Chapter 5Telescopes: “light bucket”

Telescopes have three functions

1.Gather as much light as possible: LGP ∝ Area = πR2 Why?

UV IR

Ene

rgy

Out

put

Wavelength

The light gathering power is proportional to

the area

Telescopes have three functions

2. Resolve objects: Θ = 0.25 (λ/D)Why?In astronomy, we are always concerned with angular measurement

“close together” means “separated by a small angle on the sky,”

angular resolution is the factor that determines our ability to see fine structure

Telescopes have three functions

3. Magnify EXTENDED objects. Why?

Extended objects: The most surprising property is that extended objects do not get brighter in a scope.

Also the contrast does not change (actually it only can get worse).

The only thing that changes is that the object itself gets bigger by magnification

Reflection is the

bouncing of light off a surface

How do we see an object Light rays coming from one point of

an object are focused at one point at the back of your eye

(retina).

Different points from an object are focused at different points of your retina.

So we see an image of the object produced by the lense in our eyes at the back of our eye.

A telescope is not a microscope• Microscope’s function is magnification

• Telescope’s function is gathering as much light as possible: Large lenses

• Large lenses do not result in larger magnification

The refracting telescope uses two lenses.

Where is the image of the star?

Light Bending

Problem with refraction

Different color light is refracted at different angles, causing a distortion of the image

Refractors suffer from Chromatic Aberration

Dispersive refraction leads to chromatic aberration:

More on chromatic aberration

CLL: Diffraction Rings

Chromatic Aberration

Images can be formed through reflection or refraction

Reflecting mirror

Image formation

Where do you stand to observe the object?What’s the problem with that?How to overcome the problem?

Types of reflecting telescopes:

To get around the blocking of light when you stand in the focal plane, different telescope types were invented

Do reflecting optical devices suffer from chromatic aberration

A) Yes, different color light reflects at different angles and therefore focuses at different points.

B) No, different color light reflects always at the same angle as it incidents, causing no chromatic aberration.

But: Spherical mirrors suffer from Spherical

Aberration

Parallel light from different locations of the mirror is not exactly focused at one point

Spherical aberration

can be eliminated

by a parabolic shape or a corrector

plate

CLL: Focus of a Cassegrain reflector

Optical TelescopesThe Keck telescope, a modern research telescope:

Optical Telescopes

The Hubble Space Telescope has a variety of detectors:

Discovery 5-1: The Hubble Space Telescope

The Hubble Space Telescope’s main mirror is 2.4 m in diameter and is designed for visible, infrared, and ultraviolet radiation

Discovery 5-1: The Hubble Space Telescope

Here we compare the best ground-based image of M100, on the left, with the Hubble image on the right

In astronomy, we are always concerned with angular measurement

“close together” means “separated by a small angle on the sky,”

angular resolution is the factor that determines our ability to see fine structure

Protractor-string Example

Light-gathering power: Improves detailBrightness proportional to square of radius of mirrorBelow: (b) was taken with a telescope twice the size of (a)

Telescope Size

Resolving power: When better, can distinguish objects that are closer togetherResolution is proportional to wavelength and inversely proportional to telescope size—bigger is better!

Telescope Size

Resolution Want to resolve objects which are close

together. Objects that are close together are

separated by a smaller angle than objects that are at the same distance but further apart.

Θ = 2.06 X 105 (λ/D)

Smaller the better

How to improve the resolution?• A) Increase the size of the mirror• B) Decrease the size of the mirror• C) Use light with smaller wavelength• D) Use light with larger wavelength

Smaller the better

For comparison, the angular resolution of the human eye in the middle of the visual range is about 0.5'.

What limits a telescope’s resolution?

One important factor is diffraction, the tendency of light, and all other waves to bend around corners

Diffraction introduces a certain “fuzziness,” or loss of resolution

Effect of improving resolution:(a) 10′; (b) 1′; (c) 5″; (d) 1″

5.2 Telescope Size

Image acquisition: Charge-coupled devices (CCDs) are electronic devices, can be quickly read out and reset

5.3 Images and Detectors

5.3 Images and DetectorsPhotometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation

Image processing by computers can sharpen images

5.3 Images and Detectors

Atmospheric blurring: Due to air movements5.4 High-Resolution Astronomy

“Seeing”

Solutions:• Put telescopes on mountaintops,

especially in deserts• Put telescopes in space

5.4 High-Resolution Astronomy

Active optics: Control mirrors based on temperature and orientation

5.4 High-Resolution Astronomy

Adaptive optics: Track atmospheric changes with laser; adjust mirrors in real time

5.4 High-Resolution Astronomy

5.4 High-Resolution AstronomyThese images show the improvements possible with adaptive optics:

Radio telescopes:• Similar to optical reflecting telescopes• Prime focus• Less sensitive to imperfections (due to longer

wavelength); can be made very large

5.5 Radio Astronomy

Largest radio telescope: 300-m dish at Arecibo5.5 Radio Astronomy

Longer wavelength means poor angular resolutionAdvantages of radio astronomy:• Can observe 24 hours a day

• Clouds, rain, and snow don’t interfere

• Observations at an entirely different frequency; get totally different information

5.5 Radio Astronomy

Interferometry:• Combine information from several widely spread

radio telescopes as if they came from a single dish

• Resolution will be that of dish whose diameter = largest separation between dishes

5.6 Interferometry

5.6 InterferometryInterferometry involves combining signals from two receivers; the amount of interference depends on the direction of the signal

Can get radio images whose resolution is close to opticalInterferometry can also be done with visible light but is much more difficult due to shorter wavelengths

5.6 Interferometry

Infrared radiation can image where visible radiation is blocked; generally can use optical telescope mirrors and lenses

5.7 Space-Based Astronomy

Infrared telescopes can also be in space; the image on the left is from the Infrared Astronomy Satellite

5.7 Space-Based Astronomy

The Spitzer Space Telescope, an infrared telescope, is in orbit around the Sun. These are some of its images.

5.7 Space-Based Astronomy

Ultraviolet observing must be done in space, as the atmosphere absorbs almost all ultraviolet rays.

5.7 Space-Based Astronomy

X-rays and gamma rays will not reflect off mirrors as other wavelengths do; need new techniquesX-rays will reflect at a very shallow angle and can therefore be focused

5.7 Space-Based Astronomy

X-ray image of supernova remnant

5.7 Space-Based Astronomy

Gamma rays cannot be focused at all; images are therefore coarse

5.7 Space-Based Astronomy

Much can be learned from observing the same astronomical object at many wavelengths. Here, the Milky Way:

5.8 Full-Spectrum Coverage

• Refracting telescopes make images with a lens• Reflecting telescopes with a mirror• Modern research telescopes are all reflectors• CCDs are used for data collection• Data can be formed into image, analyzed

spectroscopically, or used to measure intensity• Large telescopes gather much more light,

allowing study of very faint sources• Large telescopes also have better resolution

Summary of Chapter 5

• Resolution of ground-based optical telescopes is limited by atmospheric effects

• Resolution of radio or space-based telescopes is limited by diffraction

• Active and adaptive optics can minimize atmospheric effects

• Radio telescopes need large collection area; diffraction limited

• Interferometry can greatly improve resolution

Summary of Chapter 5 (cont.)

• Infrared and ultraviolet telescopes are similar to optical

• Ultraviolet telescopes must be above atmosphere

• X-rays can be focused, but very differently than visible light

• Gamma rays can be detected but not imaged

Summary of Chapter 5 (cont.)