Telescope Terminology
Magnification
Important: Magnification is NOT an important specification for a telescope. In fact, it's normally only the cheap "toy" telescopes that make a big deal of their magnification power.
Astronomical telescopes are not so much about about magnifying as brightening. In other words, while you do want to make objects appear somewhat bigger, the real aim is usually to make faint objects brighter (excluding things that are already bright like the Moon, Sun and brighter planets).
Having said this, it can be useful to know the maximum useful magnification. Although almost any telescope can magnify to any amount using the right eyepiece, there will be a limit beyond which the image degrades so much that it's pointless.
Aperture
The telescope's ability to collect light, which basically means how big the main lens or mirror is. The main lens or mirror is called the objective.
Larger objective = More aperture = more light.
All other things being equal, more light means a brighter and better image. Therefore the size of the objective is pretty much the most important specification of a telescope, which is why telescope descriptions almost always include the objective size, e.g. "6-inch dobsonian" or "70 mm refractor".
Note that a larger objective doesn't necessarily mean the best telescope for a particular purpose. For example, if the quality of the optics is poor then the image will be poor. Also, a larger objective means the telescope is larger, heavier and more difficult to use, so it may not be as practical as a smaller one.
Focal Length
The distance between the objective lens or mirror to the place where the light converges to a single point. Generally, telescopes with a longer focal length have higher magnification than shorter focal lengths.
Focal Ratio (f/number)
The objective's focal length divided by its size. For example, the focal ratio of a telescope with a 114 mm mirror and a focal length of 1000 mm is: 1000 / 114 = 8.77.
Focal ratio relates to the brightness of the image and the width of the field of view. Here is a rough guide to the most useful focal ratio:
- f/10 or higher - good for observing the moon, planets and double stars (high power)
- f/8 - good for all-around viewing
- f/6 or lower - good for viewing deep-sky objects (low power)