3 - 1 10/5/09 ASTRONOMY 113Laboratory Lab 3: Telescopes Perhaps no scientific instrument is as closely associated with a field of study as is the telescope to astronomy. And yet arguably the telescope is also the most misunderstood of scientific instruments, so closely associated is the telescope with the word “magnification”. First and foremost, an astronomical telescope is a “light bucket” designed to collect light. A celestial object, such as a star, emits light in all directions. After traveling for thousands of years across billions of kilometers, a tiny portion of that light rains down on the hemisphere of the Earth facing that star. Almost all of that light does nothing more than be absorbed by the atmosphere or the ground, ever so slightly heating up the Earth. An infinitesimal portion of the star’s light strikes the lens or mirror of a telescope and is directed into an eye, or onto a piece of film, or through a spectrograph. The bigger the diameter of the lens or mirror, the more light that is guided to the detector, and the more knowledge that is gained about the Universe. This is the basic fact that underlies astronomers’ never-ending quest for telescopes of larger diameter. Considered broadly, telescopes are all around you in optical devices of other names – cameras, eyes, binoculars, eyeglasses, and more. The details of the optics in each of these telescopes dictates how effectively the collected light is used. The goal of this lab is to give you a feel for optics through recreation of that marvelous device of Galileo, the telescope.3 - 2 10/5/09 Before You Come to Class ... Read the lab completely. Your time in the lab is best used observing the "sky", not reading this manual. Complete the pre-lab assignment. Bring to class this lab manual, your lab book, a pencil or erasable pen, a straight edge, and a scientific calculator. Schedule This lab is designed to be completed in two lab sessions.3 - 3 10/5/09 Section 1 – The Simple Telescope – A Primer To make a telescope all you need is one lens1, appropriately curved. This objective lens receives light from a distant object – perhaps a student across the classroom, a sailboat on Lake Mendota, the Moon, the planet Jupiter, or a galaxy - and creates an image of that object behind the lens on a flat plane called the "Image Plane''. Before working with celestial objects, let’s consider two headlights on a car on the far side of Lake Mendota at night. The car is far enough away that each headlight appears simply as a point of light. In Figure 1 the headlights are shown as points A and B. To start, Figure 1 shows only the light rays coming from headlight A. There are several important things to notice in this simple picture. a) The light rays that strike the lens are bent (refracted) in such a way that they all pass through the same point in space. This place is the focal point for headlight A. If you were to put a piece of paper here, you would see a point of light on the paper. This point of light is called the real image of headlight A. b) The light rays don’t stop at the focal point. c) Most of the light from the headlight misses the lens and thus is never sent to the focal point. 1 Either a lens or concave mirror will do, and in fact most astronomical telescopes use a mirror. Since in this lab you will be using lenses, we will continue to talk in terms of lenses. * objective lens real image of headlight A A* * B Figure 1 image plane3 - 4 10/5/09 Of course, light is also arriving at the lens from headlight B. As shown in Figure 2, because the light from B is coming from a different direction, the focal point of B is located at a different place in space. If you were to hold a piece of paper at the appropriate distance behind the lens, you would see two points of light on the paper, one at the focal point of headlight A and one at the focal point of headlight B. Now, instead of headlights, imagine a fluorescent Texaco sign on the other side of Lake Mendota. Just like the two headlights, light from every point on the sign comes to a focus at a different place behind the lens. And the ensemble of all these points of light become an image of the Texaco sign! This image behind the objective lens is a two-dimensional reproduction of the object suspended in space, and as before is called a real image. Perhaps this phrasing might now make more sense – optically speaking, the real image of the Texaco sign is no different than the sign itself. The real image can be looked at and examined with no loss of information compared to looking at the sign itself (with the exception of optical imperfections). In a sense, the sign has been “beamed” to the focal point of the lens! Once a real image is created by the objective lens, it can be carefully inspected in a variety of ways. For example, one could project the real image onto a piece of white paper, which in fact is an excellent and safe way to observe the Sun with a telescope. Alternatively, the real image can be examined by eye, perhaps with a magnifying glass. More traditionally the magnifying glass is called an eyepiece, and most likely this is how you have used telescopes. However, it is almost never the way professional astronomers use a telescope, simply because the eye is a very insensitive light detector. One could also record the image by placing photographic film or a light-sensitive silicon chip (called a charge coupled device; CCD) at the position of the image. In this case the telescope is a high-end camera – indeed, your digital camera is simply a single lens and a CCD. Or the spectrum of the light from the object could be measured by placing the entrance of a spectrograph at the position of the image. • Why do astronomers prefer these latter two uses of the telescope and almost never look at the image with their eye? objective lens * * real image of headlight A A* * B Figure 2 real image of headlight B3 - 5 10/5/09 Section 2 – Becoming Familiar with Lenses FINDING IMAGES, OR SEEING SPOTS Your lab group has been supplied with an assortment of lenses, lens holders, a white screen, and a lens rail onto which the