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Astronomy with an online telescope
Astronomy with an online telescope

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2.2 Telescopes

The next obvious step up from binoculars is a telescope and these come in a wide variety of sizes and types as well as in different combinations of mirrors, lenses and mounting systems. The next video gives a brief introduction to the three most common types of telescope optics.

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The main part of any telescope is the primary lens or mirror, often referred to as the ‘objective’. As Jo explains in the video, small telescopes often use a primary lens but larger telescopes, including COAST, use a curved mirror to collect and focus the light. This is because mirrors have numerous advantages: they are lighter and easier to manufacture than large diameter lenses; and they do not split the light into different colours, avoiding the problem of coloured fringes around bright objects (chromatic aberration) which can happen with glass lenses. The Hubble Space Telescope and all large observatory telescopes use mirrors rather than lenses for these reasons.

Whether a lens or a mirror, the purpose of the primary element of a telescope is to take all of the light coming into the telescope and focus it into a small area to form an image. For visual observing, this image is viewed through an eyepiece, and for capturing images the light will be focused onto an imaging sensor, which could be a digital camera or a specialised astronomical imaging device (often referred to as a CCD or Charge Coupled Device – essentially a very sensitive imaging sensor). Increasing the exposure time also increases the sensitivity: the photo-sensitive cells in the human eye collect light for only a fraction of a second before the signal is sent to the brain, whereas the pixels in a CCD can collect light over a much longer time, increasing the ability to detect very faint objects.

Figure 4 Some typical refracting and reflecting telescope designs.

Figure 4 shows some common telescope designs. In a lens-based (refracting) telescope the light is focused by a lens at one end of a tube, forming an image at the other end of the tube which can be viewed directly with an eyepiece or imaged with a camera or CCD sensor. In a reflecting telescope, the objective is a curved mirror, usually at the lower end of the tube. This mirror collects light entering the telescope and again focuses it to form an image. A second, smaller mirror (the secondary mirror) is often used to direct light into a camera or eyepiece, although in some cases an imaging sensor is placed inside the tube at the prime focus of the main mirror, where it receives the image directly.

The COAST telescope is of the Dall-Kirkham type, an advanced reflecting design where the secondary mirror directs the light back through a hole in the centre of the primary mirror and onto the imaging CCD, which sits at the back of the telescope.

Described image
Figure 5 The Dall-Kirkham telescope. In this type of telescope a curved secondary mirror at the front of the telescope reflects light back through a hole in the main primary mirror where additional optics deliver a highly-corrected image to a camera fitted at the rear of the telescope.

Whatever the design of the telescope, remember that the main purpose is to collect as much light as possible in order to view or take images of very faint objects. In the quest to observe ever fainter and more distant objects telescopes of ever increasing apertures are being constructed and planned, with the largest land-based optical telescopes having mirrors of up to ten metres in diameter!