Telescopes 3 - The Reflecting Telescope

A reflecting telescope makes use of mirrors instead of lenses to gather the light. Just like the refractor, the reflecting telescope has a tube. But this type of telescope has a curved mirror set at the bottom of the tube and a flat mirror set near the top. This flat mirror is angled at about 45 degrees to the curved mirror and is held in place by a "spider". This type of telescope normally has one end of the tube open to the air.

The curved mirror is known as the "primary mirror", while the flat mirror is called the "secondary mirror" or "flat". The "spider" consists of three or four metal strips, called vanes, which support the secondary mirror in the centre of the tube.

This type of telescope was first used by Sir Isaac Newton in 1668 and now nearly all reflecting telescopes are referred to as "Newtonians"

As light enters the a reflector it travels down the tube until it encounters the primary mirror, at the bottom of the tube. The curved shape of the primary mirror, more correctly known as a parabolic curve, causes the rays of light to converge in a the same way as the objective lens in a refractor. In fact the primary mirror is not unlike the magnifying side of a shaving mirror. The light is then reflected back up the tube until it encounters the flat or secondary mirror.

Because this secondary mirror is mounted at 45 degrees to the primary mirror, the light is then reflected out through the side of the tube and into the focuser, where the eyepiece is positioned. This then is our basic Newtonian Reflector.

Just as the refracting telescope might suffer the unwanted effects of chromatic aberration, reflecting telescopes can also suffer a form of aberration, known as "spherical aberration". Sounds confusing? Please read on, I will do my best to explain.

The primary mirror of a reflecting telescope has a curved, concave bowl shape to it. This shape could be described as part of a sphere. This spherical shape is easy to produce through a process of careful grinding. In the same way that a single convex lens, in a refracting telescope, would bring light of different colours to slightly different points of focus, the spherical mirror would suffer a similar problem.

But instead of splitting up white light, as the single lens would, a spherical mirror would have several slightly different points of focus, depending upon where the light has landed upon the mirror's surface. The result of this spherical aberration is a telescope that refuses to come to focus. Stars and other objects remain blurred.

To cure this problem, when the mirror has been ground to a spherical shape, an extra grinding process creates a small extra dish shape in the centre of the mirror, known as a parabola. This parabolic shape somewhat resembles that of a satellite dish. The result is to now bring the light rays to the same point of focus, regardless of where the light has landed on the mirrors surface. The result, a telescope that provides sharp images in the eyepiece.

The shape or smoothness of the flat or secondary mirror is just as important. It is thought of by some, that a perfectly flat mirror is harder to produce than a good parabolic primary mirror. The accuracy of a mirror or how well it has been ground, is measured in fractions of the wavelength of light. For example, a good telescope would be expected to have been produced to 1/4 of the wavelength of light. To put that measurement into perspective consider this. The thickness of a human hair is about 0.7mm. Visible light has a wavelength of around 0.0005mm. Imagine a fraction of this! But it is obtainable and even budget telescopes almost reach this figure.

There are various versions of the Newtonian reflector. Some are called "Dobsonians", this simply refers to the type of mount onto which the telescope is fixed. The Dobsonian mount is named after its inventor and is a simple altitude and azimuth mount. The larger reflectors may also have dispensed with a tube altogether. These scopes may use a series of rods or poles to support the flat mirror and focuser section, while the primary mirror is housed in a separate section that supports the rods and sits on a base plate.

The focuser of a reflector works in the same way as the focuser on refractors. It's designed to support an eyepiece and has a mechanism to move the eyepiece back and forth to focus the telescope.

Recap ...

The reflecting telescope uses mirrors, not lenses, to gather the light and bring the rays of light to a focal point. The main or primary mirror has a parabolic shape to ensure that all the light landing on it's surface is brought to a focus at the same point. Just before this focal point a flat or secondary mirror, which is mounted at 45 degrees, reflects the light out through the side of the tube and into the eyepiece.

Reflector telescopes can suffer from spherical aberration if the shape of the primary mirror is incorrect.

The biggest difference between refractors and reflectors is the available size of the aperture. Reflectors tend to offer the biggest range of large aperture scopes.

Telescopes Part 3The Reflecting Telescope - The power of the Mirror

Recap ...

Telescopes Part 3
The Reflecting Telescope - The power of the Mirror