Electromagnetic Radiation and Telescopes

(How we get information about the cosmos and how we gather the information)

a) bend around corners and edges.

b) separate into its component colors.

c) bend through a lens.

d) disperse within a prism.

e) reflect off a mirror.

Diffraction is the tendency of light to

Clicker Question

a) bend around corners and edges.

b) separate into its component colors.

c) bend through a lens.

d) disperse within a prism.

e) reflect off a mirror.

Diffraction is the tendency of light to

Clicker Question

Diffraction affects all telescopes and limits the sharpness of all images.

Telescopes

Light Hitting a Telescope Mirror

huge mirror near a star*

* small mirror far from a star

In the second case (reality), light rays from any single point of light are essentially parallel.

Light rays from a distant source, parallel to the "mirror axis“, all meet at one point, the focus.

CCD**

Reflecting and refracting telescopes

Optical Telescopes

Optical Telescopes - Refracting vs. Reflecting

Refracting telescope

Focuses light with a lens (like a camera).

object (point of light) image at focus

Problems:

- Lens can only be supported around edge.

- "Chromatic aberration".

- Some light absorbed in glass (especially UV, infrared).

- Air bubbles and imperfections affect image quality.

Reflecting telescope

Focuses light with a curved mirror.

<-- object image

- Can make bigger mirrors since they are supported from behind.

- No chromatic aberration.

- Reflects all radiation with little loss by absorption.

Refracting Telescope Reflecting Telescope

Yerkes 40-inch (about 1 m). Largest refractor.

Cerro-Tololo 4 -m reflector.

Chromatic Aberration

Lens - different colors focus at different places.

white light

Mirror - reflection angle doesn't depend on color.

Types of reflecting telescopes

Clicker Question

Modern telescopes use mirrors rather than lenses for all of these reasons EXCEPT

a) light passing through lenses can be absorbed or scattered.

b) large lenses can be very heavy.

c) large lenses are more difficult to make.

d) mirrors can be computer controlled to improve resolution.

e) reflecting telescopes aren’t affected by the atmosphere as much.

Clicker Question

Modern telescopes use mirrors rather than lenses for all of these reasons EXCEPT

a) light passing through lenses can be absorbed or scattered.

b) large lenses can be very heavy.

c) large lenses are more difficult to make.

d) mirrors can be computer controlled to improve resolution.

e) reflecting telescopes aren’t affected by the atmosphere as much.

Reflecting instruments like the KECK telescopes can be made larger, and more capable, than refractors.

Keck 10-m optical telescope

Mirror sizeMirror with larger area captures more light from a star. Can look at fainter objects with it.

30-100 m optical telescopes now being considered!

Image of Andromeda galaxy with optical telescope.

Image with telescope of twice the diameter, same exposure time.

The Two Main Types of Observation

Imaging (recording pictures)

Spectroscopy (making a spectrum) usually using a diffraction grating

In both cases, image or spectrum usually recorded on a CCD ("charge-coupled device")

Resolving Power of a Mirror

(how much detail can you see?)

fuzziness you would see with your eye.

detail you can see with a telescope.

(a) 10′; (b) 1′; (c) 5″; (d) 1″

"Angular resolution" is the smallest angle by which two objects can be separated and still be distinguished.

For the eye, this is 1' (1/60th of a degree). Or 100 km atdistance of the Moon.

angular resolution wavelengthmirror diameter

For a 2.5-m telescope observing light at 5000 Angstroms (greenish), resolution = 0.05".

But, blurring by atmosphere limits resolution of optical telescopes to about 1". This is called seeing.

Seeing

*

dome

Air density varies => bends light. No longer parallel

Parallel rays enter atmosphere

CCD

No blurring case. Rays brought to same focus.

* Sharp image on CCD.

Blurring. Rays not parallel. Can't be brought into focus.

Blurred image.

Example: the Moon observed with a 2.5 m telescope

1" => 2 km

0.05" => 100 m

Ground-based telescope image, 1” resolution

Hubble SpaceTelescope image,0.05” resolution

North America at night

So where would you put a telescope?

Kitt Peak National Observatory, near Tucson

Mauna Kea Observatory, Hawaii

Clicker Question

Resolution is improved by using

a) larger telescopes & longer wavelengths.

b) infrared light.

c) larger telescopes & shorter wavelengths.

d) lower frequency light.

e) visible light.

Clicker Question

Resolution is improved by using

a) larger telescopes & longer wavelengths.

b) infrared light.

c) larger telescopes & shorter wavelengths.

d) lower frequency light.

e) visible light.

Diffraction limits resolution; larger telescopes and shorter-wave light produces sharper images.

a) they don’t require chemical development.

b) digital data is easily stored & transmitted.

c) CCDs are more light sensitive than film.

d) CCD images can be developed faster.

e) All of the above are true.

An advantage of CCDs over photographic film is

Clicker Question

a) they don’t require chemical development.

b) digital data is easily stored & transmitted.

c) CCDs are more light sensitive than film.

d) CCD images can be developed faster.

e) All of the above are true.

An advantage of CCDs over photographic film is

Clicker Question

a) the quality of the telescope’s optics.

b) the transparency of a telescope’s lens.

c) the sharpness of vision of your eyes.

d) the image quality due to air stability.

e) the sky’s clarity & absence of clouds.

Seeing in astronomy is a measurement of

Clicker Question

Seeing in astronomy is a measurement of

Clicker Question

“Good Seeing” occurs when the atmosphere is clear and the air is still.

Turbulent air produces “poor seeing,” and fuzzier images.

a) the quality of the telescope’s optics.

b) the transparency of a telescope’s lens.

c) the sharpness of vision of your eyes.

d) the image quality due to air stability.

e) the sky’s clarity & absence of clouds.

Point images of a star

Smeared overall image of star

Radio Telescopes

Large metal dish acts as a mirror for radio waves. Radio receiver at prime focus.

Surface accuracy not so important, so easy to make large one.

But angular resolution is poor. Remember:

angular resolution wavelengthmirror diameter

D larger than optical case, but wavelength much larger (cm's to m's), e.g. for wavelength = 1 cm, diameter = 100 m, resolution = 20".

Andromeda radiomap with Effelsberg telescope

Effelsberg 100-m (Germany)

Andromeda galaxy –optical

Longer wavelength means poorer angular resolution.Advantages 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.

Radio Astronomy

Parkes 64-m (Australia)

Green Bank 100-m telescope (WV) Arecibo 300-m telescope (Puerto Rico)

Jodrell Bank 76-m (England)

Interferometry

A technique to get improved angular resolution using an array of telescopes. Most common in radio, but also limited optical interferometry.

D

Consider two dishes with separation D vs. one dish of diameter D.By combining the radio waves from the two dishes, the achieved angular resolution is the same as the large dish.

Example: wavelength = 1 cm, separation = 2 km, resolution = 1"

Very Large Array (NM). Maximum separation 30 km

Very Long Baseline Array. Maximum separation 1000's of km

VLA and optical image of Centaurus A

Radio dishes are large in order to

Clicker Question

a) improve angular resolution.

b) give greater magnification.

c) increase the range of waves they can collect.

d) detect shorter waves than optical telescopes for superior resolution.

Radio dishes are large in order to

Clicker Question

Resolution is worse with long-wave light, so radio telescopes must be large

to compensate.

a) improve angular resolution.

b) give greater magnification.

c) increase the range of waves they can collect.

d) detect shorter waves than optical telescopes for superior resolution.

Clicker Question

Radio telescopes are useful because

a) observations can be made day & night.

b) we can see objects that don’t emit visible light.

c) radio waves are not blocked by interstellar dust.

d) they can be linked to form interferometers.

e) All of the above are true.

Clicker Question

Radio telescopes are useful because

a) observations can be made day & night.

b) we can see objects that don’t emit visible light.

c) radio waves are not blocked by interstellar dust.

d) they can be linked to form interferometers.

e) All of the above are true.

The Very Large Array links separate radio telescopes to create

much better resolution.

Clicker Question:

When multiple radio telescopes are used for interferometry, resolving power is most improved by increasing:

A: the distance between telescopes;

B: the number of telescopes in a given area;

C: the diameter of each telescope;

D: the power supplied to each telescope

Astronomy at Other WavelengthsTelescopes also observe infrared, UV, X-rays and gamma rays.Mostly done from space because of Earth's atmosphere.

Infrared allows you to see radiation fromwarm dust ininterstellar gas.

Spitzer SpaceTelescope - infrared

Infrared also allows you to see through dust. Dust is good atblocking visible light but infrared gets through better.

Trifid nebula in visible light Trifid nebula with Spitzer

GLAST – Gamma Ray Large Area Space Telescope

Gamma rays are the most energeticphotons, tracing high-energy events inUniverse such as “Gamma-ray Bursters”.

Latest mission is GLAST – successfullylaunched this year.

X rays and gamma rays will not reflect off mirrors as other wavelengths do; need new techniques.

X rays will reflect at a very shallow angle, and can therefore be focused.

Gamma Rays cannot be focused at all; images are coarse

X-ray Optics

Hubble Space Telescope and its successor-to-be: the James WebbSpace Telescope

Advantage of space for optical astronomy: get above blurring atmosphere – much sharper images.

Center of M51: HST (left; 0.05” resolution) vs.ground-based (right; 1” resolution)

The James Web Space Telescope

Will have diameter 6.5 meters (vs. HST 2.5 meters) – much higher resolution and sensitivity. Will also observe infrared, whereasHubble is best at visible light. Expected launch 2013.

Mock-up of JWST

Clicker Question:

The biggest telescopes on Earth are:

A: Gamma-ray telescopes.

B: X-ray telescopes.

C: Optical telescopes

D: Radio telescopes

E: Infra-red telescopes

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

• Refracting telescopes make images with a lens.

• Reflecting telescopes make images 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 3

• 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 is limited.

• Interferometry can greatly improve resolution.

Summary of Chapter 3, 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 3, cont.