Telescopes How do they work? Chapter 5
1. History 2. Lenses & Hardware 3. Reflecting Telescopes 4. Refracting Telescopes
History Hans Lippershey Middleburg, Holland invented the refractor telescope in 1608 Galileo Used a small 30X scope, was the first to use a telescope in astronomy. Galileo's designs used a combination of convex and concave lenses. Observed the moon, began the modern age of Astronomy where measurement more important than philosophy.
Galileo noticed moons orbiting Jupiter phases of Venus craters on the moon sunspots This was strong evidence that Copernicus was right although Galileo wasn t willing to die for it.
History Kepler improved the design to have two convex lenses, which made the image upside-down. Kepler's design is still the major design of refractors today, with a few later improvements in the lenses and the glass to make them.
Why can t you see an object that is far away? The answer is simple: the object does not take up much space on your eye s screen (retina). For example, at 150 feet the writing on a dime does not cover enough pixels on your retinal sensor for you to read the writing. This can be corrected by bending the light with lenses.
Lenses The lens in your eyes works like a glass lens. The light bends as it goes through a different medium. Light rays are bent when they intersect glass; a curved surface can produce an image. In your eye, the image is then focused at the retina.
How does this apply to telescopes? If you had a bigger eye, you could collect more light from the object. This image could be magnified so it stretches out over more pixels in your retina. In a telescope, two pieces make this possible: the objective lens (refractor telescopes) or primary mirror (reflecting telescopes) the eye piece
The objective lens (in refractors) or primary mirror (in reflectors) collects lots of light from a distant object and brings that light, or image, to a point or focus. An eyepiece lens takes the bright light from the focus of the objective lens or primary mirror and "spreads it out" (magnifies it) to take up a large portion of the retina. This is the same principle that a magnifying glass (lens) uses; it takes a small image on the paper and spreads it out over the retina of your eye so that it looks big.
How they really work Scopes gather light through the objective (mirror or lens) bigger is better because it gathers more light The ability to see faint objects increases proportionally with the square of the radius of the objective They focuses light changing the eyepiece changes the magnification magnification is the ratio of the focal length of the objective to the focal length of the eyepiece
Diagram of a simple telescope. Parallel light rays enter from the left, pass through the objective lens, come to a focus at the focal plane, and exit through the eyepiece lens. The focal length of the objective is F, and the focal length of the eyepiece is f. www.ifa.hawaii.edu
When you combine an objective lens or primary mirror with an eyepiece, you have a telescope. Again, the basic idea is to collect lots of light to form a bright image inside the telescope, and then use something like a magnifying glass to magnify (enlarge) that bright image so that it takes up a lot of space on your retina.
A telescope has two general properties how well it can collect the light (the aperature) how much it can magnify the image (the magnification)
The Aperture A telescope's ability to collect light is directly related to the diameter of the lens or mirror -- the aperture -- that is used to gather light. Generally, the larger the aperture, the more light the telescope collects and brings to focus, and the brighter the final image.
Magnification The telescope's magnification, its ability to enlarge an image, depends on the combination of lenses used. The eyepiece performs the magnification. Since any magnification can be achieved by almost any telescope by using different eyepieces, aperture is a more important feature than magnification
A closer look at eyepieces View through an eyepiece. Note that the image is upsidedown.
Eyepiece The purposes of the eyepiece are to: produce and allow you to change the telescope's magnification produce a sharp image provide comfortable eye relief (the distance between your eye and the eyepiece when the image is in focus) determine the telescope's field of view: apparent - how much of the sky, in degrees, is seen edge-toedge through the eyepiece alone (specified on the eyepiece) true or real - how much of the sky can be seen when that eyepiece is placed in the telescope (true field = apparent field/magnification)
Filters Filters are pieces of glass or plastic that you can place in the barrel of an eyepiece to restrict the wavelengths of light that come through in the image. Set of filters for viewing, including a light pollution filter (left) and colored filters for enhancing contrast in planetary images. Filters can be used to: enhance the viewing of faint sky objects in light-polluted skies enhance the contrast of fine features and details on the moon and planets safely view the sun
There are 2 main types of Telescopes Refractor telescopes, which use glass lenses Reflector telescopes, which use mirrors instead of lenses. Both types accomplish exactly the same thing, but in completely different ways.
Refractor Telescopes Refractors are the type of telescope that most of us are familiar with. They have the following parts: a long tube, made of metal, plastic, or wood a glass combination lens at the front end (objective lens) a second glass combination lens (eyepiece) Refracting telescopes focus light rays by bending them with glass.
This is the simplest telescope design you could have. A big lens gathers the light and directs it to a focal point and a small lens brings the image to your eye.
Advantages and Disadvantages Easy to use and reliable Excellent for lunar, planetary and binary star observing especially in larger apertures. High contrast images with no secondary mirror or diagonal obstruction. Sealed optical tube reduces image degrading air currents and protects optics. More expensive per inch of aperture Heavier, longer and bulkier than equivalent aperture Newtonians and catadioptrics. Small apertures Less suited for viewing small and faint deep sky objects. Color aberration due to colors of light bending different amounts.
Refracting telescopes are not used for astronomical research (anymore) because they are large and have heavy lenses (i.e. expensive).
Reflecting Telescopes History: Isaac Newton developed the reflector about 1680, in response to the chromatic aberration (rainbow halo) problem that plagued refractors during his time. Instead of using a lens to gather light, Newton used a curved, metal mirror (primary mirror) to collect the light and reflect it to a focus. Because the mirror reflected light back into the tube, he had to use a small, flat mirror (secondary mirror) in the focal path of the primary mirror to deflect the image out through the side of the tube, to the eyepiece; otherwise, his head would get in the way of incoming light. In 1722, John Hadley developed a design that used parabolic mirrors, and there were various improvements in mirror-making. The Newtonian reflector was a highly successful design, and remains one of the most popular telescope designs in use today.
Advantages and disadvantages Lowest cost per inch of aperture Reasonably compact and portable up to focal lengths of 1000mm. Excellent for faint deep sky objects such as remote galaxies, nebulae and star clusters. Reasonably good for lunar and planetary work. Low in optical aberrations. Open optical tube design allows image-degrading air currents and air contaminants More fragile Large apertures (over 8") are bulky, heavy and tend to be expensive. Slight light loss due to secondary mirror obstruction when compared with refractors.
Reflecting telescopes focus light by bending it with mirrors
A third type But I said there were 2 types I lied Sort of. Catadioptrics Uses both mirrors and lenses to collect the image Schmidt-Cassegrain Maksutov-Cassegrain
Catadioptric telescopes Catadioptric telescopes Best all-around, all-purpose telescope design. Combines the optical advantages of both lenses and mirrors while canceling their disadvantages. Sharp images over a wide field. Excellent for deep sky observing or astrophotography with fast films or CCD s. Very good for lunar, planetary and binary star observing or photography. Closed tube design reduces image degrading air currents. Most are extremely compact and portable. Large apertures at reasonable prices and less expensive than equivalent aperture refractors.
Schmidtt-Cassegrain
Problems with earth-based telescopes Earth s atmosphere reflects certain wavelengths x-rays, gamma rays and most UV light is not transmitted by our atmosphere Earth s atmosphere blurs images the bending of light by the atmosphere depends on the temperature of the air twinkling (shimmering) effect Light pollution Solution? Put the telescope in space.
Hubble Space Telescope
Not everything is visible www.yorku.ca/eye/spectrum.gif Many modern day telescopes do not use visible light to collect images. Radio telescopes, x-ray telescopes and infrared (IR) telescopes have become a staple of modern day astronomy, producing some amazing images.
Compton Gamma Ray Observatory Chandra X-ray Observatory
Disadvantages of space-based telescopes Expensive to launch and maintain Difficult to repair Low lifetime
Examples of space-based telescopes Hubble Space Telescope 3 times better resolution can see fainter objects Chandra X-ray Observatory Compton Gamma-Ray Observatory International Gamma Ray Astrophysics Laboratory (INTEGRAL) Nuclear Spectroscopic Telescope Array (NuSTAR) Swift Gamma Ray Burst Explorer
Famous Telescopes Keck Telescope http://www.jpl.nasa.gov/events/lectures/dec04.cfm Kitt Peak Observatory http://www.jpl.nasa.gov/events/lectures/dec04.cfm
Very Large Array (VLA) radio telescopes