The Real (and Totally Misunderstood) Purpose of a Telescope

Before you drop a week’s salary on your first good astronomy telescope, here’s one essential point you must understand.  It may seem strange to start here… but once you understand this point, the rest of this guide will make sense.  The fact is, most beginners believe the purpose of a telescope is to magnify objects… to make them appear bigger.   This is not true.  So what, then, is the real purpose of a telescope?

The purpose of a telescope is to collect light.

A telescope uses a curved lens or mirror (called an objective) to collect light from distant objects and to focus that light to an image.   A bigger objective lens or mirror collects more light and creates a brighter and sharper image. The focused image formed by the objective lens of a telescope is magnified by a smaller second lens called an eyepiece.  As visual observers, we look into the eyepiece to see the bright magnified image from the objective.  But an eyepiece used with a small lens or mirror simply magnifies a dim and fuzzy image.

Schematic of a simple telescope

Schematic of a simple telescope

So, although magnification is useful, it has no effect in helping you see detail in a telescope.  The detail and brightness of an image all comes down to the amount of light collected by the objective lens or mirror.  And that depends on its diameter of the objective, also called the aperture.  Like a big bucket collects more raindrops than a small bucket, a big objective collects more light than a smaller one.

As an example, let’s look at Jupiter with two telescopes, one with a main lens of 2″ diameter and one with a main lens of 4″ diameter, and pick eyepieces for each telescope to give a magnification of 100 times (or 100x).  So each image will appear to be the same size. In the telescope with a 2” aperture, Jupiter’s largest cloud belts will be clearly observable but a little dim and fuzzy.  But in the 4″ telescope, the same cloud belts will seem to take on more structure and color, and smaller cloud belts are now visible that could not be seen in the smaller instrument. The larger telescope’s advantage in collecting light makes it possible to see more detail than is possible through the smaller telescope at the same magnification.

Jupiter at a magnification of 100x in a telescope of 2” aperture (left) and 4” aperture

Jupiter at a magnification of 100x in a telescope of 2” aperture (left) and 4” aperture

So a telescope that collects more light gives a better view.  That means choosing a telescope should be simple, right? Simply choose the telescope with the biggest objective lens or mirror that you can afford.

Well yes, but there are trade-offs.  Telescopes with bigger aperture are more expensive, heavier, and in some cases, harder to use than smaller telescopes.  They create a narrower view of the night sky than smaller telescopes.  So you need to take all of this into consideration. But we are getting ahead of ourselves.  We’ll cover all these points shortly.  Just remember for now that the purpose of a telescope is to collect light, and more light makes for a better image and a more enjoyable observing experience.   Before we get into telescopes, let’s discuss binoculars, the most useful and overlooked optical tool for observing the night sky.