• to gather light. Astronomical objects are, in general, quite faint. A bright star, like Vega, produces a flux of about 3 million optical photons per square centimeter per second. This may sound like a lot, but remember that Vega is one of the brightest stars in the sky. A star of magnitude 15 will produce about 3 optical photons per square centimeter per second.

  The total number of photons a detector sees is the product of this flux, the area of the detector, and the integration time.

  The iris of your eye has a typical diameter of 0.1 cm (1 mm), although it can expand to about 0.5 cm when dark adapted. The area of your eye ranges from about .01 to .25 square centimeters. Your eye does not integrate, rather it detects individual photons with a time resolution of about one tenth of a second. When you see Vega, your eye registers about 100,000 photons per second continuously. At V=6, the naked eye limit, the eye sees about 300 photons per second continuously.

  Telescopes provide a large collecting area, with the area proportional to the square of the diameter. A 6 inch telescope collects about 900 times as much light as the naked eye; the Keck 10 meter telescope collects about 4 time the light that the 5 meter Mt. Palomar telescope collects. In telescopes, bigger is better for light gathering power.

  The more photons you collect, the fainter you see.

• to provide good resolution.
  Resolution is the ability to separate 2 nearby objects. The resolution of a telescope is determined by the size of the aperture and the wavelength of the light. Because light is a wave, if diffracts (bends) around objects, including the aperture (opening) of the telescope. Point sources, like stars, always look fuzzy if viewed at sufficiently high resolution.

  

  The resolution, in seconds of arc, is given by the formula to the left. Theta is the resolution. lambda is the wavelength of the light. d is the diameter of the telescope. Lambda and d must have the same units (e.g, both in Angstroms, or both in centimeters).

  The image to the left illustrates a diffraction-limited image. This image of the nearby (7.2 parsecs) M2.5 star Gl 623 and its low mass companion Gl 623 (to the right) was taken with the Faint Object Camera on the Hubble Space Telescope (the press release is available at the HST Office of Public Outreach). The contrast is greatly enhanced to show the faint companion. The circular rings are the diffraction rings of the HST aperture. The separation between the stars is about 0.25 arcsec (2 AU at 7.2 pc); the orbital period is 4 years.

  In practice, turbulence in the Earth's atmosphere (called seeing) limits resolutions to about 0.25 seconds of arc. This can be accomplished with a 24 inch telescope at optical wavelengths. To do better, you need to get above the atmosphere.

Optical Telescopes

• Reflectors (use mirrors)
• Refractors (use lenses)

Mounts
• Equatorial
• Alt-Az

Foci
• Prime
• Cassegrain
• Coude
• Newtonian
• Gregorian

Tycho's observatory Stjerneborg on the island of Hven.

Mauna Kea Observatory

Why put telescope on mountaintops?

• Less atmosphere to look through.
• Improves seeing
• Improves IR transmission
• Generally mitigates light pollution
  

  1908	1988
  Los Angeles as seen from Mt. Wilson

Telescopes on Airplanes: SOFIA

Telescopes in Space: Hubble Space Telescope. Permits diffraction-limited observations and observations in the ultraviolet and X-rays.

• The Arecibo radio telescope, carved into a mountain valley in Puerto Rico, is the world's largest radio telescope. The dish is 1000 feet across. It looks straight up, but can be pointed within about 20 degrees of the zenith by moving the secondary mirror.

  Pictures from Arecibo.

• The Robert C. Byrd Green Bank Telescope, in Green Bank, WV (image courtesy of NRAO/AUI). This is the world's largest fully steerable telescope. The dish is 110 meters across. It is taller than the Statue of Liberty.
• The Very Large Array, on the plains of San Augustin, west of Socorro NM, consists of 27 25-m dishes. The individual telescopes can move along railroad tracks, to give the spatial resolution of a telescope 36 km (22 miles) across.

X-ray Telescopes
X-rays are penetrating radiation: under most conditions they will not reflect off a mirror, but will be absorbed. The will reflect at small angles, or grazing incidence. .

One of the four mirrors for the Chandra X-ray Observatory.

Since X-rays do not penetrate the atmosphere, you need to do this from a satellite. The Chandra Observatory (artists conception).

For more details on X-ray astronomy see my Astronomy Open Night presentation on the history of X-ray Astronomy

Spectroscopy

Examples of Spectra:

Continuous Spectrum
Emission Line Spectrum
Absorption Line Spectrum
Another Absorption Line Spectrum