Telescopes

• How we gather light

Devices for detecting photons

• How we collect light

Angular Resolution

• Resolving fine details

What is a telescope?

• It is an instrument that collects and focuses light (onto a photon detector!)
  

Why do we build them?

• To make much more sensitive observations
• To resolve small details on the sky

There are two basic classes of telescopes, refracting and reflecting.

Refracting

• Focuses light through a lens
  • e.g. a camera lens or
  • a magnifying glass

Reflecting

• Focuses by reflecting light off a mirror
  • e.g. a shaving mirror

Refracting telescopes use a large lens to gather and focus light.

There are three basic problems with refracting telescopes.

1. Must make a large piece of glass with no defects (bubbles, impurities, etc.).
2. Must suspend the heavy glass by the rim
3. Chromatic aberration
   • Different wavelengths of light are bent differently (like a prism!).
   • Largest refractor is ~1 meter in diameter.

Reflecting telescopes reflect light from an aluminized, curved mirror to a focus.

The mirror always has the shape of a conic section: a parabola, hyperbola, or ellipse.

• Light is reflected off the surface so it doesn't pass through the material.
• Can support from the back.
• No chromatic aberration (all light is reflected equally).

• "Gathers up" the "flux" from an object.
• The amount of light (or power) collected depends upon the area of the telescope mirror.
• Thus, bigger telescopes are better
  • They collect more light

• A dark adapted eye has diameter D ~ 7 mm.
• Mt. Palomar telescope: D = 5 m.
• It collects
  

times more light!

• Thus you could see much fainter with it.

• Looking through the Palomar telescope, you should see about 14 mags fainter
  • i.e. 20th magnitude
• But Palomar observes objects much, much fainter than this (~25th mag).
• How does it do this?

• Photon detectors are devices which respond to E-M radiation.
• Photographic film detects photons
  • Used in the "olden" days of astronomy
• Today "solid state" detectors are used, e.g. CCD's (charge-coupled devices)
  • Used in low-light level camcorders and electronic cameras

Other Wavelengths

• Solid state photon detectors in one form or another are used to detect radiation across the E-M spectrum.
• Earlier, photographic covered a very limited portion of this spectrum:
  • visible, UV and X-ray
  • But was not very efficient
    (only detecting 1 photon in about 20)

• Integration time is the exposure time of the detector.
• The dark adapted eye integrates photons for ~1/8 to 1/4 second.
• CCDs can integrate for hours.
• The long exposure time means many photons can be collected from the source.

• Angular resolution is the ability to distinguish between nearby objects.
• Measured in arcseconds or arcminutes.
• The eye has a spatial resolution of ~1 arcminute.

The angular resolution, theta, of a telescope is given by:

where lambda = the wavelength and D = telescope diameter

Some numbers

• Optical wavelengths (lambda ~ 5000 A)
  
  
  
  
• Radio wavelengths (1 mm to 100 m)
  
  
  

• Larger telescopes have better angular resolution.
• However, it is the size of the telescope relative to the wavelength that counts.
• Radio telescopes need to be very large to get "good" angular resolution.

• Telescopes are placed on mountain tops to get better seeing (thinner air).
• But atmospheric blurring limits the angular resolution ~0.5 arcsec (5000 A).
• Adaptive optics corrects the blurring due to the atmosphere in real time
• Uses a deformable mirror
• Developed for the military