The NexStar User's Guide II - Excerpt from Chapter 2
Before discussing equipment, there are a few terms we must understand.
This is the component of the optical system that collects the light from the sky allowing us to see all those faint objects that our eye cannot detect on its own. In binoculars and some telescopes this is the lens at the front of the optical tube. In other telescopes, it is a large, dish-shaped mirror at the back of the optical tube. We measure the diameter (distance across) of the objective and express this as the aperture of the instrument. Generally speaking, the larger the aperture, the more light the instrument collects, and thus, the more objects we can see. When we say the NexStar 8SE is an 8 inch scope, we are referring to the diameter of the objective.
The relationship between aperture and light gathering power is geometric, based on the area of the objective, not the diameter. When comparing two scopes, simply square the diameter of their objectives and divide. For example, to compare 80mm and 60mm telescopes, we calculate 80 x 80 divided by 60 x 60. The resulting answer of 1.78 indicates that an 80mm objective collects 78% more light than a 60mm objective.
Additionally, larger apertures are capable of resolving finer detail in objects. Thus, a larger aperture can usually split closer double stars, show more detail on the Moon and planets, and show more detail in deep sky objects. More so than light gathering power, resolution is greatly affected by the overall quality of the instrument's optics.
There is a saying among astronomers that "aperture rules". While this is certainly true, larger aperture comes at a price, both monetarily and practically. Larger aperture telescopes are more expensive than their smaller siblings. Also, larger aperture telescopes require much more effort to transport, setup, and store. Consider carefully the size of telescope you will purchase. The best telescope for you is not necessarily the most expensive or largest model you can buy. It is the one that you will use the most. An exquisite 14 inch telescope will provide incredible views, but if you only find the energy to set it up once every couple of months, perhaps you would more enjoy a 5 inch scope that takes just minutes to carry outdoors and put into use.