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A Basic Refractor Telescope
The size of the telescope is the diameter of the light-collecting
lens
“Normal” Eye
Refractive Errors
Farsightedness Nearsightedness
Limitations of Refractors
• Lens must be made of high-quality glass with no imperfections
• Larger lenses weigh a lot; lenses can be supported only around their rims
• The lens will sag under its own weight
So, the largest refractor telescope anyone has built is about 1.1 m.
A Basic Reflector Telescope
The size of the telescope is the diameter of the objective mirror
Usefulness of Reflectors
• Objective (mirror) can be made of many things, even plastic or metal
• Only one side of the mirror is polished
• Mirror can be supported from its entire back, not the rim only
So, all large modern telescopes are reflectors because they are easier
to make and maintain.
A Prime Focus Reflector
(some light blocked)
A Newtonian Reflector
(secondary mirror)
A Cassegrain Reflector
(hole in primary mirror)
Example: Hubble Space Telescope
Resolution vs. Magnification
• RESOLUTION: a telescope’s ability to distinguish fine details in an
image.
• MAGNIFICATION:
focal length of objective
focal length of eyepiece
So, magnification depends on the eyepiece used in the telescope.
Hubble Space Telescope
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