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 mirrors The 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 Question Why is a rose red? The rose absorbs red light. The rose transmits red light. The rose emits red light. The rose reflects red light.

Thought Question Why is a rose red? The rose absorbs red light. The rose transmits red light. The rose emits red light. 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 The speed of light through the liquid in your eye is slower than the speed of light in air. Refraction can cause parallel light rays to converge to a focus

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? Light-collecting area: Telescopes with a larger collecting area can gather a greater amount of light in a shorter time. 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 Tool from the Telescopes tutorial.

Thought Question How does the collecting area of a 10-meter telescope compare with that of a 2-meter telescope? It’s 5 times greater. It’s 10 times greater. 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. Emphasize that at a great distance the lights would look like one light rather than two --- but we could still see them. (Students sometimes think that below a particular angular resolution we see nothing at all.)

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 The largest mirrors in the world are in the Keck telescope on Hawaii Twin Keck telescopes on Mauna Kea Segmented 10-meter mirror of a Keck telescope

Designs for Reflecting Telescopes

Newtonian Telescope 1672 One of the most common amateur telescopes for deep sky observing. Early Newtonian used by Herschel Modern version with a Dobsonian mount

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 One of most common amateur telescopes Maksutov-Cassegrain Telescope 1941

Catadioptric Telescopes Corrector lens Schmidt-Cassegrain telescope on an equatorial mount

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 False 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 Diffraction grating breaks light into spectrum Light from only one star enters Detector records spectrum

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