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ls6 - class 6 tues feb 12 summary for lecture warning this...

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class 6. tues feb 12 summary for lecture. warning: this is not supposed to be a substitute for reading the textbook. in textbook: chapter 16 ------ Lect. Part A. -- continue on chapter 16 -- telescopes. last time -- 16.1 and 16.2, we skipped 16.3 for now, and went into 16.4. let's now skip 16.5 and 16.6 and head into 16.7 and 16.8. Refractors: generally speaking, these have not been in serious astronomical use for at lest 50 years and really more like 75 years. You need a big lens, and the bigger the lens, the harder it is to keep it the right shape! Biggest: Yerkes 40-inch. fig 16.27 in your book. Next biggest: a solar telescope in the Canary Islands that was recently built. But it's stationary! check out table 16.1 in your book for a list of comparisons betwe refr and refl. Reflectors: Generally you want to minimize the number of optical elements in the way (mirrors + lenses) before the photons get to your detector. more optical elements means more photons get absorbed which means harder to detect the object. But nowadays you have what's called a 'secondary' -- secondary mirror usually that will throw the light somewhere else. Prime Focus tel -- parabola, and just a detector at the prime focus. in the old days you had a person up there! handout #15. compare to fig 16.28 in your book. you were way up high. nowadays this doesn't really happen anymore. Newtonian -- parabola + flat secondary -- invented by Newton generally used only for amateurs, with eye or light equipment, since focus is back up at the top of the tube. Cassegrain -- parabola + hyperbola secondary -- makes long focal length in a short tube. i.e. here's how you can change focal length of the primary mirror. where do you put your eye/camera/instrument? behind the primary mirror. There's a hole in the primary for this.
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