Telescope Terminology

Transcription

1 Suffolk County Community College AST 103 Student name: Telescope Terminology T he history of mankind's understanding of the universe may be broken into two separate and distinct periods: B.T. (which stands for "Before the Telescope") and A.T. ("After the Telescope"). The invention of the telescope was the key that unlocked the vault of the cosmos. Though it is still rich with intrigue, the universe of today is no longer one to be feared, as it was to ancient cultures. Instead, we sense that is it our destiny to study, explore, and embrace the heavens. We are now able to spot phenomena that could not have been imagined just a generation ago. Such is the marvel of the modern telescope. Unfortunately, no one knows exactly who made the first telescope. Many authorities say it was Hans Lipperhey, an eyeglass maker in the Netherlands, who accidentally stumbled upon the idea around Records indicate that he first held two lenses in line with each other and noticed that they seemed to bring distant scenes closer. Subsequently, Lipperhey sold many of his telescopes to his government, which recognized its military importance. While it is certain that Galileo Galilee, an Italian physicist and astronomer, did not invent the telescope, he is credited as being the first to document its usefulness for studying the universe. With his first telescope in 1609, Galileo discovered craters on the moon, four satellites orbiting Jupiter, and that the planet Venus goes through phases just like our moon. Though he was ridiculed by his contemporaries and persecuted for heresy, Galileo's observations changed mankind's view of the universe as no single individual ever had before or has since. Since the time of Lipperhey and Galileo, the telescope has gone through some powerful changes. Countless improvements and many different designs have evolved in the ensuing 400 years. Although all telescopes do not look the same, they serve the same two purposes: 1) to collect more light than the human eye can, thereby making faint objects appear brighter, and 2) to make small things appear larger, thereby increasing resolution and fine detail. In doing these, a telescope brings the image of whatever it is aimed toward to a focus, and in the process, enlarging the view. How this is done is where the variation in design comes in. Today's astronomers use one of three basic telescope designs: the refractor, the reflector, and the compound telescope. The following illustration shows telescopes based on a refractor (lens), a reflector (mirror), and a compound (mirror/lens) system. Today, Lipperhey's and Galileo's first telescopes are called refractors, recognizable by their long, thin tubes. A large lens (the objective) is found in the front end of the telescope tube, while the observer looks through a small lens (the eyepiece) at the back end of the tube. Light passes through the objective lens and is bent, or refracted, to a focus (where all of the light rays come together into a single point). A second lens, called the eyepiece, is placed just beyond the point of convergence to refocus, and simultaneously enlarge, the image. Instead of a lens in the front of the tube, a reflector uses a large primary mirror buried deep down toward the bottom of the tube to gather light from a target and bring it to a focus. Though it may look flat, the primary mirror is actually concave; that is, it is curved inward like a bowl. 1

2 Compound Telescope 2

3 Many different reflector designs have been devised over the years. The most popular type today was invented by Sir Isaac Newton in 1681, and so is called a Newtonian reflector. In a Newtonian, light reflects from the primary mirror to a small, flat mirror (the secondary mirror or diagonal) that is tilted at 45. The light then bounces off of the secondary at a 90 angle, out through a hole in the side of the top part of the tube, and into an eyepiece. The third type of telescope is a hybrid instrument called a compound telescope that combines aspects of a refractor and a reflector. Light passes through a large front lens (referred to as a corrector plate) and on toward a primary mirror at the back of the tube. Bouncing off the primary, the light reflects back toward the front of the tube, where a convex-shaped secondary mirror returns it toward the primary. A small hole in the middle of the primary mirror lets the light pass through and out the back of the telescope, to an eyepiece. The most common type of compound instrument on the market today is the Schmidt-Cassegrain Telescope, or SCT. Refractors, reflectors, and compound instruments all share many common functions and terminology. For instance, a telescope's size is always referred to by its aperture, the diameter (usually expressed in inches or millimeters) of the instrument's main optic. A 3.1-inch (80- millimeter, or 80mm) refractor has an objective (or front) lens 3.1 inches across, while the mirror in a 4-inch (100mm) reflector measures 4 inches in diameter. The length of a telescope is determined by its focal length, the distance from the objective lens or primary mirror to its focal point, where the light rays converge. As with aperture, focal length is commonly expressed in either inches or millimeters. 3

4 If you are familiar with photography, then you also know that camera lenses are referred to by two numbers, such as 50mm f/1.8 or 200mm f/4. The first number in each pair specifies the lens focal lengths, while the f/ number refers to the lens focal ratio. Telescopes share this terminology. To calculate a telescope's focal ratio, divide its focal length by its aperture. For instance a 6-inch telescope with a focal length of 48 inches has a focal ratio, or f-number, of f/8, while an 8-inch telescope with a focal length of 40 inches has a focal ratio of f/5. A telescope's power or magnification is also a ratio of two numbers. Calculating magnification is simple, but first, we need to know the telescope's focal length as well as the focal length of the eyepiece to be used. Different eyepieces have different focal lengths, but all are usually specified in millimeters. Look on the barrel of an eyepiece. You might see a number like "25mm" or "10mm." Those numbers are the eyepieces' focal lengths in millimeters. To determine magnification, divide the telescope s focal length by that of the eyepiece. First, be sure that all the units of measure are the same, either inches or millimeters; the units cannot be mixed. For example, a 6-inch f/8 telescope has a focal length of 48 inches or 1,200 millimeters. Therefore, in this example, a 25mm eyepiece yields 48x (read 48- power), while a 12mm eyepiece produces 100x. Conversion To convert inches to millimeters, multiply the number of inches by Therefore, 6 inches is the same as 152 mm, since 6x25.4=152. If you want to see an object twice as big, you use an eyepiece with a focal length half as long. But it's not always that easy. Many people think that a telescope s power or magnification is its single, most important feature. In a word: NO! Sure, you need to magnify the image to see detail, but too much magnification will work to you detriment. The problem is that a telescope will gather just so much light, based on the instrument s aperture. As magnification rises, that light is being spread over an increasingly large area. As this happens, image brightness dims and focusing becomes more difficult. Beyond a certain point, an image becomes a blurry blob. How much magnification is necessary? An often-cited rule is not to exceed 60 power per inch of aperture, though this will vary depending on the quality of telescope optics as well as the steadiness of the earth s atmosphere. With poor-quality optics or unsteady sky conditions (e.g., stars appear to twinkle fervently), this number may be reduced by as much as 50 percent, while top-quality optics and/or exceptionally stable sky conditions may make it possible to exceed the 60-power rule by 50 percent. But never use magnification just for its own sake. Use the lowest magnification required to see what you want to see. 4

5 Exercises 1. The photos below show three different types of telescopes. Based on the discussion in class, write each telescope's optical design (name) on the line below its photo. Also label the location of the objective lens/primary mirror and eyepiece. A B C Type Type Type 2. The table below includes optical specifications for each of the telescopes in question 1. Fill in the missing data based on the discussion in class as well as earlier in this lab. (Remember your units of measure!) Telescope Aperture Focal Length Focal ratio (f/ #) Inches mm Inches mm A 10 inches 50 inches 1250 B 120 mm f/8.3 C 6 inches 1,500 mm 5

6 3. As mentioned earlier, a telescope's magnification depends on two numbers: the telescope's focal length and the eyepiece's focal length. Using the information in class as well as in this exercise, calculate the magnification in each of the telescopes above for the eyepieces listed in the following table. Copy the telescope focal lengths from question #2 into the table below before completing your calculations. [Note the addition of a fourth telescope, D, shown at right).] Telescope Telescope focal length (mm) Eyepiece #1 focal length Eyepiece #1 magnification Eyepiece #2 focal length Eyepiece #2 magnification Eyepiece #3 focal length Eyepiece #3 magnification A B C 25 mm 17 mm 10 mm D 450 mm 4. Examine each of the photos of the planet Saturn below. If photo #1 was taken at a magnification of 50x, estimate the magnifications of the other images. #1 50x #2 #3 #4 #5 Hint: magnification is simply a ratio. Measure each image, calculate their relative scale (size) to image #1, and then multiply that factor by #1's stated magnification. 5. If you were using telescope A to view Saturn, what eyepiece focal lengths (rounded to the nearest whole millimeter) would you need in order to achieve these magnifications? View #1 View #2 View #3 View #4 View #5 Eyepiece focal length 6