The classic Newtonian telescope

5.7 The classic Newtonian telescope

The Newtonian telescope was invented by Sir Isaac Newton who did not believe that one could overcome the problem of chromatic aberration that was suffered by simple refracting telescopes of the time. Sadly, it is not thought that he made any astronomical observations with it. In the Newtonian, a primary mirror refl ects the light to a focus that would lay in the centre of the tube so, to avoid

obstructing the light path with the head, a secondary mirror, often called the fl at, refl ects the light sideways to form an image just outside the tube where the focuser and eyepiece are placed. As the secondary mirror has to intercept the converging light cone at an angle of 45° it must have an elliptical outline with its major axis 1.414 (the square root of 2) times longer than its minor axis.

In Figure 5.11, let us assume that the focal plane is 10 mm outside the telescope tube which has an external diameter of 230 mm and which houses a primary mirror of focal length 1600 mm and diameter 200 mm. The centre of the secondary mirror is thus at a distance of (10 ⫹ 115) mm from the focal plane, that is 125 mm. One can then calculate the minimum length of the minor axis

Observing the Universe

Figure 5.11 The Newtonian telescope.

using similar triangles. If D is the diameter of the primary mirror, d is the minor axis of the secondary mirror, F is the focal length of the primary mirror and k is the distance of the image plane from the telescope axis, then:

D /F ⫽ d/k

With the given values:

d ⫽ D ⫻ k/F ⫽ 200 ⫻ 125/1600

⫽ 15.6 mm.

The corresponding major axis is 15.6 ⫻ 1.414 ⫽ 22 mm. Why does this give the minimum size of the secondary mirror? Imagine putting

the pupil of your eye at the focal point of the telescope. You would be able to see the whole of the primary mirror surface. However, if you move your eye to one side you will no longer be able to see the whole of the surface as the secondary mirror will not be large enough. The effect of this is to reduce the effective area of the primary mirror for image points away from the axis, causing what is called vignetting of the image away from the axis. The actual size of the secondary mir- ror is thus increased to minimize this effect, the amount dependent upon the main use of the telescope.

As described above, the ‘image’ of a star is a disc surrounded by a number of circles, the so-called Airy disc. With an unobstructed aperture (as with a refrac- tor) little light (16%) falls in the rings but, as the secondary mirror of a refl ecting telescope gets larger, more light falls into the rings and the effect is to reduce the effective resolution of the telescope (though the diameter of the central part of the disc actually gets smaller). The secondary mirror causes a second problem – it has to be supported. Usually this is done with a four vane ‘spider’. This is what causes the ‘cross’, formed by diffraction spikes, seen around bright stars in many images you may have seen. Thus light is moved away from where it should be and so the image is slightly degraded.

Introduction to Astronomy and Cosmology