AST 105 Solar System Astronomy Test-Your-Understanding Questions PDF

AST 105: Solar System Astronomy Fall 2023 Test-Your-Understanding Questions CHAPTER 6 (Astronomy 2e; Fraknoi, Morrison, Wolff) How do reflecting telescopes differ from refracting telescopes?...

AST 105: Solar System Astronomy Fall 2023 Test-Your-Understanding Questions CHAPTER 6 (Astronomy 2e; Fraknoi, Morrison, Wolff) How do reflecting telescopes differ from refracting telescopes? Which type is more commonly used by professional astronomers and why? Reflecting and refracting telescopes differ in many ways Reflecting telescope Refracting telescope Main optical component Mirror Lens Chromatic aberration No Yes Support Can be supported from the back Only from the edges Largest size (single mirror or lens) 8.4 m ~1 m (40 inches) Segmented Yes No Adaptive optics Yes No Due to the size limitations and the issue of chromatic aberration, where light at different wavelengths is focused at different locations, professional astronomers almost exclusively use reflecting telescopes. How much greater is the light collecting area of a 6-meter telescope than that of a 3-meter telescope? Here we want to compare the area of the two telescopes. Recall that the quoted size of a telescope references the mirror’s diameter, 𝑑𝑑, and that the area of a circle is given by: 𝑑𝑑2 𝐴𝐴 = 𝜋𝜋𝑟𝑟 2 = 𝜋𝜋 4 Since we want to compare the sizes, we can use a ratio: 2 𝑑𝑑6m 𝐴𝐴6m 𝜋𝜋 4 𝑑𝑑6m 2 6m 2 = 2 = 𝑑𝑑 = = 22 = 4 𝐴𝐴3m 𝑑𝑑3m 3m 3m 𝜋𝜋 4 Thus, the light collecting area of the 6-meter telescope is 4 times greater than that of the 3-meter telescope. The largest observatory complex in the world is on Maunakea, the tallest mountain on Earth. What are some factors astronomers consider when selecting an observatory site? Don’t forget practical ones. Should astronomers, for example, consider building an observatory on Denali (Mount McKinley) or Mount Everest? Generally, astronomers are looking for sites that are dry and have weather that produces as many clear nights per year. Mountaintops are preferred to reduce the amount of atmosphere that the incoming light from celestial objects must traverse. Although it is preferable to choose sites that are far from the light pollution of big cities, observatories cannot be place in areas that are too remote or else it will be very challenging to have staff and equipment to maintain and operate them. This is why Denali and Mount Everest are not good choices, besides the weather considerations. AST 105: Solar System Astronomy Fall 2023 People are often bothered when they discover that reflecting telescopes have a second mirror in the middle to bring the light out to an accessible focus where big instruments can be mounted. “Don’t you lose light?” people ask. Well, yes, you do, but there is no better alternative. You can estimate how much light is lost by such an arrangement. The primary mirror (the one at the bottom in Figure 6.6) of the Gemini North telescope is 8 m in diameter. The secondary mirror at the top is about 1 m in diameter. Use the formula for the area of a circle to estimate what fraction of the light is blocked by the secondary mirror. We can approach this question in a similar fashion to the previous one comparing telescope sizes and use a ratio: 2 𝑑𝑑1m 𝐴𝐴1m 𝜋𝜋 4 𝑑𝑑1m 2 1m 2 1 = 2 = = = = 0.015625 ≈ 1.6 % 𝐴𝐴8m 𝑑𝑑8m 𝑑𝑑8m 8m 64 𝜋𝜋 4 In other words, the secondary mirror in this configuration only blocks about 1.6 % of the incoming light which is an acceptable loss given the light collecting area of an 8-meter telescope. What is the advantage of an equatorial mount over and altazimuth mount? With an equatorial mount, the fork structure that holds the telescope is lined up with the direction of the NCP (or SCP in the southern hemisphere) which means that once the telescope is pointed at a target, it only needs to rotate along the direction of Right Ascension, the target’s Declination will not change. In contrast, an a telescope mounted on an altazimuth mount needs to constantly move in both altitude and azimuth to track a target across the night sky.