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1 Lecture 22 Telescopes January 10b, 2014 2 The Hubble Space Telescope 3 Why Use a Telescope? • All astronomical objects are distant so a telescope is needed to – Gather light -- telescopes sometimes referred to as “light buckets” – Resolve detail (angular resolution) – Magnify an image (least important of the three) • Uses combination of lenses and/or mirrors 4 Refracting Telescope • Refraction = as light passes from one medium to another (e.g. air to glass) it is bent • Light is gathered and focused by a curved lens. 5 Refracting Telescope • First telescopes were of this type • Not made for astronomical use any more – Very difficult to make large, defect-free lenses – Weight of large lenses makes them deform over time. 6 Reflecting Telescope • A curved mirror is used to collect and focus the light. 7 Reflecting Telescope • Used in modern telescopes – Mirror can be supported from the back. – Do not need large, defect free glass since surface is coated with reflective material. – Can be shaped to minimize aberrations – Can be rapidly deformed to compensate for atmospheric turbulence (adaptive optics) – Can be manufactured at much larger scales than refracting telescopes 8 Focal Length • Distance from lens or mirror to focus = focal length. • Rarely will put an eye or detector at the prime focus (image is too small) • An eyepiece (usually a lens) will be used to magnify the image. 9 Eyepiece and Magnification focal length of objective Magnificat ion focal length of eyepiece 10 What should the eyepiece focal length be for a telescope with a 5.0-m focal length objective if a magnification of 250× is desired? A. 2.0 cm B. 5.0 m C. 50 m D. 1,250 m 11 What should the eyepiece focal length be for a telescope with a 5.0-m focal length objective if a magnification of 250× is desired? A. 2.0 cm f objective M B. 5.0 m f eyepiece C. 50 m f 5.0 m objective D. 1,250 m f eyepiece M 250 0.020 m 2.0 cm 12 Telescope Design • For a reflecting telescope, a secondary mirror is used to reflect the image to a detector outside of the telescope. Prime Focus Newtonian Cassegrain 13 Observing the Image • On research telescopes astronomers rarely look through the telescope • Detector is put at the focus to record a digital image. • Data are processed on a computer CCD Chip 14 Telescope Properties: Gathering Light • The larger the area of the primary mirror, the more light can be collected and the fainter the object we can detect. Light Gathering Power Diameter 2 15 Gathering Light Sometimes many smaller mirrors are combined to make one large-area mirror Keck telescopes: primary mirrors made of 36 hexagonal mirrors 16 How much more light can a 12-inch reflecting telescope gather compared with an 8-inch reflecting telescope? A. 1.5× B. 2.25× C. 8× D. 144× 17 How much more light can a 12-inch reflecting telescope gather compared with an 8-inch reflecting telescope? new 4 D old 4 D A. 1.5× B. 2.25× C. 8× D. 144× 2 new 2 old 2 12 in 2.25 8 in new 2.25 old 18 Telescope Properties: Angular Resolution • The smallest separation in angle which can be distinguished by the telescope • Angular resolution is limited by atmospheric blurring and light diffraction by the primary lens or mirror 19 Angular Resolution due to diffraction • Absolute limit of angular resolution: 2.5 10 5 D = best angular resolution in arcseconds = wavelength (meters) D = diameter of mirror (meters) • We want to be small • Shorter wavelengths = better resolution • Larger mirror = better resolution 20 Angular Resolution 21 What is the maximum angular resolution of the UWSP 16 inch (0.406 m) diameter telescope? Assume a wavelength of 500 nm. A. B. C. D. 50 arcsec 25 arcsec 4.1 arcsec 0.31 arcsec 22 What is the maximum angular resolution of the UWSP 16 inch (0.406 m) diameter telescope? Assume a wavelength of 500 nm. A. B. C. D. 2.5 10 50 arcsec D 25 arcsec 9 500 10 m 5 4.1 arcsec 2.5 10 0.406 m 0.31 arcsec 0.308 arcsec 5 23 Angular Resolution due to atmospheric blurring • Resolution often limited by motions in the atmosphere (“twinkling”) – Need site with calm, dry weather, little atmosphere above the telescope to reduce twinkling effect. – Adaptive optics: sensors monitor distortions due to atmosphere and correct the shape of the mirror 10 to 100 times per second 24 Adaptive Optics Object viewed through typical telescope Object viewed with adaptive optics 25 Spectroscopy • Light coming though telescope is separated by prism or diffraction grating to produce a spectrum. 26 Observing at Other Wavelengths • Wavelengths other than visible are very useful since they are often produced by different objects or processes than optical light • Atmosphere blocks some types of light – Low opacity: Optical and Radio – Medium opacity: Infrared and UV – High opacity: Gamma Rays, X-rays & some UV 27 Infrared Telescopes • Telescope design much like optical telescope, but with different detector. • Infrared light can pass through dust • Used to observe star formation, center of galaxies, low T objects (i.e. planets) • Best if telescope is placed above much of the atmosphere. 28 IRAS 29 Radio Telescopes • • • • Can observe day or night Not affected by Earth’s atmosphere Radio light can pass though dust in space Because wavelength is long, we need large telescope to get good resolution 30 64 m telescope at Parkes Obs. in Austrailia 305 m telescope at Arecibo Observatory in Puerto Rico 31 Interferometers • More than one radio telescope is used to increase resolution. – Creates a large effective diameter – Image made only after much computer processing Very Large Array in New Mexico 32 Ultraviolet, X-rays and Gamma Rays • All blocked by atmosphere • Telescopes must be above atmosphere Compton Gamma-ray Observatory 33 Hubble Space Telescope Chandra X-ray Telescope Spitzer Infrared Telescope 34 Sky at Many Wavelengths