Telescopes

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Telescopes

A telescope is a truly wonderful thing. It opens up wonderful views of the amazing objects that surround us in the Universe. In this section there is a brief overview of the different types of telescope, recommendations on how to buy an instrument and what you can expect to see.

Types of Telescope

There are three main design types of telescope - refracting, reflecting and catadioptric.

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Refracting Telescopes

This type is probably what most people would think of if they were asked to visualize a typical telescope. It was first developed by Galileo in 1609 and the basic design remains the same today.

A refracting telescope uses an objective lens which determines the size of the instrument. So a 3 inch telescope has an objective lens 3 inches in diameter. The object lens collects rays of light from the object being looked at and then refracts (bends) them to form an image at the focus where another lens magnifies the image.

This is much easier to understand in a diagram

Catadioptric Telescopes

A catadioptric telescope really has the most horrendous sounding name, but it is simply a telescope that uses a combination of refraction and reflection. The name derives from the fancy name for a curved mirror - catoptric and for a lens - dioptric.

Actually, catadioptrics (or cats for short) are some of the most popular telescopes on the market because they provide a great compromise between refractors and reflectors by combing the best features of both.

However, to make matters even more complicated, there are several sub-types of cats - let’s just look at two of the more popular, Schmidt-Cassegrain and Maksutov-Cassegrain.

$Diagram of a Refracting Telescope$

Reflecting Telescopes

This kind of instrument was invented by Isaac Newton in 1668 in order to overcome a major drawback of the refracting telescope - chromatic aberration. This just means that the lens cannot bring all the different colours of light to focus at the same point. Today this problem has been pretty much resolved by using achromatic lenses but at Newton’s time these had yet to be invented.

The Newtonian reflecting telescope uses a concave mirror to focus the light. The mirror is located at the bottom of a tube, the light is reflected back up the tube to a small secondary mirror tilted at 45° which then directs the image towards an eyepiece.

Another type of reflecting telescope was invented just after the Newtonian by Guillaume Cassegrain. It uses the secondary mirror to reflect the light back through a hole in the primary mirror.

The size of a reflecting telescope is determined by the diameter of the main mirror.

Simplified Diagram of a Refracting Telescope

More detailed diagram of a Refracting Telescope - Click to Enlarge

Diagram showing the Light paths in a Newtonian Reflector Telescope

A Typical Reflector Telescope

Telescopes for Beginners >>

SCTs - Schmidt-Cassegrain Telescopes

The Schmidt camera was designed in 1930 by Bernard Schmidt to take wide angled photographs of the sky. This design was modified in 1940 by James Gilbert Baker by combining it with a Cassegrain telescope to develop the Schmidt-Cassegrain telescope.

As you can see from the ray diagram below, light is reflected by a secondary mirror through a hole in the primary mirror to the eyepiece, as in a Cassegrain. However, there is also a large ‘corrector plate’ across the aperture. This is a thin lens which is designed to correct the spherical aberration in the spherical primary mirror it is combined with.

SCTs are short in length, making them easy and comfortable to use.

Ray Diagram for a Schmidt-Cassegrain Telescope

Exaggerated cross section of a Schmidt corrector plate. The real curves are hard to detect visually giving the corrector plate the appearance of being an optically flat window.

This diagram shows how the Schmidt corrector plate, C, works in combination with the spherical primary mirror to bring the light to a focus at F.

Schmidt-Cassegrain Telescope

Maksutov-Cassegrain Telescopes

Maksutovs are a different type of cat. The main difference is in the type of correcting lens. They use a deeply curved meniscus lens to correct for spherical aberration. Like SCTs Maksutovs are light and portable and they also offer high contrast, high resolution views.

Further Reading

Choosing and Using a Schmidt-Cassegrain Telescope: A Guide to Commercial SCTs and Maksutovs (Patrick Moore's Practical Astronomy Series)

What Can You See with a Telescope?

Binoculars are great but do have limitations. Even a relatively inexpensive telescope will open up the wonders of our amazing universe. Just don’t expect to see images like those taken by the Hubble Telescope!

The following gives you an idea of what you can see with a typical 6 inch Newtonian reflector, given reasonably good conditions:

The phases of Venus and Mercury. Depending on where Mars is in its orbit, you can see surface details. Jupiter is a great target - the ever changing Jovian atmosphere is fascinating. Saturn's rings and moons. Uranus and Neptune.