Telescopes
This set of notes by Nick Strobel covers: the basics of telescopes and effects
of the atmosphere on images. Vocabulary terms are italicized.
Telescopes--instruments used to gather and focus light. Visible and
large parts of radio spectrum can be observed anywhere on ground; small parts
of infrared (IR) must be observed above water layer in atmosphere (elevations
of 2750 m (9000 ft) or above); gamma-rays, X-rays, UV, most IR, and rest of
radio must be observed in space. The part collecting the light is called the
objective.
- Refractor--use lens to bend light. First ones built.
Disadvantages: a) Chromatic Aberration--redder colors bent less than
blue colors so see rainbow of colors around the image. Use multiple
compensating lenses to counteract this OR have really long objective focal
length to minimize effect (see drawing). This is why the early refracting
telescopes were made very long. b) Support for lens only at ends of lens. Tends
to sag under own weight. 40 inches is maximum size of refracting objective
built.
- Reflector--use mirror to reflect light. All modern research ones
are this type. Advantages: a) No chromatic aberration. b) Support for objective
all along one side so they can be BIG! Parabollic-shaped mirror focuses all
parallel light rays to single point (remember that celestial objects are so far
away that all the light rays from an object hit the Earth as parallel rays).
Focus is before the eyepiece, so the image in astronomical telescopes (even with the refractor above) is upside down. Terrestrial-viewing telescopes use
other lenses to re-invert the image right-side up. Spherical Aberration--mirror not curved enough (shaped like part of a sphere) so all
of the light is not focussed to a single point. Hubble Telescope objective
suffers from this so it uses corrective optics to compensate.
B
- Light Gathering Power--telescope acts as a light bucket so the
bigger the objective, the more light is collected and the image is brighter.
Faint objects only seen with BIG objective telescopes.
- Resolving Power--ability of telescope to see really small details
so objects that are close together in the sky are easily seen as separate.
Absolute minimum resolvable angle in arcseconds = 1.22 * 206,265
* (observation wavelength) / (objective diameter). The wavelength and
diameter must be measured in the same length units. Radio wavelengths are LARGE
so the radio telescope objective must be LARGE to get decent resolving power.
The atmosphere will usually smear images so this theoretical resolving power
not reached with telescopes on ground. Speckle interferometry can get rid of
atmospheric distortion. Adaptive optics is another way to remove atmospheric
distortion.
- Magnification--ability of telescope to make image bigger.
Least important power because it magnifies telescope distortions and
atmospheric distortions so a small fuzzy faint blob only becomes a big fuzzy
fainter blob. Magnifying power = (focal length of objective) / (focal length of
eyepiece); both focal lengths in same units.
- The air is in turbulent motion and light from celestial objects is bent
randomly in many ways over time periods of tens of milliseconds. Images dance
about (twinkle) and images are blurred. This atmospheric effect is called
seeing. Good seeing is when the air is stable and the twinkling is
small.
- The air also absorbs and scatters different wavelengths in different
amounts. Redder light is scattered less by atmosphere molecules and dust than
bluer light. Since blue light scattered more, we have a blue sky and objects
appear reddened when viewed through lots of air. Some forms of light
scattered at every wavelength and do not make it to the surface--- extinction of that radiation at every wavelength. Get on high mountaintops
to look through less atmosphere and less distortion. Even better, observe from
space!
- Gases in atmosphere can create absorption lines light from celestial
object. Need to remove atmospheric spectral lines from spectroscopy data,
otherwise you'll find a hot star with molecular nitrogen, oxygen and water
lines!
last updated 29 Aug 95
Nick Strobel --
Email:
strobel@astro.washington.edu
(206) 543-1979
University of Washington
Astronomy
Box 351580
Seattle, WA 98195-1580