It is most commonly thought that telescopes are used to magnify the night sky. While this is true, more importantly the telescope is used to capture as much light as possible to make the faint objects visible.

The Basics

The main body of a telescope works by focusing the incoming light to form an image. This image is viewed through an eyepiece, which itself acts as a magnifying glass. The opening through which the light enters the telescope is called the aperture. A larger aperture means the telescope can gather more light.

Light enters the telescope and passes through a lens at the opening of the tube. The lens bends the light together. The light is brought to a single point called the focus. This is designed to happen near the end of the tube. An eyepiece (not shown) is slotted into the end of the telescope and the eyepiece can be slid in and out by the focuser. There are other designs of telescope, but this Refractor design has been shown here as it is the simplest.

What do the numbers all mean?

Looking at the specification of a telescope you will see words such as Aperture, Highest Useful Magnification, Focal Length. This section will help you understand what it all means.

The Aperture is the most important number when buying a telescope. A telescope with a larger aperture can view fainter objects than a telescope with a smaller aperture. The aperture is measured as a diameter, so a 120mm aperture telescope gathers 4x as much light as a 60mm aperture telescope. The aperture size is also sometimes called the Primary Mirror Diameter or the Objective Lens Diameter.

The Highest Useful Magnification is the theoretical upper limit that the magnification used should not exceed. Doing so will not reveal any more detail. It is often calculated as about 2x magnification for every 1mm of aperture. So a 120mm aperture telescope has an upper limit of about 240x magnification. This limit is also sometimes called the Highest Practical Power.

The Focal Length is the distance at which a telescope focuses an image. Eyepieces also have focal lengths. It is the combination of both telescope focal length and eyepiece focal length that gives the magnification. Eyepieces come in a range of focal lengths, from around 50mm down to about 2mm. Having several eyepieces allows a telescope to achieve a range of magnifications.

There is a simple formula to calculate the magnification:

So with my telescope that has a 1000mm focal length and with my four eyepieces of 4mm, 10mm, 25mm and a 50mm focal lengths, I can achieve: