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Chapter 4: Telescopes
Optical telescopes: Reflectors and refractors
E
O
Refractors use lenses
E: eyepiece
O: objective
Reflectors use mirrors:
Newtonian
Cassegrain
Objective (O) = Main light-gathering surface (lens or
mirror)
Aperture = a = Diameter of objective ("size" of telescope)
Focal length = f = Distance from objective to image
("length" of telescope)
Eyepiece (E) = Lens used to inspect image from objective
1) LIGHT GATHERING POWER (how faint you can see)
LGP ~ a2
(Bigger = better)
2) MAGNIFYING POWER (how large image is)
MP = f(O) / f(E)
(Bigger = better)
So can change this with eyepice choice, not a fundamental
property of telescope
Practical limit  50 x O (in inches)
3) RESOLVING POWER (size of detail, sharpness of focus)
Size of detail ~ 1/a (Smaller = better)
i.e., larger ‘a’ allows smaller details to be seen
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Resolution vs.
magnification
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Airy disk. Light coming through a circular aperture
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Resolution vs.
magnification
Atmospheric seeing
The planet Uranus imaged in the infrared
Spitzer Space Telescope
An infrared telescope in orbit
around the Sun!
43 m diameter
How to improve resolution? Radio interferometry
The Very Large Array, Socorro, New Mexico
VLBI
Chandra Xray observatory
X-ray telescope design (Chandra X-ray Observatory)
It is very difficult to focus (redirect) high energy photons,
because they are so readily absorbed by matter. Thus it
is difficult to tell where they came from, and to
reconstruct the image.
Chandra image of X-rays emitted from hot
gas surrounding galaxy Cygnus-A
An Xray image of a black hole, by Chandra.
X-ray image
Visible (optical)
Hot stars give off a lot more Xrays than cooler stars do.
Observing gamma rays
Space-based telescopes (Compton Gamma ray
Observatory