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Chapter 3
• 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)
• 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
Galileo noticed
moons orbiting Jupiter
phases of Venus
craters on the moon
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 (Dobsonian)
– 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
• More expensive per inch
of aperture
• Heavier, longer and
bulkier than equivalent
aperture Newtonians and
• Small apertures
• Less suited for viewing
small and faint deep sky
• Color aberration due to
colors of light bending
different amounts.
Reflectors (mirror)
Advantages and disadvantages
• Lowest cost per inch of
• Reasonably compact and
portable up to focal
lengths of 1000mm.
• Excellent for faint deep
sky objects such as remote
galaxies, nebulae and star
• Reasonably good for lunar
and planetary work.
• Low in optical aberrations.
• Open optical tube design
allows image-degrading
air currents and air
• More fragil
• Large apertures (over 8")
are bulky, heavy and tend
to be expensive.
• Slight light loss due to
secondary obstruction
when compared with
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
• 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.
• Combining information from multiple
• Used in radio-telescopes
Problems with earth-based
• 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.
Disadvantages of space-based
• Expensive to launch and maintain
• Difficult to repair
• Low lifetime
Examples of space-based
• Hubble Space Telescope
– 3 times better resolution
– can see fainter objects
• Chandra X-ray Observatory
• Compton Gamma-Ray Observatory