- Amateur astronomers today: Very
important in spotting unexpected events that could occur anywhere
in the sky (novas and supernovas, nearby asteroids, comets, ...), even
- 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
- Image collection: Photography
and digital images (with CCDs), instead of astronomers' eyes;
Often produce spectra.
- New windows into the sky: 1950s,
Radio astronomy; 1980s, 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
8.4 m, in operation since 2005.
- Space telescopes: The 2.4-m
(school-bus size) Hubble Space Telescope, since 1990; It can see objects
up to magnitude 30! What makes it so good is its location. Next will be the Gaia Space Telescope in 2013 and James Webb Space Teescope in 2018. But the technology for ground-based telescopes
is also making a lot of progress...
Wide telescopes are good for two different reasons.
- Light-gathering power: Depends
on mirror area; 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
- 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.
- 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 many observatories on it!), the Chilean Andes
(with the ESO's VLT and other observatories), the Canary islands, Arizona;
And use advanced technology.
- Active optics: Computer-controlled
real time "star detwinkling" by changing the telescope
- 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 CCDs or spectrographs.
|* Like reading a book from half a mile away!
page by luca bombelli <bombelli at olemiss.edu>,
modified 3 jul 2013