Lecture #8: Astronomical Instruments Astronomical Instruments: Optics: Lenses and Mirrors. Detectors. Ground Based Telescopes: Optical, Infrared, and Radio. Space Based Telescopes. Spacecraft Missions. The Main Point Astronomers are constantly trying to maximize the resolution of their observations, but there are fundamental limits because of wavelength, telescope size, and the blurring effects of our atmosphere. Astro 102/104 1 Astro 102/104 3 Some Basics Astronomers rely on instruments that collect, magnify, and/or disperse light. How much light can be collected is determined by the aperture of the collecting instrument. A combination of lenses or mirrors are used to focus light onto a detector. Dispersion is obtained using a prism or grating. Detectors include photographic plates, photosensitive resistors, and silicon semiconductors. Astro 102/104 4 Familiar Optics Telescopes operate on the same principles as your eyes. Astro 102/104 5 1
Resolution Spatial Resolution is a measure of the smallest details that can be identified by an instrument. Resolution is limited by: Diffraction of light by lenses and mirrors: Depends on the wavelength. Depends on the aperture or diameter of the instrument. The"seeing" or amount of turbulence in the atmosphere. Resolution typically expressed in arcseconds: 1 = 60 arcminutes = 3600 arcseconds (3600"). Typical human resolution: ~ 60" (~1/60 ). Astro 102/104 6 Resolution: Theory The diffraction limit of any optical instrument is: θ ~ 2.5x10 5 ( λ / D), where θ = the best possible resolution, in arcseconds. λ = the wavelength of the observation. D = the diameter of the instrument's aperture. Examples Best resolution of the Hubble Space Telescope at 500 nm? θ ~ 2.5x10 5 ( 500x10-9 m / 2.4 m) = 0.05" What size telescope needed to obtain θ = 0.001" at 500 nm? D ~ 2.5x10 5 (λ / θ) = 2.5x10 5 (500x10-9 m / 0.001) = 125 m (!) Astro 102/104 7 } must be in the same units How much resolution is needed? Weasel answer: it depends! Depends on the angular separation between the things you are trying to detect/discern. Angular separation: depends on actual separation. depends on distance away. α = ( s / 2πd) x 360 The angular separation must be greater than the resolution to detect features of interest. Astro 102/104 8 Resolution example: A vehicle is approaching at night. At what distance will you be able to tell if it's a car or a motorcycle? Recall: for humans, θ ~ 60. Want α > θ (angular separation > resolution of your eyes) α=(s/2πd) x 360 =60" =0.017 Typical headlights, s ~ 1.5 m. So d = (360 x 1.5 m) / (2π x 0.017 ) ~ 5 km ( ~ 3 miles). Astro 102/104 9 2
Another Resolution Example What is the smallest feature that you can discern (α=θ) on the Moon using (a) your eyes? s =2πdα/360 = 2π(380,000 km)(0.017 )/360 = 113 km (70 miles) (b) binoculars? (5 cm aperture) θ ~ 2.5x10 5 ( λ / D) = 2.5x10 5 (500x10-7 cm / 5 cm) = 2.5"=0.0007 s =2πdθ/360 = 2π(380,000 km)(0.0007 )/360 ~ 5 km (~ 3 miles) (c) The Fuertes telescope on campus? (30.5 cm aperture) s ~ 750 meters (~ 2500 feet) BUT: θ ~ 0.4", so s really ~ 1875 m for θ ~ 1" (d) The Hubble Space Telescope? s ~ 100 meters! Astro 102/104 10 Resolution: Real World For telescopes on the Earth, the resolution is limited by the shimmering and turbulence of our atmosphere (what astronomers call "seeing"): Turbulent "cells" (atmospheric motion) exist on scales of ~ 1 arcsec and greater. Size of telescope that gets θ ~ 1 at 500 nm: 12.5 cm (!) Telescopes larger than this can't do any better than ~ 1" resolution, despite what theory predicts. Then WHY build larger telescopes? Astro 102/104 11 Light-Collecting Area The size of a telescope is usually described by the diameter of the primary mirror. The light-collecting area is proportional to the square of the diameter. A 10-m telescope has 100 times the lightcollecting power of a 1-m telescope. Can detect much fainter objects. Astro 102/104 12 Detectors First astronomical detector: the human eye: But our eyes (& brains) don't store reliable long-term records of the observations. Our eyes also have short integration times--not very sensitive to very faint signals. Our eyes also only detect visible wavelength light. Next big leap: photographic film or plates: Provide a lasting record of observations. But films are inefficient--only ~1% of photons contribute to the resulting image. Astro 102/104 13 3
Modern Detectors Astronomers now use electronic detectors: Most common: CCD (charge-coupled device): Built out of silicon semiconductor material. Photons enter the silicon and initiate energy level transitions that are recorded as a voltage within a circuit: up to 90% efficiency! The voltage is proportional to the number of incident photons. Silicon is sensitive to photons from ~400 to ~1000 nm. Millions of CCD detectors are assembled into an array on a single chip to construct images; each detector is a picture element or pixel. Similar chip technology used for digital cameras! Other types of substrates besides silicon are used to detect photons at other wavelengths (InGaAs, HgCdTe, InSb,...) Astro 102/104 14 Refracting Telescopes First telescopes used lenses to refract (bend) and focus light: refracting telescopes. Refractors provide good magnification, but have some distinct limitations: Long and cumbersome tubes. Hard to accurately polish both surfaces of very large lenses. Some distortion of colors occurs in lenses. World's largest refractor: 40 inches. Galileo's telescope (1610) Astro 102/104 15 Cornell's Fuertes Observatory 12-inch refractor (~1921) Yerkes Observatory 40-inch refractor (1897) Reflecting Telescopes Modern large telescopes use curved mirrors to focus light: reflecting telescopes. Easier to build very large mirrors than large lenses. Easier to build supporting structures and mounts. binoculars Larger collecting area means studying fainter objects. No distortion of colors. Astro 102/104 16 Prime Focus Newtonian Cassegrain Astro 102/104 17 4
Palomar Observatory 5-meter reflector, California (1948) Very Large Telescope 4 x 8 m, Chile (2000) Astro 102/104 18 Keck Observatory 2 x 10 meter, Hawaii (1993) Radio Telescopes Radio waves can be focused and collected too (it's light after all). Recall resolution: θ ~ λ / D So for best resolution (lowest θ) at radio wavelengths (large λ) need large D! Example: To achieve θ ~ 60" (human eye) at λ=5 cm: D=200 m! Astro 102/104 19 NSF's Arecibo Observatory in Puerto Rico, the largest telescope in the world (300 m diameter!), operated by Cornell University. Observing Sites Where you put your telescope is important! Lights, clouds, water vapor, other gases hamper astronomy. Strong desire for isolated, high, dry observing sites. Even so, many observations can only be done from space. Astro 102/104 Green Bank Telescope (100 m), West Virginia. 20 Astro 102/104 21 5
Airborne Observatories Telescopes in Space Avoids problems of ground based sites. Full access to the entire electromagnetic spectrum. True diffraction-limited performance. BUT: Expensive, risky, infrequent,... Astro 102/104 22 Astro 102/104 23 Space Missions Ultimate way to improve resolution: Go there! Humans have sent a small armada of spacecraft out into the solar system over the past ~40 years: 50+ missions completed prior to 1989. About 30 missions operating right now. About 20 missions approved but not yet launched. About 50 more missions under detailed study. Huge list at http://spacescience.nasa.gov/missions/ We will study results from many of these missions. Astro 102/104 24 Spacecraft Orbits Same principles as planetary orbits. Substantial force needed to reach Earth orbital velocity (~8 km/sec). Nearly 9,000 tracked objects in Earth orbit! (NORAD) Even more force needed to reach Earth escape velocity (~ 11 km/sec). Tens of spacecraft have been sent out to explore! Use clever gravity assist tricks to visit multiple bodies. Astro 102/104 25 6
Summary Lenses or mirrors are used to collect and magnify light for astronomical observations. Resolution is proportional to wavelength and inversely proportional to aperture diameter. Light-collecting area is proportional to the square of the aperture diameter. Two kinds of telescopes: refractors and reflectors. Many spectacular telescope facilities on Earth. More and more observatories in space! Missions to the planets are the ultimate field trip! Astro 102/104 26 Next Lecture... Categorizing the Planets. Distances and a Solar System Scale Model. Overview of Planetary Surfaces and Interiors: Density and Interior Composition, Structure. Planetary Surface Processes. Dating Planetary Surfaces: Relative Dating: Counting Craters. Absolute Dating: Radioactive Decay. Reading: Chapters 7, 9.1. Astro 102/104 28 7