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Astro Week 8 Lecture 2.docx

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Patrick Mc Graw

Astro Week 8 Lecture 2 Review: Telescope Basics  Powers of a telescope:  Light-gathering power (depends on size) o Gathers more light than our eyes are able to gather o Bigger telescopes can gather more light  Resolving power: Ability to distinguish fine details: depends on optical quality, diffraction limit, and the atmosphere. o diffraction = ways waves bend, spreads out the light waves  Magnifying power: actually the least important. Depends on focal length of eyepiece compared to objective. o Can change by changing the eyepiece  Diffraction limit: a basic limit on resolving power-- depends on wavelength compared to the size of the telescope. Telescope Basics  Refracting telescopes use an objective lens to focus light and produce an image, and an eyepiece lens to magnify the image and make it visible.  Reflecting telescopes have a curved primary mirror instead of an objective lens. o primary mirror does main job of focusing lights  The basic ideas are the same for both--- both have a part that does the main job of focusing light, and an eyepiece to magnify the image. Modern Telescopes  Modern research telescopes are reflecting telescopes.  Largest visible-light telescopes currently in use have a diameter of about 10m.  Most capture images with Charge coupled devices (CCDs): essentially a digital camera. Review: Things that limit resolving power  Optical quality: defects in lenses and/or mirrors. o Optical quality can be improved by making better lens/smoother mirror  Diffraction: depends on the ratio of the wavelength of light to the aperture size. The bigger the telescope compared to the wavelength, the better. o Diffraction can be improved only by making telescope bigger  Atmospheric distortion (seeing). The same thing that makes stars twinkle also makes them blurry. What about seeing: several solutions to the problem  Minimize distortion by placing telescopes in dry climates and on mountaintops.  Put telescopes in space above the atmosphere.  Adaptive optics technology (developed in the 1990's) uses a computer and small motors to compensate for atmospheric distortion by continuously changing the shape of the secondary mirror. o Take mirror in telescope and distort it to counteract the distortion of atmosphere Adaptive Optics  Requires a bright source in a known location for the computer to focus on and calculate the correction.  Can use a known star: Guide star  Or, create an artificial guide star by bouncing a laser beam off of the upper layers of the atmosphere. Observing non-visible light  Earth's atmosphere is transparent to visible light and radio waves.  With adaptive optics, we may soon be able to get visible-light images from the ground that are as clear as we get from the Hubble Space telescope.  But, to observe other types of light that don't get through the atmosphere, we still need telescopes in space. Why look at other types of light?  Infrared and radio waves can pass through clouds of gas and dust that are opaque to visible light. This can allow us to see more of our own galaxy, for example.  Different types of sources emit different ranges of wavelengths. For example:  Objects that are cooler than stars (like planets, or warm dust clouds) emit mostly infrared  Different types of sources emit different ranges of wavelengths. For example:  Cosmic microwave background shows us the last remaining heat from the Big Bang and gives us clues to the early history of the Universe.  Different types of sources emit different ranges of wavelengths. For example:  Some radio sources include active galactic nuclei, pulsars (neutron stars).  Different types of sources emit different ranges of wavelengths. For example:  Many stars emit at least some X-rays (including the Sun.) Matter falling into black holes emits a lot of X-rays.  Short gamma-ray bursts from distant galaxies may come from the explosions of very large d
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