Physics Bootcamp

Historically, refracting telescope was the first telescope to be used for observing astronomical objects. Although the accounts vary as to who invented the first refracting telescope, but records suggest that Galileo Galelei built his own refracting telescope and was the first one to make observations in astronomy using a telescope.

Refracting telescope is similar to a microscpe. It also uses two lenses - the first of these lenses facing the object is called the objective or objective lens, and the second lens near the eye is called the eyepiece. While in microscope, both lenses were used to magnify, the task of the objective in a telescope is to bring the far away object near and then eyepiece is used to magnify this real image.

Figure 45.20 shows the optical setup of a telescope. The objective forms a real image \(\text{Q}_1\) of a star that lies in the direction of \(\theta_\text{o}\text{.}\) This image forms in the focal plane of the second focal plane of the objective lens. The first focal plane of the eyepiece is very close to it and left of it sso that when we use the real image as the object for the eyepiece, we get a virtual image in the direction \(\theta_\text{i}\text{.}\)

Typical eyepieces have focal lengths of \(2.5\text{ cm}\text{,}\) and \(1.25\text{ cm}\text{.}\) If the objective of the telescope has a focal length of \(1\text{ meter}\text{,}\) then these eyepieces would result in magnifications of 40-times, and 80-times respectively. The angular magnifications would make the image appear 40-times, or 80-times closer.

In a refracting telescope objective lens collects light from the source. For fainter objects in the sky you need large diameter objective lens. The largest refracting telescope in the world is the 40 inch diameter Yerkes telescope operated by the University of Chicago and located at Lake Geneva, Wisconsin (Figure 45.22).

It is very difficult and expensive to build large refracting telescopes. You need large defect free lenses, which in itself is a technically demanding task. Refracting telescope basically looks like a tube with support structure to rotate it in different directions.

A refracting telescope suffers from a number of defects. The aberration of lenses causes the image to be blurred. Also as the lenses become thicker for larger lenses more light is absorbed making the faint stars more difficult to observe. Large lenses are also very heavy and deform under their own weight. Some of these problems with refracting telescope is addressed by avoiding refraction for collecting light. Instead a curved mirror is used for the objective. Such telescopes were first designed by Isaac Newton. These telescopes are called the reflecting telescopes which we discuss next.

In the reflecting telescope, also known as a reflector, light rays from a distant source fall upon the surface of a concave mirror fixed at the bottom end of the tube. The concave mirror focuses the rays on its focal plane. To observe the focused image Newton used a design in which the focused light from the concave mirror was reflected to one side of the tube into an eyepiece as shown in Figure 45.23. This arrangement is common in many amateur telescopes and is called Newtonian design.

Some telescopes reflect the light back towards the middle of the concave mirror by using a convex mirror as shwon in Figure 45.24. In this arrangement the light-gathering concave mirror has a hole in the middle through which light emerges to be incident on the eyepiece lens. This arrangement of the objective and eyepiece is called Cassegrainian design.\ Most big telescopes, including Hubble telescope (see Figure 45.25), are of this design. There are other arrangements also possible. Sometimes one puts a light detector right at the spot where light is focused by the curved mirror.

Hubble telescope shown in Figure 45.25, put in the orbit around earth in 1990, is another large reflecting telescope with the primary mirror of diameter 2.4 meter. That makes \(f_\text{o}= 2.4/4 = 0.6\text{m}\text{.}\)

An important use of a telescope is to be able to resolve images of two nearby stars. The image seen through a telescope is affected by a number of factors, such as poor optics and blurring by atmosphere. When a telescope, such as Hubble, is placed above the atmosphere and optical aberrations discussed below are eliminated, resolving two point-like objects is still limited by diffraction effects, which cannot be eliminated since it is fundamental aspect of wave nature of light.

Although Raleigh criterion provides a useful metric of resolution of images of two nearby stars, one can often go beyond it. There is another criteria, called Sparrow criterion, which looks at the presence of any dip in the image and use that dip to separate the images of the two stars. We will not go into that here.