Telescopes
Chapter 3
Objectives
• To know the general types of telescopes and the
advantages and disadvantages of each one.
• To know the primary parts and functions of each
part of a telescope.
• To know the importance of the diameter of the
objective and to know how the magnification of a
telescope is related to the focal lengths of the
objective and eyepiece.
• To know the advantages and disadvantages of
earth and space-based telescopes.
Galileo Galilei (1564-1642)
History
• invented by Dutch lens maker in 1608
• Galileo: small 30X scope
• Observed the moon and “began” the modern age of
Astronomy where measurement was more important than
philosophy
Galileo noticed
•
•
•
•
moons orbiting Jupiter
phases of Venus
craters on the moon
sunspots
This was strong evidence
that Copernicus was right
although Galileo wasn’t
willing to die for it.
How a telescope works
• gathers light through the objective (mirror or lens)
– bigger is better because it gathers more light
– ability to see faint objects increases proportionally with
the square of the radius of the objective
• focuses light
• viewed through an eyepiece (changing the
eyepiece changes the magnification)
• magnification is the ratio of the focal length of the
objective to the focal length of the eyepiece
General types of telescopes
• Refracting (objective is a lens)
• Reflecting (objective is a mirror)
– Newtonian
– Cassegrain
• Catadioptrics
– uses mirrors and lenses
– Schmidt-Cassegrain
– Maksutov-Cassegrain
Refractors (glass lens)
Advantages and Disadvantages
• Easy to use and reliable
• Excellent for lunar,
planetary and binary star
observing especially in
larger apertures.
• High contrast images with
no secondary mirror or
diagonal obstruction.
• Sealed optical tube
reduces image degrading
air currents and protects
optics.
• More expensive per inch
of aperture
• Heavier, longer and
bulkier than equivalent
aperture Newtonians and
catadioptrics.
• Small apertures
• Less suited for viewing
small and faint deep sky
objects.
• Color aberration due to
colors of light bending
different amounts.
Reflectors (mirror)
Advantages and disadvantages
• Lowest cost per inch of
aperture
• Reasonably compact and
portable up to focal
lengths of 1000mm.
• Excellent for faint deep
sky objects such as remote
galaxies, nebulae and star
clusters.
• Reasonably good for lunar
and planetary work.
• Low in optical aberrations.
• Open optical tube design
allows image-degrading
air currents and air
contaminants
• More fragil
• Large apertures (over 8")
are bulky, heavy and tend
to be expensive.
• Slight light loss due to
secondary obstruction
when compared with
refractors.
Cassegrain reflector
Catadioptric telescopes
• Best all-around, all-purpose telescope design. Combines
the optical advantages of both lenses and mirrors while
canceling their disadvantages.
• Sharp images over a wide field.
• Excellent for deep sky observing or astrophotography with
fast films or CCD’s.
• Very good for lunar, planetary and binary star observing or
photography.
• Closed tube design reduces image degrading air currents.
• Most are extremely compact and portable.
• Large apertures at reasonable prices and less expensive
than equivalent aperture refractors.
Schmidtt-Cassegrain
Problems with earth-based
telescopes
• Earth’s atmosphere reflects certain wavelengths
– x-rays, gamma rays and most UV light is not
transmitted by our atmosphere
• Earth’s atmosphere blurs images
– the bending of light by the atmosphere depends on the
temperature of the “air”
– “twinkling” (shimmering) effect
• “Light pollution”
• Solution? Put the telescope in space.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html
Disadvantages of space-based
telescopes
• Expensive to launch and maintain
• Difficult to repair
• Low lifetime
Examples of space-based
telescopes
• Hubble Space Telescope
– 3 times better resolution
– can see fainter objects
• Chandra X-ray Observatory
• Compton Gamma-Ray Observatory