Among a variety of aspects to consider when choosing a telescope, aperture is one of the most important for buyers to consider. The aperture affects the ability of the telescope to gather light and helps to make otherwise dark elements in the sky visible. However, increasing the aperture increases the diameter and weight of the telescope, so choosing too large an aperture makes the scope less portable.
The aperture also serves as a rule of thumb for selecting the appropriate magnification for the telescope. Generally, magnification needs to be limited to 50 times the aperture in inches. If users push the magnification beyond this limit, the images loose the clarity needed for viewing in the telescope. In many cases, a telescope offering 75x magnification allows the user enough power to see Saturn's rings or Jupiter's major cloud formations.
Most quality telescopes use interchangeable eyepieces. Changing the eyepiece affects the overall magnification. However, increasing the magnification reduces the brightness of the image. For very small objects in the sky, the tradeoff is often worth the change in brightness. Dividing the focal length by the power of the eyepiece provides the magnification of the telescope. Adding more eyepieces helps to ensure a larger range of magnification while maintaining the level of light necessary to see darker objects.