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?
- 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.
- 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
- 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.
- Bending depends on wavelength, so different colors focus at different
- Lens must be supported at edges. Heavy glass lens sags under its
own weight, distorts optics.
- 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
- 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
- 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
- One solution: put telescope in space. (Difficult. Expensive.)
- Another solution: constantly bend mirror to compensate for
atmospheric motions. (Difficult. Expensive.)
What do you do with light collected by telescope?
First detector: human eye.
Better: film or photographic plates.
- Use secondary lens ("eyepiece") to make converging rays parallel again,
send to eye.
- Short "exposure time" (fraction of second).
- Not recorded, can't study result. Have to rely on observer's
description or drawing.
Much better: Electronic detectors (CCDs).
- 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
- 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.
Two basic classes:
Sensitivity of an instrument depends on quality of its optics,
efficiency of its detector.
- 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.
A powerful telescope is only useful if it has powerful instruments.
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
- 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;
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.
Go to A161 Home Page
Updated: 2005 April 30 [dhw]