• Amateur astronomers today: Very important in spotting unexpected events that could occur anywhere in the sky (novas and supernovas, nearby asteroids, comets, ...), even backyard astronomers.

  Some History

• First telescopes: 1608 in the Netherlands, magnified 2-3 times objects on land; spread quickly and were available in Italy by 1609.
• First use in astronomy: Galileo improved the design in 1609 until it magnified × 20, then turned it to the sky.
• Major improvements: [Kepler, convex ocular lens and inverted images; Huygens]; Newton, use of mirrors; Tracking; Interferometry; Space telescopes; Adaptive optics.
• Image collection: Photography and digital images (with CCD's), instead of astronomers' eyes; Often produce spectra.
• New windows into the sky: 1950's, Radio astronomy; 1980's, Infrared astronomy; other forms of radiation...

Optical Telescopes: Types

• Refracting telescopes: Use lenses (refraction); Important concepts are focal length and aperture; Largest one is 40" Yerkes Observatory telescope (1897).
• Problems: Chromatic aberration, absorption, weight, machining.
• Reflecting telescopes: Use mirrors (reflection); Lighter, easier to manufacture.
• Earthbound telescopes: Examples are the Mt. Palomar 200" telescope, the Gran Telescopio Canarias, 10.4 m, the Keck I and II 10.0-m telescopes on Mauna Kea, Hawaii; The largest single mirror is the LBT's 8.5 m, in operation since 2005.
• Space telescopes: The 2.4-m (school-bus size) Hubble Space Telescope, since 1990; Can see objects up to magnitude 30! But what makes it so good is its location...
• The future: The James Webb Space Telescope, 100 times as sensitive, 10 times sharper than HST, to be deployed in 2013 (when the HST comes down), 1 million miles from Earth; Ground-based telescopes are also making a lot of progress.

  Telescope-Related Performance Factors

• Light-gathering power: Depends on mirror area (a 10-m diameter mirror collects more than 1 million times more light than a human eye); Long exposure times can help, but may have undesired effects.
• Effect of diffraction: Why do stars often look like they have spikes or circles/halos around them?
• Angular resolution: Depends on wavelength and diameter; [resolution (arcsec) = 0.25 wavelength (microns) / D (m)].
• Values: For our eyes, about 1' at best; For a ground-based telescope, the best so far is around 1.0"; For the HST, around 0.05".*
• Magnification: Depends on the eyepiece or image processing; Although often desired, it is not the primary function of the telescope itself.

Environment-Related Performance Factors

• Atmosphere: Turbulence causes fluctuations in scattered light ("twinkling"), and worsens the resolution; The seeing disk depends on weather and location.
• Light pollution: Caused by scattered artificial light in the atmosphere; Keeps getting worse, but some communities are passing protective laws.
• What to do? Look for high, dry, dark places with steady air. Good ones are Mauna Kea in Hawaii (with 13 observatories and more to come!), the Chilean Andes (with the ESO's VLT and other observatories), the Canary islands, Arizona; And use advanced technology.

  Telescope Control Technology

• Active optics: Computer-controlled real time "star detwinkling" by changing the telescope alignment.
• Adaptive optics: Computer-controlled modifications of the telescope's secondary mirror shape.
• Interferometry: Use several telescopes together; greatly improves the resolution, although it may not increase the light collection efficiency by much. With optical telescopes the technology is still being developed, but it has been used for many years with radio telescopes.

Imaging Technology and Uses of Telescopes

• CCD's: Electronic images with millions of pixels on silicon wafers; Advantages: efficiency, storage and image processing.
• Consequences: Much faster and more efficient observations; easier supernova and minor planet sightings by amateurs.
• Colors: Color information can be obtained by using filters.
• Note: Most professional telescopes have no eyepieces! Can have CCD's or spectrographs.