Telescopes

Visible Astronomy

The Basis of Optics

Optics is the study of instruments that can change the direction in which light travels.

Refraction. Light travels at different speeds in different media, and attains its maximum speed of 300,000 km/s in vacuum. In air light travels slightly slower than this, while in glass and water light travels slower still. Therefore, if a beam of light is traveling in air and then enters glass the light beam will slow down. This slowing down (or speeding up of light, if it is going in the other direction) causes the direction of the light beam to change at the boundary of the two media. This is called refraction.

Reflection. Light bounces off a polished surface in the same way for all colors. This is called reflection. The angle of reflection is the same as the angle of incidence.

Optics and Images

Image.

If we can arrange for the light rays from an object to come together in the same relative alignment as when they left the object we shall have created an image of the object. An image can be created either by refracting or reflecting light, or by a combination of both. The trick is to get all the light from an object to arrive at the same place and create a faithful representation of the object.

Lens. A lens is an optical device that brings light rays together. When the light rays come from a distant object the lens causes the light rays to come together at a region called the focal point. The distance between the center of the lens and the focal point is the focal length. A concave mirror can also be used to bring light rays together at a focal point, provided that the object is very far away.

Telescopes

A telescope is an optical instrument that has two basic optical elements: an objective and an image viewer (for example, an eyepiece or a photographic film).

Objective. This is a large lens, or mirror, that collects light from a distant object and creates an image at the focal point that is a faithful representation of the object.

Eyepiece. The image formed by an objective is usually a bit too small to see all the detail it contains. We therefore use an eyepiece, which is just a sophisticated magnifying glass, to enlarge the image.

Refractor. This is a telescope with a lens as the objective. The largest refractor in the world, has a 40 inch objective, and is located at the Yerkes Observatory in Williams Bay, Wisconsin, USA. One doubts that a larger refractor will ever be built. The problem is that a lens can only be supported along its edge and so there is no obvious way to prevent the heavy lens from sagging in the middle, and thereby ruin its optical surfaces.

Reflector. This is a telescope with a (concave) mirror as the objective. All very large telescopes are reflectors because mirrors are much easier to support than lenses.

Functions of a Telescope

Light-gathering power (LGP). Gathering as much light as possible is the principal function of an astronomical telescope. We need lots of light because the objects of most interest are very faint. The larger the area of the objective the more light we can collect. For example, a telescope with a 10 cm objective collects four times as much light as a telescope with a 5 cm objective because the area of a 10 cm objective is four times greater than that of a 5 cm objective. To compare the LGP of two telescopes, or that of the eye with a telescope, we just compare the areas of their objectives. For the eye, ``the objective" in this case is really just the pupil.

Resolution. This is the smallest angular separation between two objects that can be seen. If the objects are closer than the resolution of the telescope then the objects will appear as one object. If, however, the objects are further apart than the resolution they will appear as two distinct objects. The resolution depends on the wavelength of the light entering the objective as well as on the diameter of the objective. The bigger the diameter of the objective the smaller the resolution, that is, the finer the detail that can be seen. If we measure the resolution in arcsec (1 arcsec = 1/60 arcminute) then the theoretical resolution (TR) is given by

where l is the wavelength of the light and D is the diameter of the objective, both measured in meters. In practice, on earth telescopes can rarely, if ever, reach their theoretical resolution because of the blurring of the images due to atmospheric disturbance. This blurring is called seeing. When astronomers speak of ``good seeing" they mean that the air is particularly steady and consequently the images seen are sharper. Usually, the air is more stable up a high mountain than at sea level. Therefore, all large telescopes, like the Keck Telescopes, are located on mountaintops.

Magnifying power. This is just the (apparent) increase in size of an object relative to how it looks to the unaided eye. The magnifying power is chosen so that the smallest detail in the image can be seen easily by the eye, when looking through the eyepiece. A greater magnification than this is not useful because one will see no more detail than is contained in the unmagnified image.

Invisible Astronomy

Atmospheric absorption

Light is but a small part of a spectrum of different radiation, called electromagnetic radiation. This radiation travels at the speed of light. The spectrum ranges from gamma rays and x-rays to ultraviolet, light, infrared, microwave and radio. The only difference between the different kinds of radiation is their wavelengths: gamma rays have the shortest wavelength and radio the longest. We happen to have eyes that respond to light; rattlesnakes, on the other hand, respond to infrared. The earth's atmosphere is opaque to (that is, it absorbs) different parts of the electromagnetic spectrum, while being transparent to other parts.

Opaque to: Gamma rays and x-rays, and much of the ultraviolet and infrared

Transparent to: Visible, microwaves and radio. The ozone in the atmosphere absorbs the ultraviolet. The water vapor and carbon dioxide absorbs the infrared.

Radio Telescopes

In 1930 Karl Jansky, while working for the Bell Telephone Company, discovered radio waves coming from space. In the 1940s Grote Reber made the first maps of the sky that showed the source of some of the radio waves. Today, radio telescopes are an important tool for observing astronomical objects. They do for radio waves what optical telescopes do for light, that is, they are used to collect radio waves and draw them to a focus.

Radio Interferometers

The big problem with the use of radio waves, rather than light, is that the wavelength of radio waves is about 100,000 times longer than that of light! Consequently, for the same size of objective a radio telescope will have a resolution that is a 100,000 times worse than it light counterpart. The only way to achieve better resolution is to make radio telescopes very, very large. One way to make such large radio telescopes is to make a large number of small ones and spread them over a large area. Such an array of small radio telescopes is called a radio interferometer. The size of the area determines the resolution of the interferometer.

Space Astronomy

Ultimately, the best way to view the heavens is to move above the earth's atmosphere. Then the entire electromagnetic spectrum is available for observation. Indeed many different instruments have been placed in earth orbit to collect different kinds of radiation: infrared - the Infrared Astronomical Satellite (IRAS); ultraviolet - the International Ultraviolet Explorer (IUE); x-rays - the Roentgen Satellite (ROSAT), launched in 1990; gamma rays - the Gamma Ray Observatory (GRO), launched in 1991, and of course for light - the spectacularly successful Hubble Space Telescope (HST), launched in 1990.

Image Processing

Photography

Charge-Coupled Devices (CCDs)

These devices are electronic versions of photographic film. When light falls on a picture element (a pixel) of a CCD the light causes electrons in the pixel to be liberated. The number of liberated electrons depends on the intensity of the light that falls, that is, on the number of photons that collide with the pixel: more photons give rise to more electrons. The charge that is collected in each pixel is measured and recorded. The distribution of the charge from one pixel to another forms a picture. Here is a beautiful CCD image of the moon.