Telescopes: peeking at our wonderful, mysterious, universe time-traveling to a distant past 2LT Van Henson CAP California Wing, Gropup 2 Squadron 156 Aerospace Education January 20, 2021 Medulla Nebula Supernova Remnant -- image credit Russell Croman https://www.rc-astro.com/ • • • • • Medulla Nebula Supernova Remnant the remains of a star that exploded around 10,000 years ago looks about the size of the full moon but is extremely faint over 130 hours of exposure time through two different telescopes special filters that isolate light from sulfur (yellow), hydrogen (red), and oxygen (blue) Contents 1. What is light? 2. Reflection and Refraction 3. Prisms, rainbows, and the like 4. Basic telescopes and what they are good for: 1. Refractor 2. Reflector 3. Optical aberrations 5. Twinkle, Twinkle, little star 6. The big kids: a) b) c) Hubble Adaptive Optics Guide Star During the planetary conjunction of Jupiter and Saturn a few weeks ago, I dragged out my old 2.5” refracting telescope. This is approximately what I saw: During the planetary conjunction of Jupiter and Saturn a few weeks ago, I dragged out my old 2.5” refracting telescope. This is approximately what I saw: What is light? • The superficial answer: a narrow band in the electromagnetic spectrum What is light? • Whoa! Not so fast! It is considerably more complex! • Light is a wave, yes. Interference patterns from the double slit experiment show that… • But it also acts like separate “particles” of light make up the waves, these are called photons Hologram of a single photon reconstructed from raw measurements (left) and theoretically predicted (right). Credit: FUW Reflecting light • For our purposes, light is a wave that can be described by rays, that is, lines perpendicular to the wavefront • When the wavefront hits a surface, it acts like a point-source for a new wave. Interference of these waves with the original produce coherent reflection • Law of specular reflection: the angle of incidence equals the angle of reflection: Reflecting light • Specular reflection: Light reflects off smooth and shiny objects (glass, polished metal, mirror, still water) at angle of reflection equal to angle of incidence • Diffuse reflection: Light reflects off uneven objects (rippled water, frosted glass, etc.) at angle of reflection equal to angle of incidence everywhere relative to the normal to the surface Refracting light • The speed of light is different in different media (air, vacuum, water, oil, etc). When light passes from one medium into another, it is refracted such that the wavefronts change direction, and the “rays” are bent: • Going from high-speed medium (air) to lower speed (water) the rays are bent toward the normal θinc > θrefr • From lower to higher, θinc < θrefr • This phenomenon produces some cool effects and proves to be exceptionally useful! Refracting light • But we can use these reflection and refraction properties Refracting lenses to focus light • But we can use refraction to build lenses that focus light: Monochromatic aberrations affect all wavelengths the same1. Defocus 1. Defocus 2. Spherical aberration 2. Spherical aberration 3. Coma 3. Coma 4. Astigmatism 4. Astigmatism 5. Field curvature 5. Field curvature Refracting light of different wavelengths • The different colors we see are different wavelengths of the EM spectrum. In air or vacuum, they all travel at the same speed, c. • In slower media the different colors of light travel at slightly different speeds, and therefore refract at different angles. • This effect is how prisms separate out colors of light, and how rainbows occur • This breaking of white light into a rainbow is called dispersion Chromatic aberrations affect different wavelengths in differing degree 1. Perfectly focused ring 2. Axial aberration: different wavelengths (colors) focused at differing “depths” along line perpendicular to the focal plane (distortion of lens varies with wavelength) – typical with long focal length lenses 3. Transverse aberration: different wavelengths (colors) focused on different positions on the focal plane – typical of short focal length lenses So, at long last: telescopes!! • What kinds? And what are they good for? The refractor telescope • Simplest design • Galileo didn’t invent the refractor, he built one from a description of a Belgian inventor. • What did Galileo see in his telescope? • Galileo saw the craters of the moon. • He saw Jupiter and the four largest moons of Jupiter. • He saw “appendages” on Saturn • He saw phases of Venus The refractor telescope The Newtonian reflector telescope • Newton didn’t invent it • Galileo knew about it • Lenses were easier to make than mirrors • Newton built the reflector telescope to prove his theory that white light is composed of spectrum of colors • Refractors suffered chromatic aberrations; Newton believed the lenses were at fault • Wanted to build a telescope without lenses The Newtonian reflector telescope The Newtonian reflector telescope A Newtonian reflector telescope, circa 1873 Note the large structure to allow observer access to the eyepiece The entire assembly rotates around two axes, parallel and perpendicular to the Earth’s axis When we look at the heavens, we’re looking back in time • Light travels through space at 186,000 miles per second. • Light-second is the distance light travels in one second (186,000 miles) • Light-minute is the distance light travels in a minute • Light-year is the distance light travels in one year (approx. 5,865,696,000,000 miles) • So when we see light from stars and galaxies we are seeing what was going on some time ago! • Suppose the North Star actually went nova and blew up 300 years ago. • We won’t see that for another 20 years! “What I like is the way they twinkle.” We can correct the aberrations, BUT… twinkle, twinkle! Any optical telescope on earth “sees” through this variable atmosphere; whatever it looks at, the image is marred by this phenomenon… How can we fix this? Idea! Get the telescope outside the atmosphere! • The Hubble space telescope • Launched in 1990 • Has beamed awesome images to us The “Pillars of Creation” • in the Eagle Nebula • columns of cold gas driven by stellar winds • Columns about 4 light-years tall… Idea! Get the telescope outside the atmosphere! • The Hubble space telescope • Launched in 1990 • Has beamed awesome images to us Jupiter’s “Northern lights” • An aurora Jupiter’s atmosphere • in the ultraviolet band Idea! Get the telescope outside the atmosphere! A “Seyfert” spiral galaxy • 150 million light-years away • In the constellation of Libra • Unusually bright center, causes by “hungry, supermassive black holes,” billions of times the size of the sun, pulling in gas and dust Great! What could go wrong? • The Hubble space telescope • Launched in 1990 • Has beamed awesome images to us • Like any man-made object, it gets old, wears out, needs repair • Two space-shuttle missions to repair it • And we no longer have the space shuttle • So... Idea! If we can’t keep fixing the telescope, let’s fix the atmosphere! • Let’s use adaptive optics to remove the effect of the atmosphere on the image! Does it work? The key is knowing what the image should look like! Focus the telescope on a bright star; that should always be a point. Keep it looking like a point; everything else will be in focus! What if there is no star in view?? No star in view? No problem! Just make one. • Enter “Guide Star.” a) b) c) d) Train your telescope at the part of the sky you want to see Shine an intense, very narrow laser beam up from the telescope It forms a “dot” on the ozone layer Use the dot as if it were a star, and focus the adaptive optics on keeping it a point source. Sources • http://www.hyperiontelescopes.com • https://en.wikipedia.org/wiki/Optical_aberration • https://theconversation.com/hubble-in-pictures-astronomers-top-picks • https://www.nasa.gov/image-feature/the-pillars-of-creation • https://www.bluelightlbvlockingglasses.com.au • https://www.zmescience.com/science/what-is-photon-definition-04322/ • https://giphy.com/gifs/light-G7Pc0fNwuVzYk • http://opticampus.opti.vision/cecourse.php?url=nature_light/ • https://byjus.com/reflection-of-light • https://www.explainthatstuff.com/howmirrorswork.html • https://dynamicscience.com.au • https://Mediagiphy.com • https://wikiwand.com] • https://www.wired.com/2012/08/photography-focus/ • https://www.britannica.com/technology/lens-optics • https://www.celestron.com • https://mocomi.com/why-do-stars-twinkle/ • https://www.nasa.gov/mission_pages/hubble/story/index.html • https://theconversation.com/hubble-in-pictures-astronomers-top-picks-40435
