The Purpose of a Telescope

The 8.2-m primary mirror of Yepun, Unit Telescope 4 of ESO's Very Large Telescope, after its recoating in early March

The 8.2-m primary mirror of Yepun, Unit Telescope 4 of ESO’s Very Large Telescope, after its recoating in early March

In this next article about tools for astronomy, we cover an essential but often overlooked point about telescopes.  It may seem strange to cover this, but once you understand this point, you’ll understand the trade-offs involved in choosing a good telescope for stargazing. The fact is, most beginners believe the purpose of a telescope is to magnify objects, to make them appear bigger. This is not true. What, then, is the purpose of a telescope?

The purpose of a telescope is to collect light.

That’s all there is to it. By collecting more light, a telescope makes astronomical objects like stars, galaxies, and planets brighter, sharper, and easier to see visually or to image.

A schematic of a simple telescope

A schematic of a simple telescope

To collect light from distant objects, a telescope uses a curved lens or mirror (called an objective) and focuses that light to an image at the focal point. 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.

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.

Let’s use 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.

An approximation of the image of Jupiter in a 2" (left) and 4" telescope.

An approximation of the image of Jupiter in a 2″ (left) and 4″ telescope.

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. See the images above.

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 also give a narrower view of the night sky than smaller telescopes.  So you need to take all of this into consideration. 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. In the next article, you’ll look at the trade-offs and main considerations for choosing a telescope for astronomy.

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