with a long or a short focal length and the effects on the what you can see or image.

Compacting telescopes

By bouncing light between multiple mirrors and lenses, some telescope types can achieve a longer effective focal length than the actual size of their tube.  More on telescope types later.

Barlows & focal reducers

Focal length is an inherent property of a telescope and is always fixed.  It is what it is.  But barlows and focal reducers can be added to a telescope that influence the focal length of a telescope and give astronomers a little more flexibility in how they want to observe or image the night sky:

• Barlows multiply the telescope’s focal length by a factor to achieve a longer effective focal length
• Focal reducers divide the telescope’s focal length by a factor to achieve a shorter effective focal length.