Ch 5 -how do telescopes work

TOOL OF THE TRADE

The whole idea is to collect radiationfrom distant objects, not just observe them.

We have come a long way with telescopes.

Now telescopes are not only on earthbut in space as well.

MODERN TELESCOPE

What is measured:

1. Brightness

2. Spectra

3. Position

8 meter mirror

at Mauna Kea,

Hawaii

TOOLS:

Astronomers are rarely at the “eyepiece.” They are on computer

terminals.

POWERS OF A TELESCOPE

• Collecting Power

-Bigger the telescope, more light

collected.

• Focusing Power-Use mirrors or lenses to bend the

path of light rays to create images

• Resolving Power

- Picking out the details of an image

LIGHT GATHERING POWER

Light collected brighter image

is proportionalto the collectorarea.

Small changesin collector radius gives large change in number of photons caught. Because A = pr2

REFRACTING TELESCOPES

Remember:

Refraction is

The bending of light becauseof lightchanging speed in different mediums.

REFRACTION IS RESPONSIBLE FOR

Dispersion: causes

different colors to

travel at different

speeds through the

same material. Responsible

for the

distortion of

the Sun near

the horizon.

TWINKLING OR SCINTILLATION

Temperature and density differencesin pockets of air shift the image of a star.

REFRACTING TELESCOPES

Telescopes that use lenses to collect and focus light called refractors.

Disadvantages:

1. Large lens: Expensive

2. Large lens: sags in the center

3. Dispersion causes images to have

colored fringes.

4. Many lens materials absorb short

wavelength light.

CHROMATIC ABERRATION

The colors of white light refract

differently so they focus differently.

REFLECTING TELESCOPES

Use Mirrors.

Can make BIG, BIG Mirrors.

Used almost exclusively by Astronomers today.

Multiple

mirrors

and

very

thin.

REMEMBER RESOLVING POWER?

For a given wavelength resolution is

increased for a larger telescope.

Interferometers will increase resolution by combining observations

from two or

more

widely spaced

telescopes.

INTERFEROMETER DIFFERENCE

McGraw Hill Picture

Seetwo

starsin B.

LIGHT GATHERING POWER

Compare Two Telescopes: A to B

LGA = Diameter of A

LGB Diameter of B

2

Example: Suppose we compare a

6 cm telescope with a 32 cm telescope. How much more light will the large telescope gather?

Use A as the larger telescope:

(32/6)2 = 28.6 times