Observing Projects
Obtain a telescope during the daytime along with several eyepieces of various focal lengths. If you can determine the telescope’s focal length, calculate the magnifying powers of the eyepieces. Focus the telescope on some familiar object, such as a distant lamppost or tree. DO NOT FOCUS ON THE SUN! Looking directly at the Sun can cause blindness. Describe the image you see through the telescope. Is it upside down? How does the image move as you slowly and gently shift the telescope left and right or up and down? Examine the eyepieces, noting their focal lengths. By changing the eyepieces, examine the distant object under different magnifications. How do the field of view and the quality of the image change as you go from low power to high power?
On a clear night, view the Moon, a planet, and a star through a telescope using eyepieces of various focal lengths and known magnifying powers. (To determine the locations in the sky of the Moon and planets, you may want to use the Starry Night™ program if you have access. You may also want to consult such magazines as Sky & Telescope and Astronomy or their Web sites.) In what way does the image seem to degrade as you view with increasingly higher magnification? Do you see any chromatic aberration? If so, with which object and which eyepiece is it most noticeable?
Many towns and cities have amateur astronomy clubs. If you are so inclined, attend a “star party” hosted by your local club. People who bring their telescopes to such gatherings are delighted to show you their instruments and take you on a telescopic tour of the heavens. Such an experience can lead to a very enjoyable, lifelong hobby.
Use the Starry Night™ program to compare the field of view, magnification, and quality of image provided by different optical instruments when observing various celestial objects. Open Favourites > Explorations > Field of View to examine several solar system objects. Click the Find tab on the left side of the View window. Remove any text from the edit box at the top of the Find pane, click on the magnifying glass icon at the left side of the edit box, and select the Orbiting Objects item from the dropdown menu that appears to bring up a list of solar system objects. Select each of the following objects in turn: the Moon, Jupiter, and Saturn. Double-click on the name of the selected object in the Find pane to center it in the view. Click the down arrow to the right of the Zoom panel in the toolbar and select first the 7 × 50 Binoculars from the dropdown menu. Note the change in the quality of the view of the object under observation. Open the Zoom dropdown menu again and select the 25mm Plossl eyepiece on a Sample 4″ refractor, followed by the 25mm Plossl eyepiece on a Sample 8″ Schmidt-Cassegrain telescope, followed by the 10mm Plossl eyepiece on an 8″ Schmidt-Cassegrain. Before finding the next object, use the Zoom dropdown menu to return the field of view to 120°. (a) Describe the change in the level of detail visible in the observed object as the field of view decreases. Open the Find pane again. Click on the magnifying glass icon on the left-hand side of the edit box at the top of the Find pane and select Messier Objects from the dropdown menu that appears. From the Messier Objects listed in the Find pane, double-click the entry for Ring Nebula to center this object in the view. If nothing appears in the view, open the Options side pane and click the checkbox to the left of Messier Objects under the Deep Space layer. Once more, use the Zoom dropdown menu to examine this object through the various instruments. (b) Assuming that the 8″ Schmidt-Cassegrain telescope has a focal length of 2000mm, what is the magnification produced by the 25mm and the 10mm eyepieces? (c) What is the ratio of the magnification of the 10mm eyepiece to that of the 25mm eyepiece on the 8″ Schmidt-Cassegrain telescope? (d) What is the nominal field of view of the 10mm and 25mm eyepieces on the Schmidt-Cassegrain telescope (shown next to the eyepiece name in the Zoom dropdown menu)? (e) What is the ratio of the field of view of the 10mm eyepiece to the 25mm eyepiece? (Note that this is the inverse ratio to that which you calculated for the magnification yield of the two eyepieces.) (f) Describe the relationship between magnification and field of view for a particular telescope.
Use Starry Night™ to explore the difference between magnification and resolution. Select Favourites > Explorations > Resolution. The view is centered upon the full Moon as it might appear to the naked eye from Earth. Right-click the image of the Moon and select Magnify from the contextual menu to see the Moon as it might appear in good binoculars or a small telescope. (a) Describe the difference between the naked-eye image of the Moon and the magnified image. (b) What happens to the field of view when the image is magnified? (c) Zoom in to a field of view about 10′ wide (this is equivalent to increasing the magnification of the telescope). How does this further magnification affect the quality of the image, that is, the ability to distinguish close details on the lunar surface? (d) Zoom in to a field of view about 3′ wide. Does the clarity of the image improve, deteriorate, or remain unchanged with this further magnification? (e) Slowly Zoom in further to a field of view about 1′ wide. What happens to the quality of the image with this increase in magnification (zoom)? (e) Explain your observations based on the concept of resolution.
Use Starry Night™ to explore the effect of light pollution on the night sky. This exercise will also help you to determine the brightness of the faintest stars that are likely to be visible to the naked eye from your location under your present sky conditions and to estimate the fraction of possible stars that you can see with the unaided eye. This exercise is best done outdoors on a dark, clear night with a laptop computer and Starry Night™ set to function in night vision mode (select Options > Night Vision). Click the Home button to ensure that the view is correct for your present location and time. Open the Options side pane and expand the Local View layer. Turn the Daylight option off and then place the cursor over the words Local Light Pollution and click the Local Light Pollution Options… button that appears. This will open the Local View Options dialog window. Move this dialog window to the side of the view. In the Local View Options dialog window, click the checkbox to the left of the Local Light Pollution option to turn this feature on. You can now use the slide bar to adjust the Local Light Pollution level until the view matches your night sky. When you are satisfied that the view matches your sky, click on OK to dismiss the Local View Options dialog window. Be sure not to change the Zoom from the standard field of view, which is 100° wide. Move the cursor over some of the stars that appear on your screen to display their properties in the HUD (Heads-up Display). (If necessary, open the Preferences dialog from the File menu in Windows or Starry Night menu on a Mac and add the apparent magnitude option to the Cursor Tracking (HUD) options.) (a) Make a note of the apparent magnitudes of some of the faintest stars in this view and compare these values to the faintest apparent magnitude of about +6 that can be seen under ideal conditions by the human eye. (Reminder: Magnitude values increase as stars become fainter.) (b) The second goal is to estimate what fraction of the visible stars you can see under these conditions, compared to the total number of stars you might see under ideal conditions. Open the FOV pane and click the Add… button in the Other (This Chart) layer. Select Rectangular… from the popup menu and use the FOV Indicator dialog window to create a rectangular indicator with a width and height of 10° and click OK. If necessary, expand the Other (This Chart) layer and click the checkbox for the 10° indicator you created. This will display the indicator in the center of the screen. Without changing the Zoom from the standard 100°—wide field of view, use the hand tool to drag the view so that a region of sky with a reasonable number of stars lies within the central 10° indicator. Count and record the number of stars in this square with your present setting. Now open the Local View Options dialog window again and set the Local Light Pollution to less (which essentially gives you ideal conditions) and repeat the count of visible stars. Divide the first number you count by the second. What fraction of the stars that would be visible under ideal conditions were you seeing? (c) To see how much light pollution occurs in large cities, adjust the slide bar for Local Light Pollution in the Local View Options dialog window to the far right for maximum light pollution and repeat the star-counting within the same limited sky region. Compared to viewing under ideal conditions, what fraction of the stars are observers in large urban centers seeing?
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Collaborative Exercises
Stand up and have everyone in your group join hands, making as large a circle as possible. If a telescope mirror were built as big as your circle, what would be its diameter? What would be your telescope’s diffraction-limited angular resolution for blue light? Would atmospheric turbulence have a noticeable effect on the angular resolution?
Are there enough students in your class to stand and join hands and make two large circles that recreate the sizes of the two Keck telescopes? Explain how you determined your answer.