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Chapter 5 Observatories & Space missions. 5.1 Optical Observatories. 5.1 a: Observatory Sites: One limitation: the time needed for optics to reach equilibrium shape when exposed to severe temperatures. Southern Hemisphere. Dry High Dark Steady atmosphere.
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5.1 Optical Observatories • 5.1 a: Observatory Sites: • One limitation: the time needed for optics to reach equilibrium shape when exposed to severe temperatures. • Southern Hemisphere. • Dry • High • Dark • Steady atmosphere. • Adaptive optics modify the shape of the mirror to cancel the blurring effects of the atmosphere.
5.1 Optical Observatories • 5.1 b: The new Generation of Optical Telescopes • Twin Keck telescopes (10 m each), mirror made of many smaller segments. • Four 8-m telescopes (Very Large Telescope VLA) in Chile. • With segmented mirrors, there is no limit on size • 30-m plan (California) • 100-m plan European Southern Observatory.
5.2 Wide-Field Telescopes • 5.2 a: Schmidt Telescopes • Combines the best features of reflector with that of refractor. • Spherical large mirror. • Thin lens (correcting plate) • FOV 7 deg. Instead of 2 arc min for the 5-m telescope.
5.2 Wide-Field Telescopes • 5.2 b: The Forthcoming Large Survey Telescope • LSST • 6.5 m mirror • Surveys the whole sky every week. • Data will be available on the internet.
5.3 Hubble Space Telescope (HST) • 2.4 m mirror • Launched in 1990 • 2 billion $ cost • 559 km orbit above Earth • Maintenance trip every 3 years • 3 advantages: • Above Earth’s atmosphere, resolution only limited by mirror size (0.1 arc sec). • HST can detect fainter objects, darkness • UV & IR are detectable.
HTS facts • Launch date April 24, 1990, 8:33:51 am • Launch vehicle Space Shuttle Discovery • Mission length 20 years, 7 months • Mass 11,110 kg • Type of orbit Near-circular low Earth orbit • Orbit height 559 km (347 mi) • Orbit period 96–97 minutes (14-15 periods per day) • Orbit velocity 7,500 m/s • Acceleration gravity 8.169 m/s2 • Wavelength Optical, ultraviolet, near-infrared • Diameter 2.4 m • Collecting area 4.5 m2 • Focal length 57.6 m
HST Maintenance missions • Dec 1993 • Feb 1997 • Dec 1999 • Mar 2002 • May 2008 • Retirement 2011
List of Space Telescopes • http://en.wikipedia.org/wiki/List_of_space_telescopes
5.3 a: The Next Generation Space Telescope • NGST • Planned 8-m mirror • Reduced to 6-m mirror (better, cheaper, faster) • It would go to one of Lagrangian points. • There, it will not go through day & night cycles. It will be able to observe for a longer fraction of time.
Lagrange Points • Lagrange points are locations in space where gravitational forces and the orbital motion of a body balance each other. • There are five Lagrangian points in the Sun-Earth system and such points also exist in the Earth-Moon system. http://www.esa.int/esaSC/SEMM17XJD1E_index_0.html
5.4 Recording the Data • 1- Films (silver emulsion – chemical reaction – negative) • 2- Electronic devices (Photometry) • 3- CCD charge-coupled device: • when light hits the surface of the chip, electrons are released. • Discrete area of a chip is called pixel • HST has 800 x 800 pixels array
5.6 Observing at Short Wavelengths • Ordinary films can be used • HST is the largest for UV observation • X rays pass through mirrors ? • X rays can still bounce off a surface if they strike at very low angel (Grazing Incidence).
5.7 Observing at Long Wavelengths • IR image of Earth • HST observes at IR
Radio Telescopes • Radio waves cause electrical changes in antennas. • Large dishes are needed for 2 reasons: • Larger surface area more sensitive • Larger dish better resolution
Radio Telescopes • 1-m optical telescope is 2 million wavelengths across • 100-m radio telescope is 1000 wavelengths across if used to detect radio waves 10 cm in wavelength. • Radio telescopes used to study millimeter length radio waves do not have to be as physically large as telescopes meant to study longer wavelengths.
