Chapter 5Telescope FundamentalsTelescope FundamentalsRefractionTypes of Reflecting TelescopesResolving PowerInterferometersSlide 8Slide 9Slide 10Slide 11Detecting LightDetecting Beyond the Visible SpectrumSlide 14Hubble Space TelescopeHubble Space TelescopeFalse color images…What is the primary reason scientists put telescopes in space?Challenges and New Directions for Ground-Based TelescopesChallenges and New Directions for Ground-Based TelescopesChallenges and New Directions for Ground-Based TelescopesBasic Optics (Mini-Lab next Wednesday)Basic Optics (Mini-Lab next Wednesday)Basic Optics (Mini-Lab next Wednesday)The HW Quizzes are difficult.The HW Quizzes are helpful.The workload from the HW quizzes is too high.Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Chapter 5TelescopesTelescope Fundamentals•Light-gathering power•The bigger a telescope is, the more photons it collects, and the brighter the image is.•ELT (“Extremely Large Telescope”)•Currently being built in Chile by European countries.•40 meters across.Telescope Fundamentals•Telescopes work by taking a large amount of light and focusing it to a much smaller area.•Lenses refract light, and these are used in “refracting telescopes.”•Mirrors reflect light, and these are used in “reflecting telescopes.”•Most big telescopes are reflecting telescopes.•Difficulties with refracting telescopes•Large diameter lenses are expensive•Large lenses sag, distorting the image•Chromatic abberation: different colors of light focus at different points.Refraction•The bending of light when passing between two different mediums.•Important for refracting telescopes.Types of Reflecting Telescopes•This doesn’t change how the telescope fundamentally works, just changes where you need to be to see the image.Resolving Power•Just collecting a lot of light (light-gathering power) doesn’t make an image sharper, just brighter.•The ability to resolve small details is called the resolving power.•The resolving power is limited by diffraction, which is created when a wave passes through a small opening.•How big does your telescope have to be?To see light of wavelength λ (nanometers) two objects separated by an angular distance of α (arc seconds) you need a telescope at least 0.2*λ/α centimeters in diameter.•Smaller details require larger telescopes.Interferometers•If you enlarge the opening of a telescope, you can limit the diffraction and sharpen the image.•Interferometers measures light simultaneously from multiple telescopes spread apart.•In terms of resolving power, an interferometer acts like a single telescope that is as big as the individual telescopes are far apart.<——OrdinaryTelescope——>InterferometerTo detect a single very dim star, does your telescope need a high light-gathering power or a high resolving power?A. High light-gathering powerB. High resolving powerC. BothD. NeitherTo detect whether there are 2 or 3 bright stars in a closely-packed area, what does your telescope need?A. High light-gathering powerB. High resolving powerC. BothD. NeitherTo detect how many dim stars there are clustered in a small area of the sky, what does your telescope need?A. High light-gathering powerB. High resolving powerC. BothD. NeitherWhat has a better resolving power: a single 100 meter telescope or an interferometer of three 10 meter telescopes arranged in a triangle spaced 5 miles apart?A. Single telescopeB. InterferometerC. Can’t tell from information provided.Detecting Light•In the early days, this involvedactually looking into the telescope and writing down what you saw.•Photographic film was used to record images from the late 1800s until the 1980s. This was not very efficient.•Now, we use “charge-coupleddetectors” and computers to keep track of everything. 20x as efficient as photographic film.•This information can be stored digitally, allowing the use of computer-aided analysis of the images, rather than looking through them all manually.Detecting Beyond the Visible Spectrum•Radio telescopes: Similar to visual spectrum telescopes. Earth-sized interferometers.•Infrared telescopes: Biggest challenge is that the telescope itself emits IR radiation as heat.•X-ray telescopes: X-rays are absorbed by mirrors, but can be reflected at low angles (like skipping rocks on a pond). These telescopes are shaped like funnels.•False-color images•We can’t see X-rays or infrared, so how do we depict what the telescope sees?•Not all photons make it to the Earth’s surface.•Some telescopes need to be in space or high in the atmosphere to actually see what they’re trying to see.•Astronomers submit proposals and sign up for time slots. Space-based telescopes can be operated remotely.Hubble Space Telescope•Most widely known telescope.•Can measure some infrared and ultraviolet, but mostly produces very sharp visible spectrum images (i.e., the pictures are what your eyes would see)•Even though the atmosphere lets in visible light, it still causes blurring, so the HST can get sharper images than a ground-based telescope.Hubble Space Telescope•Most widely known telescope.•Can measure some infrared and ultraviolet, but mostly produces very sharp visible spectrum images (i.e., the pictures are what your eyes would see)•Even though the atmosphere lets in visible light, it still causes blurring, so the HST can get sharper images than a ground-based telescope.False color images…A. Show how the image would look to a human eyeB. Show how the image would look to a color-blind personC. Use color to show the brightness of wavelengths we can’t seeD. Use color to show the wavelength of wavelengths we can’t seeE. Both C and D can be trueWhat is the primary reason scientists put telescopes in space?A. It is cheaper than building one on the groundB. Because space is awesomeC. To show off to scientists in other countriesD. The atmosphere absorbs and/or distorts much of the EM spectrumE. Both A and D are trueChallenges and New Directions for Ground-Based Telescopes•Atmospheric Blurring: variations in the air’s density causes small refractions, making the star appear to twinkle (called “scintillation”)•Scientists pick dry regions with few clouds for ground-based telescopes. Many are on mountain peaks to get above the “haze” of hot locations.•Astronomers can correct for atmospheric blurring through shooting a laser out through