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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INSTRUCTOR:

So as you learn more about the night sky looking around just with your eyes, you might start thinking about some equipment to use. Binoculars are the first obvious thing, but there's also a huge variety of telescopes out there. And choosing which one can be quite a difficult choice. It's worth having a go with lots and lots of different types.

The first type of telescope that was developed, and one of the most common, is a refractor. And this particular type of telescope's nice that simple. You've got a lens up at the front here. The light passes down the telescope itself, and then the eyepiece that you're looking in is down at the back here. So simple, but very, very effective.

Your next step up is then a Newtonian style reflector. So this style of telescope, again, your light is obviously coming in at the front. But this time, it's travelling all the way down the tube to the main mirror that's all the way down here at the back. The light then comes back up, bounces off the little mirror mounted in the tube here at a 45 degree angle, and out to your eyepiece here.

So these are the two most common types. But telescopes are progressing very, very rapidly with their technology, just as the rest of astronomy is. And so there's various different combinations now that use both mirrors and lenses. And we've got one of those combinations here. This is what's referred to as a Schmidt-Cassegrain. You've actually got a lens right at the front, but then your large primary mirror at the back. And that primary mirror actually has a hole in it. So as your light bounces up and down the tube, your eyepiece again is down on the back here.

On this particular telescope, we either have an eyepiece on the back of it, or we quite often attach a camera, because we're here today in the George Abell observatory actually on campus in Milton Keynes at the Open University. So you don't have to do all of your astronomy from Teneriffe. You can do a lot from the UK as well.

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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.

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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.

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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.

Figure 5 The Dall-Kirkham telescope. In this type of telescope a curved secondary mirror at the front of the telescope reflects light ...

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!