LECTURE 12: Telescopes

• What are the two important functions of a telescope?
• What is the difference between refracting and reflecting telescopes? What are the advantages of reflecting telescopes?
• What determines a telescope's light gathering power?
• What limits the angular resolution of most ground-based telescopes?
• How is light from a telescope detected and recorded?
• What are the advantages of putting a telescope in space?

BASICS

• Two basic functions of a telescope:
  • Gather lots of light.
  • Magnify -- i.e., spread out in angle.
• First allows you to detect fainter (usually more distant) objects or to examine light from brighter objects in more detail.
• Main motivations for very large telescopes.
• Second allows you to examine structure of objects, e.g., features on the surface of the Moon or planets.

REFRACTING TELESCOPES

• Use lens (usually glass) to collect light over large area.
• Lens refracts (bends) light. Design it so:
  • Parallel light rays from a distant point come together at a focus point.
  • Parallel rays from another distant point come together at a different focus point in the same plane ("focal plane").
• Simple design, still used for small amateur telescopes.
• Limitations:
  • Bending depends on wavelength, so different colors focus at different distances.
  • Lens must be supported at edges. Heavy glass lens sags under its own weight, distorts optics.

REFLECTING TELESCOPES

• Invented by Newton.
• Use large, curved primary mirror to collect light, bring to focus.
• Put camera or person in front of primary at prime focus OR
• Use secondary mirrors to reflect light to eyepiece or instrument.
• Different designs with different placements of secondary mirrors.
• Mirror usually polished glass with thin layer of aluminum.
• Can support mirror from behind to prevent sagging.

Largest refracting telescopes: 1-m primary lens.
Largest reflecting telescopes: 8-10 m primary mirror.

LIGHT GATHERING POWER

Light gathering power:

• Number of photons collected proportional to exposure time multiplied by area of primary lens or mirror.
• Area = (\pi / 4) x D², where D = mirror/lens diameter.
• Diameter of primary determines light gathering power, hence ability to study faint objects.

ANGULAR RESOLUTION AND SEEING

• Magnification depends on optics of secondary lenses or mirrors.
• More magnification = higher angular resolution, ability to see structure.
• Maximum resolution achievable is \lambda / D radians, light wavelength divided by diameter of primary. Consequence of wave nature of light.
• Human eye (0.2-cm pupil): about 1 arc-minute.
• 10-cm (4-inch) telescope: about 1 arc-second.
• Achieving best resolution requires very accurate mirror surface, with correct shape to ~ 1/20 wavelength of light.

• Large telescope can in principle do much better than 1 arc-second BUT
• Blurring by air currents in atmosphere usually smears images over several arc-seconds.
• This blurring is called seeing.
• From best sites seeing can be as good as 0.5-1 arc-seconds.
• For good seeing and dark skies: put telescopes on high mountaintops.
• Atmospheric seeing limits the angular resolution of ground-based optical telescopes.
• One solution: put telescope in space. (Difficult. Expensive.)
• Another solution: constantly bend mirror to compensate for atmospheric motions. (Difficult. Expensive.)

DETECTORS

What do you do with light collected by telescope?

First detector: human eye.

• Use secondary lens ("eyepiece") to make converging rays parallel again, send to eye.
• Limitations:
  • Short "exposure time" (fraction of second).
  • Not recorded, can't study result. Have to rely on observer's description or drawing.

• Put in focal plane, so extended image forms on plate.
• Incoming photons cause permanent chemical change.
• Build up image over time, collect more light.
• Permanent, objective record.
• But inefficient: best film/plates miss 99% of incoming photons.

• Incoming photons knock electrons out of silicon. These are counted electronically.
• Excellent efficiency: up to 80% of incoming photons counted.
• Data in good form for analysis by computers.
• CCDs have revolutionized optical astronomy over last two decades.
• Also personal cameras and camcorders.

INSTRUMENTS

Two basic classes:

• Camera: record an image of a portion of sky.
• Use filters to get images at different wavelengths.
• Can combine results from different filters to recreate color image.
• Spectrographs: look at single object, use prism or grating to disperse light in wavelength, measure spectrum.

SPACE TELESCOPES

Why put telescopes in space?

1. Look at wavelengths that are blocked by the atmosphere.

• Atmosphere blocks gamma-ray, X-ray, ultraviolet, much of infrared.
• Fortunate for life, unfortunate for astronomy.
• Many astronomical objects emit EM radiation at these wavelengths.
• Only way to study this radiation is from space.
• Detection technology specialized, so need different telescopes for each of these regimes.

2. Overcome atmospheric seeing.

• Principal strength of Hubble Space Telescope.
• 2.4-m mirror, less light gathering power than largest ground-based telescopes.
• But no atmosphere, so images have 0.05 arc-second resolution instead of 1 arc-second.
• See much more structural detail than any ground-based telescope.
• Hubble also sensitive at some ultraviolet and infrared wavelengths; also important.

3. Go to planets.

• Space probes can actually go to planets in solar system.
• Get images and spectra from close up, much better resolution.
• Successful landings on Moon, Mars, Venus.
• Irrelevant to other areas of astronomy, where objects are much, much further than most distant planets.