Name:      Date:

Telescopes

TOOLS OF THE ASTRONOMER

Telescopes of All Kinds

Throughout history people have been interested in the night sky. It is sort of contagious, to look at the night sky and wonder what is up there and to marvel at its beauty. In large cities there seems to be less of an interest in the night sky. Perhaps because of the many bright lights there is less to see. Perhaps in the hustle and bustle of the modern world it is felt that there is less mystery, less yet to be explained. regardless of the perspective of the viewer, there are still mysteries to be discovered, new ideas to be explored. In fact, we are learning the answers to questions that only yesterday we did not know how to ask.

Until about the time of Galileo, about 1600 AD, all the astronomical observations were done with the naked eye. Now, some of these were so carefully done that tremendous information was learned about the heavens. Kepler's three empirical laws of planetary motion were surmised from naked eye, painstaking observations. Galileo's contributions were a quantum leap beyond what could be seen with the naked eye alone. Then, for centuries, refinement and development of bigger and better telescopes built on the access to enlarged and more refined images.

It was not until this century that the acquisition of information accross the entire electromagnetic spectrum became a reality. Radio, microwave, infrared, UV and even x-ray and gamma ray radiation is collected and examined to learn more about the heavens and the universe.

All telescopes, whether they be optical, radio, infrared, or whatever, gather light. This is the single most important function of a telescope. We assume that all light, whether it be short wavelength (eg x-ray, ultraviolet) or long wavelength (eg radio, microwave) carries energy in the form of waves. The most fundamental aspect of astronomy is that objects in the heavens radiate light energy. It is that light energy that comes to us. It contains all the information about the object, perhaps it temperature, chemical composition, age, even its history and perhaps it ultimate destiny. Our job is to be smart enough to break down the "code" and interpret the light energy so we can say with confidence, certain things about the object. It is far more than just looking at pretty pictures of galaxies and the like.

Visible light is measured in units called Angstroms. An angstrom is 0.0000000001 metres in wavelength. Because we don't want to write so many zeros (indeed we'd lose track of them easily) we use a system called power of ten notation. So the above numer (1 angstrom) can be written by counting places from the decimal point. In this case it is written as 10^-10. Your instructor will discuss this with you. One might also refer to the video Powers of Ten.

Visible light (light we can see with our naked eye) includes violet or bluish light at the short end, about 3500 angstroms or 3.5 X 10^-7 metres. At the other extreme is red light, approaching nearly 7000 angstroms or 7.0 X 10^-7 metres. Visible light is often called the optical spectrum. The optical spectrum is sort of in the middle of the extremes of short and long wavelength. Radio waves are of tyhe order of metres in wavelength and x-rays of the order of fractions of an angstrom.

Radio astronomy began in the 1930s with Karl Jansky. Typical wavelengths in the radio region are for example, 200 metres for an AM radio wave, 3 metres for an FM wave and of the order of centimetres for microwaves. eg, our microwave ovens use waves about 12 cm in length. We cannot "see" radio or microwaves. Nor can we see ultraviolet and shorter waves. We can only see directly, waves of the optical or visible spectrum. But instruments can detect the existence of these various waves. Rado waves are usually detected with parabolic dishes. But then again, parabolic mirros are used in optical telescopes, so in fact, they are very similar. Fundamentally, all light is similar in that it has wavelength and frequency and all these waves, whether they are radio or visible, or ultraviolet or whatever, travel at the same speed, the speed of light.

Purpose of Telescopes:

Telescopes are used for three main purposes:

1. To gather light, sometimes too dim to be seen with the naked eye.
2. To see more detail than the naked eye can see.
3. To make things appear closer (bigger)

The fundamental purpose of all telescopes (optical, radio, infrared, etc.) is to gather light. All telescopes, regardless of the wavelength of light they are intended to gather must have:

1. Objective - this is a large mirror or lens to collect the light. It may be a receiving dish or some other similar device. The size or collecting area is what is important. The greater the area, the stronger the signal, or basically, the "brighter" the light. The ten metre Keck Telescope atop Mauna Kea is about 400 inches or ten metres across. The radio telescope at Aricebo, Puerto Rico is about 1000 feet across!
   
2. Eyepiece - In an optical telescope this is the smaller lens that magnifies the image. In radio or infrared, etc telescopes, this is determined by gain or an electronic signal enhancer. Frequently eyepieces are replaced with sensitive instruments, cameras, photodetectors, etc.

Optical Telescopes:

There are two basic types of optical telescopes. These are Reflector and Refractor. The refractor type uses lenses, typically ground from glass while the reflector uses mirrors to form the images.

Sky and Telescope magazine has information about:

1. Telescope Basics
2. Refracting Telescopes
3. Reflecting Telsecopes
4. Schmidt Cassegrain Reflecting Telescopes
5. Maksutov Telescopes
6. Dobsonian telescopes
7. Binoculars

Be able to describe the difference between the various kinds of telescopes and to identify them from the list above.

Light Gathering Ability:

The amount of light energy that falls onto a telescope (the objective lens or the dish, mirror or whatever the case may be) depends on the surface area of the aperature. The bigger the area of the aperature the brighter the object appears. If the aperature is circular, the area of the aperature merely depends on the area of a circle or

Area = p r²

So, if we want to compare two telescopes, we compare the energy gathered by the ratio of the areas or in this case, the ratio of the square of the radii or diameters:

Energy Scope #1/Energy Scope #2 = d(1)² / d(2)²

Resolving Power

Resolving Power is the measure of the ability of a telescope to produce fine detail, or basically to separate images into its distinct parts. For example, at night while driving on a lonely road you may have seen an approaching automobile. While it is very distant it appears that it has a single headlight. When it gets close enough you can tell that it has two headlights. The separation of the image into two distinct headlights is a function of resolution. The greater the ability of a telescope to resolve two objects, the more we can see and learn. In practice, when a person observes an object in a telescope, there must be a balance of power or magnification and resolution. Typically the greater the power or magnification the poorer the resolution and vice versa. There is a relationship in physics called Rayleigh's Criterion which describes the amount of separation that can be resolved, measured in seconds of arc. A second of arc is a fraction of an agle the object "subtends" or takes up across the sky. Fort example, the moon or the sun subtend an angle of about 1/2 degree. A degree has 60 minutes ( ' ) of arc and each minute of arc contains 60 seconds ( " ). Thus one degree has 60 X 60 or 3600 seconds of arc. This allows us to describe very small angular dimensions. In astronomy, the greater the "resolving power", the smaller angular size it can resolve. We define this using Rayleigh's Criterion:

RP = 2.52 X 10⁵ l / D seconds of arc

This is basically a measure of the size of object that can be resolved. Note that the bigger the wavelength involved, the result is bigger and the resolving power less. The bigger the aperature D, the smaller an object or the greater the resolving power. So the ideal is to use a small wavelength and large aperature. Then the smaller angular size can be resolved and the resolving power is greater.

For visible light (the middle of the visible spectrum is about 5500 angstroms) this reduces to Dawe's Limit:

RP = 4.56 / D seconds of arc (D in inches)

and

RP = 11.5 / D seconds of arc (D in centimetres)

Radio Telescopes:

The National Radio Astronomy Observatory has a host of sites and telescopes.

Radio telescopes detect very long wavelength compared to visible light, of the order of 107 longer. One might ask why would anyone want to observe in the radio spectrum? The answer is that visible light is stopped by interstellar material throughout the galaxy and space. Radio waves are not. So to peer into the heart of the galaxy, radio telescopes allow us to tackle the problem.

Magnifying Power:

This tells us how much larger an object appears. For an optical telescope this is given by:

MP = focal length (objective) / focal length (eyepiece)

The focal length is found by finding the point at which an object located verry very far away (infinity for practical purposes) focuses at a point. Your instructor will show you how to do this.

Questions:

1. You have the equipment in front of you with which to "make" a telescope. All that is required is an objective lens and an eyepiece. Matter not that the image may look inverted. For astronomers looking at stars this is no big deal.
   1. Focus an "infinitely" far away object on a screen. Write down the focal lengths of the objective lens and the eyepiece:

      Objective Focal Length ______________________ cm

      Eyepiece Focal Length _______________________ cm

   2. Is the image Erect or Inverted? Explain what you see:
   3. By looking at an object with your telescope, estimate the magnification. Calculate the magnification using data in part a. Compare the results and discuss:

Resolution:

We discussed resolution and the the Rayleigh Criterion. For Optical Telescopes we can consider the following:

These two images were taken of the same objects. Which of the two exhibits better resolution and why can we say that?

The two pictures above are of the Andromeda Galaxy. One is obviously of better resolution. Which one would you say was taken with a bigger aperature telescope? Why do you say this? Discuss.

		Phys 104 Page
St. Kate's Home Page	Physics Home Page