Chapter 6 Optics and Telescopes

Guiding Questions

1. Why is it important that telescopes be large?2. Why do most modern telescopes use a mirror rather than a lens?3. Why are observatories in such remote locations?4. Do astronomers use ordinary photographic film to take pictures

of the sky? Do they actually look through large telescopes?5. How do astronomers use telescopes to measure the spectra of

distant objects?6. Why do astronomers need telescopes that detect radio waves and

other non-visible forms of light?7. Why is it useful to put telescopes in orbit?

Two Basic Types of Telescopes

• Refractors– Use lenses to concentrate incoming light at a focus.

• Reflectors– Use mirrors to concentrate incoming light at a focus.

The goal is always the same – gather as much light as possible and concentrate it at a focus.

Concept of Refraction: light

slows and changes direction when it enters a denser

medium.

A refracting telescope uses a lens to concentrate incoming light at a focus.

A refracting telescope actually uses two a lenses: an objective and an eyepiece.

The two lenses are separated by the sum of their focal lengths.

A lens creates an extended image of an

extended object.

lenses reverse images

Three main functions of a telescope

• Brighter image“light gathering power” is proportional to the area of

the objective lens (r2)

Three main functions of a telescope

• Brighter image“light gathering power” is proportional to the area of

the objective lens (r2)

• Finer detail“resolution” or resolving power is proportional to

the radius of the lens (r)

Poor and Great Resolution (improved by using adaptive optics)

Telescope images are degraded by the blurring effects of the atmosphere and by light pollution

Three main functions of a telescope

• Brighter image“light gathering power” is proportional to the area of

the objective lens (r2)

• Finer detail“resolution” or resolving power is proportional to

the radius of the lens (r)

• Bigger imagemagnification = (objective lens focal length / eyepiece lens focal length).

Refracting telescopes have drawbacks

• Spherical aberration• Chromatic aberration

• Sagging due to gravity distorting the lens

• Unwanted refractions• opaque to certain

wavelengths of light

Special achromatic compound lenses and lens coatings can often fix this aberration.

REFLECTING telescopes solve many problems of refracting telescopes.

Reflecting telescopes use a mirror instead of a lens to focus images.

The secondary mirror in the tube does not cause a hole in the image.

It does however make it a little dimmer because it reduces the total amount of light reaching the primary mirror.

Drawback of Using Spherical Mirrors in Reflecting Telescope

•Spherical Aberration

(can be corrected with a correcting lens)

The largest research telescopes

in the world are reflectors.

The Keck I telescope on Mauna Kea on the Big Island of Hawaii uses 36 hexagonal mirrors to make a total diameter of 10 m.

(Note the astronomers standing on either side of the platform.)

A Charge-Coupled Device (CCD)

An electronic device counts the photons arriving at a telescope’s focus

Ordinary Photographs vs. CCDs

Spectrographs record the spectra of astronomical objects.

Spectrographs can be built into telescopes.

Observations at wavelengths other than visible light are revealing previously invisible sights.

Visible light image radio wavelength image

Infrared observations reveal hot gases.

UV

Ordinary visible

infrared

Map of Orion region

Telescopes in orbit around the Earth detect radiation that cannot penetrate the

atmosphere.

Other Space ObservatoriesChandra X-ray Observatory

XMM-Newton X-ray ObservatoryCompton Gamma Ray Observatory

The Entire Sky at the Visible Wavelengths

The Entire Sky at the 21-cm Wavelengths

The Entire Sky at the Infrared Wavelengths

The Entire Sky at the X-ray Wavelengths

The Entire Sky at the Gamma Ray Wavelengths

Guiding Questions

1. Why is it important that telescopes be large?2. Why do most modern telescopes use a large mirror rather than

a large lens?3. Why are observatories in such remote locations?4. Do astronomers use ordinary photographic film to take pictures

of the sky? Do they actually look through large telescopes?5. How do astronomers use telescopes to measure the spectra of

distant objects?6. Why do astronomers need telescopes that detect radio waves

and other non-visible forms of light?7. Why is it useful to put telescopes in orbit?