Scholars Online Astronomy - Chapter 6: Optics and Telescopes
Reading: Astronomy, Chapter 6: Optics and Telescopes
Study Notes: notes on your assigned reading from the text
- Section 6.1 In refracting telescopes, light entering a lens bends to the focal point, with is the focal distance away from the center of the lens. Curvature determines where the focal point is, and the distance between the focal point and the observed image determines the magnification. Refracting telescopes use lenses to focus the light. The bigger the focussing surface, the more light is gatthered; the greater the magnification, the dimmer the object viewed. There are limitations on the ability of materials to focus clearly: most lenses must be made in layers of different materials to avoid problems with chromatic aberration.
- Section 6.2 Reflecting telescopes use mirrors and lenses to focus light. The advantage of reflecting telescopes is that they fit in smaller spaces, so larger mirrors can be used to gather light. Both reflectors and refractors have to be mounted so that they can be moved to track the moving night sky.
- Section 6.3 Observatories are usually located away from cities and high in the mountains to avoid difficult observing conditions caused by light pollution, air pollution, and humidity. Large sites allow for the use of multiple telescopes placed some distance apart; using computers to control these telescopes results in observations made as though a single telescope the size of the distance between the multiple mirrors had been used.
- Section 6.4 Astronomers use various devices to capture the images of the celestial phenomena they need to observe. Charge-coupled devices record the intensity and frequency of incoming light very accurately, transmitting the information electronically to the observer, who may be located in the same building, across the compound, or on the other side of the world. Astrophotography using film and special lenses is largely the province of amateur astronomers.
- Section 6.5 Spectroscopes are also used to survey the spectrum of objects; these scan low to high ranges of the electromagnetic spectrum and record intensities. They do not produce an image of the object!
- Section 6.6 Ranges of electomagnetic radiation below and above the visible range are also studied. Radio and infrared telescopes allow us to view cool objects. Ultraviolet, X-ray, and gamma-ray telescopes allow us to view extremely hot objects.
- Section 6.7 Better resolution is obtained by telescopes orbiting above the earth's atmosphere for two reasons. Pollutants in the atmosphere and weather conditions block light from reaching earth-bound telescopes. But even the clearest atmosphere still contains elements that absorb light in particular frequencies, so that sources emitting light in these ranges are very faint at the earth's surface. Many orbiting telescopes have been launched, most with instruments dedicated to specific narrow ranges, such as the Compton Gamma Ray Observatory, or the International Ultraviiolet Explorer. The most famous general purpose space telesecope is the Hubble Telescope or HST.
Key Formulae to Know
- Magnification of telescope M:
- Light Gathering Power P:
- Angular Resolution θ:
Read the following weblecture before chat: Telescope Basics
- Planetarium exercise: If your planetarium program models different telescopes, select at least three (use a low powered binocular, a 4" and an 8" refractor if possible) and compare the field of view, magnification, and quality of image provided by each. Use each to view the Moon (pay attention to details along the terminator and crater definition), Saturn (pay attention to the rings!), and Jupiter (pay attention to the moons!). Identify the smallest crater on the Moon where you can see both sides as distinct. Estimate the resolution of each telescope.
If you are not able to use a planetarium program use the telescope simulator at Stelvision. Enter the diameter of the telescope for
- binoculars: 60 (about 2.3" -- this is he smallest value allowed)
- small telescope: 100mm (about 4")
- large telescope: 203mm (about 8")
With each telescope, view the moon. Remember that the Moon is about 31 arcseconds. Use this information to estimate the field of view diameter.
Use the detailed simulator with your 203mm telescope setting and vary your eyepieces. How does magnification change?
- Optional Websites: Compare the visible light view of the "Cigar galaxy" (M82) with the infrared view taken by NASA's Spitzer Space Telescope. What can you see in the white-light picture that isn't visible in the infrared? What can you see in the infrared that isn't visible in the white-light picture? How does our theory of light emission explain the difference? What do the differences tell you about the composition, shape, and temperatures of this galaxy?
Chat Preparation Activities
- Essay question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer before chat starts for this session.
- Go over the list of Key Words and Key Ideas at the end of the chapter. If you don't remember the definition of the key word, review its use (the page number on which it is explained is given).
- Read through the Review Questions and be prepared to discuss them in class. If any of them confuses you, ask about it!
- Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.
- Required: Complete the Mastery exercise with a passing score of 85% or better.
- Go to the Moodle and take the quiz for this chat session to see how much you already know about astronomy!
Read through the lab for this week; bring questions to chat on any aspect of the lab, whether you intend not perform it or not. If you decide to perform the lab, be sure to submit your report by the posted due date.
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