The Very Large Array (VLA) in New Mexico Observations at wavelengths other than visible light are revealing previously invisible sights Visible light image radio wavelength image Astronomers use different instruments to look at light of different wavelengths - sometimes, we even have to go above Earth’s atmosphere. SOFIA - the Stratospheric Observatory for Infrared Astronomy Observations at other wavelengths are revealing previously invisible sights UV Ordinary visible infrared Map of Orion region Consider Orion as Seen in Different Wavelengths of Light! http://www.cnn.com/2001/LAW/02/20/scotus.heatdetector.01.ap/index.html High above Earth’s atmosphere, the Hubble Space Telescope provides stunning details about the universe Hubble Space Telescope Views of Orion Nebula showing stars hidden in clouds http://oposite.stsci.edu/pubinfo/pr/97/13/A.html Yesterday’s Sun as seen in visible light from Earth and from space in X-rays by satellites High Energy Gamma Rays - Compton Gamma Ray Observatory (GRO) Satellite The Sky’s emission of Gamma Rays But, we receive GRBs from every direction !! The fact that GRBs come from every direction imply that GRBs don’t come from our galaxy, but from other galaxies spread in every direction! Three main functions of a telescope • brighten (called light gathering power) • see fine detail (called resolution) and least important, • magnify magnification = (objective lens focal length / eyepiece lens focal length) magnification = (objective lens focal length / eyepiece lens focal length) A larger objective lens provides a brighter (not bigger) image Two Fundamental Properties of a Telescope 1. Angular Resolution • smallest angle which can be seen = 1.22 / D where = wavelength; D = diameter of the aperture Angular Resolution • The ability to separate two objects. • The angle between two objects decreases as your distance to them increases. • The smallest angle at which you can distinguish two objects is your angular resolution. Two Fundamental Properties of a Telescope 1. Angular Resolution • smallest angle which can be seen = 1.22 / D 2. Light-Collecting Area • The telescope is a “photon bucket” A = (D/2) 2 D A Angular Resolution: 1.22 where is in radians. D radians 180 deg so 57.30 deg/radian 1.22 57.30 D 69.91 69.91 deg 3600 D 251676 D arcsec D hence: deg seconds deg Parts of the Human Eye • pupil – allows light to enter the eye • lens – focuses light to create an image • retina – detects the light and generates signals which are sent to the brain A camera works in the same way where the shutter acts like the pupil and the film or CCD acts like the retina! Lenses bend Light Focus – to bend all light waves coming from the same direction to a single point Light rays which come from different directions converge at different points to form an image. Telescope Types • Refractor – focuses light using lenses • Reflector – focuses light using mirrors – used exclusively in professional astronomy today A refracting telescope uses a lens to concentrate incoming light Similar to a magnifying glass Refracting telescopes have drawbacks • Spherical aberration Too spherical Refracting telescopes have drawbacks • Spherical aberration • Chromatic aberration Special achromatic compound lenses and lens coatings can often fix this aberration Refracting telescopes have drawbacks • Spherical aberration • Chromatic aberration • Sagging due to gravity distorting the lens • Unwanted refractions • opaque to certain wavelengths of light Refractor Yerkes 40-inch telescope; largest refractor in the world Reflector Gemini 8-m Telescope, Mauna Kea, Hawaii Reflectors MMT – Mt. Hopkins, AZ SUBARU – Mauna Kea, HI Reflecting telescopes use mirrors to concentrate incoming starlight Newtonian Focus Prime Focus Cassegrain focus coude’ focus Astronomer’s face two major obstacles in observing the skies • Light Pollution from Cities Tucson, Arizona in 1959 and 1980 Astronomer’s face two major obstacles in observing the skies • Light Pollution from Cities • Effects of Twinkling from Earth’s atmosphere Rapid changes in the density of Earth’s atmosphere cause passing starlight to quickly change direction, making stars appear to twinkle. Irregularities in the atmosphere’s density cause the light to not arrive at the telescope as plane parallel waves. Atmosphere Lens Advanced technology is spawning a new generation of equipment to view the universe • CCDs (charge-coupled devices) • Large telescopes on remote mountain tops – Maunakea – Cerro Pachon in Chile • Orbiting space observatories • Adaptive Optics to counteract the blurring of Earth’s atmosphere Adaptive Optics (AO) • It is possible to “de-twinkle” a star. • The wavefronts of a star’s light rays are deformed by the atmosphere. • By monitoring the distortions of the light from a nearby bright star (or a laser): – a computer can deform the secondary mirror in the opposite way. – the wavefronts, when reflected, are restored to their original state. AO mirror off AO mirror on • Angular resolution improves. • These two stars are separated by 0.38 • Without AO, we see only one star. A.O. Movie… Instruments in the Focal Plane How do astronomers use the light collected by a telescope? 1. Imaging – – use a camera to take pictures (images) Photometry measure total amount of light from an object 2. Spectroscopy – use a spectrograph to separate the light into its different wavelengths (colors) Imaging • Filters are placed in front of a camera to allow only certain colors to be imaged • Single color images are superimposed to form true color images. A little color theory: So why do TV’s use RGB? Shouldn’t they use RYB? Spectroscopy • The spectrograph reflects light off a grating: a finely ruled, smooth surface. • Light interferes with itself and disperses into colors. • This spectrum is recorded by a CCD detector.