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Mirrors or Lenses? - Beginner's Telescope Advice

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Mirrors or Lenses? - Beginner's Telescope Advice

NEIL PHILLIPSON of ASTRONOMIA helps us to understand refractors, reflectors and the “other” kind of telescope

I often tell people that we are something of a ‘dating service for telescopes’ – as we spend a great deal of time helping customers to decide between the various types – Reflectors, Refractors and the mysterious Catadioptric. No telescope is best for everything, so this month I’m going to try to explain the differences between them and what you can do with them.

Refractors

Ever since Galileo invented the refractor in 1609, it’s been this iconic design that we most associate with the telescope. Using a large front “Objective” lens, refractors bend light from distant objects to reach a focus within the optical tube, which then exits via an eyepiece, greatly magnified and brightened.

Unfortunately, passing white light through just one lens separates the colours much like a prism – so most telescopes actually contain a ‘doublet’ – two lenses sandwiched together, which corrects this problem (known as ‘Chromatic Aberration’ or ‘False Colour’). Spectacle lenses are made exactly the same way.

Refractors usually have a relatively long focal length but smaller aperture. This, combined with good contrast, makes them ideal for observing the Moon and planets.

Reflectors

Isaac Newton completed his first reflecting telescope in 1668, which changed things significantly. His design consists of a relatively short optical tube, with a mirror at the bottom. At the top is a ‘spider’ support which holds a flat secondary mirror at 45 degrees, diverting the image to the eyepiece at the side.

With mirrors there is no chromatic aberration – eliminating one of the key problems with refractors. In turn, the ability to bend light through tight angles using a mirror meant a much shorter focal length could be achieved – so reflectors can have lower magnification for a given aperture.

Because mirrors are cheaper to make than lenses, reflectors generally cost much less per inch of aperture. Their low magnification and comparative brightness makes them great for deep sky observations.

Catadioptrics

Catadioptric telescopes combine the benefits of both lenses and mirrors. The most popular types are the Schmidt-Cassegrain and Maksutov-Cassegrain.

Cassegrain telescopes pack a huge amount of focal length into a very small space, by effectively folding the light path back on itself, with two curved mirror surfaces. Their compact, lightweight tubes don’t require heavy duty mounts either – one reason they work well on small GOTO mounts.

The combination of aperture and focal length makes catadioptrics suitable for almost all types of observation. Picking the right one means you get all the power of a much larger telescope in a very compact package.

CommentsComments

  • himanshu mishra
    himanshu mishra 21/08/2017 09:37

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  • kevin Bennett
    kevin Bennett 25/11/2017 09:16

    A good all-round beginner's telescope is a Newtonian reflector (below). http://www.outsourcedataservices.com/ It's a simple design and is relatively cheap for the size of mirror you'll get for your money – ideal if you're just starting out.

     

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John Eds
John Eds
  • Member since: 11/12/2016
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