Telescopes: Portals of Discovery

How do light and matter interact?

• Emission• Absorption• Transmission

– Transparent objects transmit light– Opaque objects block (absorb) light

• Reflection or Scattering

Reflection and Scattering

Mirror reflects light in a particular direction

Movie screen scatters light in all directions

Reflection of light with mirrorsThe flat surface enables an incoming light beam to perfectly bounce

Angle of Incidence = Angle of Reflection

Reflection and Scattering with Matter

Interactions between light and matter determine the appearance of everything around us

Thought QuestionWhy is a rose red?

a) The rose absorbs red light.b) The rose transmits red light.c) The rose emits red light.d) The rose reflects red light.

Thought QuestionWhy is a rose red?

a) The rose absorbs red light.b) The rose transmits red light.c) The rose emits red light.d) The rose reflects red light.

Refraction (occurs during transmission)

• The universal speed of light is measured in a vacuum.

• The speed of light is slower in different substances.

Refraction• E.g. Light travels

slower through glass or water than through air.

• Refraction is the bending of light when it passes from one substance into another

Focusing Light

• Refraction can cause parallel light rays to converge to a focus

• The speed of light through the liquid in your eye is slower than the speed of light in air.

Image Formation

• The focal plane is where light from different directions comes into focus

• The image behind a single (convex) lens is actually upside-down!

What have we learned?• How do light and matter ineract?

– 4 forms of interaction, including reflection and transmission

– Refraction is when light slows and bends when travelling through a substance

• How does your eye form an image?– It uses refraction to bend parallel light rays so

that they form an image.– Cameras focus light like your eye and record

the image with a detector (CCDs in digital cameras).

Telescopes: Giant Eyes

• Our goals for learning• What are the two most important properties

of a telescope?• What are the two basic designs of

telescopes?• What do astronomers do with telescopes?

What are the two most important properties of a telescope?

1. Light-collecting area: Telescopes with a larger collecting area can gather a greater amount of light in a shorter time.

2. Angular resolution: Telescopes that are larger are capable of taking images with greater detail.

Light Collecting Area

• A telescope’s diameter tells us its light-collecting area: Area = π(diameter/2)2

• i.e The larger the aperture size, the better the telescope

• The largest telescopes currently in use have a aperture diameter of about 10 meters

Bigger is better

Thought QuestionHow does the collecting area of a 10-meter telescope compare with that of a 2-meter

telescope?

a) It’s 5 times greater.b) It’s 10 times greater.c) It’s 25 times greater.

Angular Resolution• Distant objects look very close

together; i.e. a small 'angular separation'

• Angular resolution is the minimum gap between objects that the telescope can distinguish.

• Eventually the car is so far away that you see the headlights as one light.

-you have reached the limit of your eye to resolve the angular separation.

Angular Resolution• Ultimate limit to

resolution comes from interference of light waves within a telescope.

• Larger telescopes are capable of greater resolution because there’s less interference for the amount of light

What are the basic designs of telescopes?

• Refracting telescope: Focuses light with lenses

• Reflecting telescope: Focuses light with mirrors

• Catadioptric telescope: Focuses light with both lenses and mirrors

Refracting Telescope

• Refracting telescopes need to be very long, with large, heavy lenses

Basic Refractor Telescope 1608

Galileo’s Telescopes 1609

Chromatic Aberration• Light focuses imperfectly in a lens because the

light must transmit though glass• All lenses suffer from chromatic aberration (slight blurring caused by the short wavelength light

focusing before the long wavelengths)

Reflecting Telescope

• Reflecting telescopes do not suffer chromatic aberration• Reflecting telescopes can have much greater diameters.• Most modern telescopes are reflectors

Mirrors in Reflecting Telescopes

Twin Keck telescopes on Mauna Kea

Segmented 10-meter mirror of a Keck telescope

The largest mirrors in the world are in the Keck telescope on Hawaii

Designs for Reflecting Telescopes

Newtonian Telescope 1672One of the most common amateur telescopes for deep sky observing.

Modern version with a Dobsonian mount

Early Newtonian used by Herschel

Cassegrain Reflector Telescope, 1672

More efficient, but requires a more complex secondary mirror.(hyperbolic rather than flat)

Some Cassegrain Correctors -Catadioptrics

Schmidt-Cassegrain Telescope 1930

Maksutov-Cassegrain Telescope 1941

One of most common amateur

telescopes

Catadioptric Telescopes

Schmidt-Cassegrain telescope on an

equatorial mount

Corrector lens

What do astronomers do with telescopes?

• Imaging: Taking pictures of the sky• Spectroscopy: Breaking light into spectra• Timing: Measuring how light output varies

with time

Imaging

• Early astronomers had no choice, they observed by eye and recorded with descriptions and drawings.– Visible light only.

• Camera - permanent record, non-subjective, reproducible world-wide in publications.– Can be built to detect any wavelength.

Imaging

• Astronomical detectors generally record only one color of light at a time

• Several images must be combined to make full-color pictures

Imaging• Astronomical

detectors can record forms of light our eyes can’t see

• Color is sometimes used to represent different energies of nonvisible light

Imaging• True color image

– The colors are the same as if you were viewing with your own eyes

• False color image– The colors have been

arbitrarily assigned to assist image interpretation

The famous image of the Eagle nebula is false color!

(Nebula in true color)

Spectroscopy• A spectrograph

separates the different wavelengths of light before they hit the detector

Diffractiongrating breakslight intospectrum

Detectorrecordsspectrum

Light from only one starenters

Spectroscopy• Graphing

relative brightness of light at each wavelength shows the details in a spectrum

Timing

• A light curve represents a series of brightness measurements made over a period of time

What have we learned?• What are the two most important properties of a

telescope?– Collecting area determines how much light a

telescope can gather– Angular resolution is the minimum angular

separation a telescope can distinguish• What are the basic designs of telescopes?

– Refracting telescopes focus light with lenses– Reflecting telescopes focus light with mirrors– Catadioptric telescopes focus light with both.– The vast majority of professional telescopes are

reflectors

What have we learned?

• What do astronomers do with telescopes?– Imaging– Spectroscopy– Timing