Chapter 4: Telescopes

E

O

Optical telescopes: Reflectors and refractors

Refractors use lensesE: eyepieceO: 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 telescopePractical 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 imageVisible (optical)

Hot stars give off a lot more Xrays than cooler stars do.

Observing gamma rays

Space-based telescopes (Compton Gamma ray Observatory