Stour Astro


Choosing a Telescope

.. By Chris Stellis

A unique feature of Astronomy, when compared to other sciences, is the ability to look directly back into the past. When we observe any "body", whether it be the Sun, another star, a galaxy, a planet or the Moon, we see it as it was when the light or radio waves, with which we make the observation, left that body. In the case of the Sun, Moon and planets, this time difference is insignificant in comparison to the age of the Universe. Travelling at 186,000 miles second, it takes light, or any other form of electromagnetic radiation, less than a second and a half to reach us from the Moon and the journey time from the Sun is still only 8 minutes.

Astronomy is a fascinating hobby for old and young alike. However, newcomers can sometimes be disappointed by what they see in a telescope, especially the cheap, 'bargain' telescopes often found in high street and department stores. Don't expect to see the planets, particularly Jupiter, Saturn and other celestial objects, in full colour just like those they see on TV or in magazines, taken by the big telescopes at Mount Palomar, Hawaii or the Hubble Space Telescope and then heavily processed.

Choosing a telescope can be a confusing experience and there are so many things to take into consideration:

What type of telescope should I buy?

What size mirror do I need?

What's the 'f' number?

How much magnification will I get?


What type of telescope should I buy?

First thing to consider is what a telescope actually does. A telescope is an optical aid, it's used to gather light. The larger the aperture (objective lens size or primary mirror size), the more light is collected and the fainter the object you will see. The size of the aperture determines the telescopes resolving power or resolution.

There are two main types of telescope, the Refractor and the Reflector (Newtonian). The refractor uses lenses to focus the light and a reflector uses mirrors to focus the light. There is a third type of telescope called a catadioptric which uses a combination of mirrors and lenses to fold the optics and form an image.

The refractor is the telescope that most people think of as a telescope. Light passes through a convex lens and is focused towards the end of the tube where it forms an inverted image through the concave lens. An eyepiece is then used to magnify this image.



  • Easy to use and reliable due to the simplicity of design.
  • Little or no maintenance.
  • Excellent for lunar, planetary and binary star observing especially in larger apertures.
  • Good for distant terrestrial viewing.
  • High contrast images with no secondary mirror or diagonal obstruction.


  • More expensive per inch of aperture than Newtonians or Catadioptrics.
  • Heavier, longer and bulkier than equivalent aperture Newtonians and catadioptrics.
  • Less suited for viewing small and faint deep sky objects such as distant galaxies and nebulae because of practical aperture limitations.
  • Focal ratios are usually long (f/11 or slower) making photography of deep sky objects more difficult.
  • Some colour aberration in achromatic designs (doublet).
  • Poor reputation due to low quality imported toy telescopes; a reputation unjustified when dealing with a quality refractor from a reputable manufacturer.

The reflector is an open tube with a curved mirror at its base. Light hits the curved (parabolic) mirror and is reflected and focused back up the tube to a secondary (flat) mirror which redirects the image into the convex lens and to the eyepiece. The observer looks through the side of the telescope rather than through it as with the refractor. One task that will be required is checking the alignment of the mirrors in the reflector, this is known as collimation. A telescope that is poorly collimated will show stars with comet tails and the planets will look like there's two of them and focusing will also be difficult. This can be a daunting task for the novice so I'd recommend getting someone to go through the steps with you the first time.


  • Lowest cost per inch of aperture compared to refractors and Catadioptrics since mirrors can be produced at less cost than lenses in medium to large apertures.
  • Reasonably compact and portable up to focal lengths of 1000mm.
  • Excellent for faint deep sky objects such as remote galaxies, nebulae and star clusters due to the generally fast focal ratios (f/4 to f/8).
  • Reasonably good for lunar and planetary work.
  • Good for deep sky astrophotography.
  • Low in optical aberrations and deliver very bright images.


  • Need more maintenance.
  • Slight light loss due to secondary (diagonal) obstruction when compared with refractors

The more popular catadioptric design is the Schmidt-Cassegrain telescope (SCT). Here, the light enters through a thin aspheric Schmidt correcting lens, then strikes the spherical primary mirror and is reflected back up the tube and intercepted by a small secondary mirror which reflects the light out an opening in the rear of the instrument where the image is formed at the eyepiece.

Schmidt-Cassegrain Advantages

  • Best all-around, all-purpose telescope design. Combines the optical advantages of both lenses and mirrors while cancelling their disadvantages.
  • Excellent for deep sky observing or astrophotography with fast films or CCDs.
  • Very good for lunar, planetary and binary star observing or photography.
  • Excellent for terrestrial viewing or photography.
  • Focal ratio generally around f/10. Useful for all types of photography.
  • Closed tube design reduces image degrading air currents.
  • Most are extremely compact and portable.
  • Durable and virtually maintenance free.

Schmidt-Cassegrain Disadvantages

  • More expensive than Newtonians of equal aperture.
  • More light loss due to secondary mirror obstruction compared to reflectors.



What size mirror do I need?

The simple answer to this question is the largest you can afford! Remember a larger mirror will collect more light, hence you will be able to see fainter objects. However, the larger the mirror the bigger and heavier it will be. If the telescope is too big and heavy you may not be too keen on getting it out that often.

If you live in a light polluted area I would recommend that you invest in a telescope that has an eight inch (200mm) primary mirror. Alternatively if you live under dark skies you may have similar results with a six inch (150mm) primary mirror.

So be aware that light pollution can affect the ability of your telescope to see faint objects.


What's the 'f' number?

The 'f' number refers to the telescopes focal ratio. It can be calculated by dividing the focal length of the telescope by its aperture. My telescope has a focal length of 2000mm and an aperture of 200mm giving a focal ratio of f/10

2000mm / 200mm = f/10

A high focal ratio of say f/9 to f/15 gives a narrower field of view but good contrast, so the image produced tends to be magnified well with bright objects showing more detail. A telescope with a high focal ratio is best for viewing objects like the planets and the Moon.

A low focal ratio of around f/4 to f/6 produces a wider image but of low contrast so it's best for a telescope designed for deep sky viewing.


How much magnification will I get?

If you're looking at a telescope that claims to have 900X magnification or similar then look somewhere else. These claims are false, very misleading and will lead to disappointment in your equipment.

There's a simple way to work out the maximum useful magnification a telescope will give you:

For every inch of aperture multiple by 50 so 6" X 50 = 300X Magnification

Remember these are maximums and not necessarily what you can push your telescope too. Atmospheric turbulence will often prevent you from achieving maximum magnification. I find that my telescope will not cope well with more than 200X magnification even though the theoretical limit is 400X magnification for my 8" SCT!

To work out how much magnification an eyepiece will give you need to know the focal length of your telescope. I've given a few examples below based on my telescope that has a focal length of 2000mm:

Magnification = Telescope Focal length / Eyepiece Focal length

Eyepiece Focal length Magnifiation
40mm 50
32mm 63
26mm 77
16mm 125
9mm 222
6mm 333

You can insert a special lens called a Barlow lens between the eyepiece and telescope to double or even triple the magnification thereby getting more mileage out of your limited number of expensive eyepieces. With two eyepieces and one Barlow lens you have four magnifications available.

Remember that your eyepiece can't exceed the maximum magnification of the telescope. If it does you'll end up with an out of focus image. Also, the shorter the eyepiece focal length the shorter the eye relief meaning for high power viewing you really have to get your eye close to the eyepiece.


Choosing a telescope is a very subjective undertaking. There's no "best" telescope for everyone. The one that's right for you will depend on your lifestyle and your astronomy goals. Spending a little time analysing your motivations will help you make an intelligent choice. It helps to understand the different types of telescopes so you can determine what should influence your decision.

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