I: TELECSOPE IN GENERAL

1. Telescope: An instrument for gathering and focusing light.
2. In general, constructed with one bigger lens for refractor, called the objective lens, or one bigger mirror for reflector, called primary mirror, and a smaller lens called eyepiece.
3. The objective lens or primary mirror is to use for focusing the image and eyepiece is to enlarge the focused image.
4. It makes the distant objects look larger, nearer and brighter.
5. Major types: Refractor, Newtonian Reflector and Schmidt-Cassegrain.
6. Major mounting: Azimuth, Equatorial and Dobsonian.
7. Other astronomical telescope (mostly non-optical): HST, radio-telescope, X-ray telescope (Chandra), etc.

II: REFRACTOR

1. Using convex lenses to gather light.
2. Law of refraction: Light ``bend'' when pass through different mediums.
3. Not invented by Galelio, but he's first person who used telescope to see the sky.
4. Problem of chromatic aberration for single lens. To correct this aberration, make multiple combination lenses as objective, make the price higher.
5. World largest refractor: 40-inch (1m) telescope in Yerkes Observatory. (//astro.uchicago.edu/yerkes)
6. Binoculars is one special type of refractor telescope, with the combination of two identical refractors.

III: REFLECTOR

1. Using concave mirror to gather light.
2. Law of reflection: The light reflected same angle as the incident light.
3. Invented by Newton. Most popular reflector is called Newtonian.
4. No chromatic aberration problem, but has other minor aberration.
5. Eyepiece is located at side of telescope for Newtonian.
6. World largets reflector: 10m Keck Telescope in Mt. Mauna Kea, composed with 36 hexagonal segments.

IV: PROPERTIES AND ABILITIES OF TELESCOPE

1. Diameter:
   • Also known as aperture.
   • Very important for telescope because it determines the collecting area and the resolution of telescope.
2. Collecting Power:
   • Telescope will have higher collecting power if it has bigger collecting area.
   • Area is proportional to Diameter-squared:
     A = pi D² / 4
   • Higher collecting power means telescope can gather more light, or more photon per second, then the image will be brighter and be able to see fainter objects.
3. Resolution:
   • Also called resolving power.
   • The ability to see fine details or to reveal the separation between two close objects from a distance.
   • Most important property of telescope, because for higher resolution, the telescope can see finer structures.
   • Resolution depends on the wavelength of light and the aperture.
   • If theta is the angle of resolution, or resolving angle (measured in second of arc), lambda is the wavelength (in meter) and D is the aperture of telescope (in meter), then the relationship is:
     theta = 2.5 X 10⁵ lambda / D
   • The larger the aperture of a telescope, the smaller the resolving angle, then the better or stronger resolving power.
4. Magnification:
   • The ability to make image larger.
   • Important in daily usage but less important in astronomical observations.
   • Defined as the ratio of the focal length of objective lens or primary mirror (fo) and the focal length of the eyepiece (fe):
     M = fo / fe
   • Eg: only have 10m telescope, but no 100X telescope.

Back to main page.