Atmosphere • Filters Part of the Electromagnetic Radiation • to the Earth's Surface – Optical Window 400 – 800 Nm
Atmosphere
• Filters part of the electromagnetic radiation
• To the Earth’s surface – optical window 400 – 800 nm – radiowindow 1mm – 15 m
• Disturbances – refraction (light bends in the atmosphere) – skintillation (twinkling star) – seeing
Observational tools
• Optical telescopes – Observations of visible light
• Radio and submillimeter telescopes – Observations of the radio region (radio window) – Satellites and probes – Other parts of the electromagnetic radiation – planet, asteroid and comet probes
Telescope types
Refracting telescope
• A lens acts as the objective, front side of the telescope
• First refracting telescopes at the start of the 17th century – chromatic aberration, focal points for different wavelengths don’t connect • correction: achromatic lenses
• Structural problem: supporting the lens – Largest diameter 102 cm
• Astrographs: size of the image field 5o – mapping of large areas
Reflecting telescope
• A paraboloid mirror (main mirror) covered with a thin aluminum layer collects the light
• Main mirror reflects the light back to the incident direction – Image at the focal point (prime focus) • Difficult to reach – Usually a secondary mirror that has been placed before the prime focus -> directs light somewhere else • Image at the secondary focus • The shape and position of the secondary mirror defines the focus type
• Coma error
Telescope foci
Prime focus
• Location where the reflected light forms an image • Observations difficult because focus is in front of the main mirror • Placement of large equipment difficult
Newton focus
• Secondary mirror – plane mirror – at 45° angle – reflects light out coming from the main mirror outside of the telescope • pros: no prime focus • cons: difficult to reach, placement of equipment
Cassegrain focus
• Secondary mirror – surface hyperboloid – light reflects out from the hole in the middle of the main mirror – pros: focus easy to reach • cons: support structures must be strong enough
Caude focus
• Secondary mirror – Structure same as cassegrain focus – New mirrors that reflect the light out parallel to the axes, light directed at the wanted location • Pros: Observational tools can be placed to a permanent position • Cons: light lost in reflections
Nasmyth focus
• Nasmyth focus is the same as coudé focus – equatorial coudé – azimuthal nasmyth • Focus named after James Nasmyth (1808-1890)
Telescope mounts
Equatorial mount
• To direct the telescope, two axes are needed • Equatorial mount 1. First axis parallel to the rotation axis of the Earth 2. Second axis parallel to the equator • Pros: Easy to follow a target, rotate one axis • Cons: Heavy structures (expensive!)
Azimuthal mount
• Azimuthal mount 1. First axis is vertical 2. Second axis is horizontal (azimuth) • Pros: No massive support structures • Cons: Target followed by rotating both axes, image field must be rotated, all with a right speed, done by a computer • 1975 first big telescope (BTA telescope at Caucasus)
Coelostat telescope
• Set telescope • Light directed into the telescope using rotating mirrors • Used in solar telescopes • Another method is to move the secondary mirror – Arecibo telescope – Hobby-Eberle telescope
Radio telescopes
• Radio astronomy was born in 1930 • Structure same as in optical telescopes • Studies the radiation emitted by molecules • antenna lines – bad resolution of a single telescope – better resolution with antenna lines
Wavelengths
Gamma region
• Wavelength shorter than 10-11 m • First observations at the end of 1960s – OSO 3 observed gamma radiation of the Milky Way – Observational targets – pulsars – gamma bursts (GRB targets)
X-ray
• Wavelength 0.01 – 10 nm – hard zone 0.01 – 0.1 nm – soft 0.1 – 10 nm • First observation 1962 – mapping from 1970s (HEAO1 and 2) – now Chandra and XMM-Newton • Observational targets – Compact stars – Sun
UV
• Wavelength 10 – 400 nm – EUV 10 – 91.2 nm • Sun UV radiation in 1946 – satellites starting from 1962 • Observational targets – comets – interstellar matter – star surface layers
Visible
• Wavelength 400 – 800 nm • Observational tools – naked eye – telescopes – detectors • Observational targets – planets – stars – interstellar gas – galaxies
Infrared
• Wavelength 800 nm – 1mm – Near-infrared, wavelength less than 5 μm – submillimeter region, wavelength 0.1 – 1 mm • Observations carried out from the ground and space • Observational target – interstellar dust – stars – cosmic background radiation
Radio
• Wavelength over 1 mm • First observations of the radio radiation of space in1932 (Karl Jansky) • Significant wavelengths – hydrogen 21 cm line – carbon monoxide(CO), multiple lines at 1-3 mm regions • Observational target – molecular clouds – pulsars – cosmic background radiation
Other forms of radiation
• Cosmic radiation – energetic particles – mostly protons and alpha particles • Neutrinos – weak interactions – no mass? – observed from the Sun and SN1987A • Gravitational radiation – two masses in an accelerated motion