1. Telescopes Lecture

2. Standards
Understand how knowledge about the universe comes from evidence collected from advanced technology (e.g., telescopes, satellites, images, computer models)
Understand the scientific foundations of common technologies (e.g., telescopes)

3. Introduction
Telescopes are used to look at astronomical objects, such as
planets
stars
nebulae
galaxies
details of the universe

4. Introduction
Astronomy is not an experimental science, it is an observational science.
Can’t bring a star into the lab to study
Experimentation is done indirectly in one of two ways:
Different observations of the same object in different parts of the spectrum
Theoretical models – using computers and technology

5. Introduction Telescopes are time machines.
When we look at stars, we are seeing light that is decades, centuries or millions to billions of years old.
We are looking into the past.
We never observe the universe as it is, but as it was

6. Introduction Light from our sun takes 8 minutes to reach Earth.
We therefore see the sun as it existed 8 minutes in the past.
The closest galaxy to us is Andromeda. The light we receive from it is 3 million years old.
We are seeing it as it was 3 million years ago!

7. Introduction
Without optical telescopes, we can see 6000 stars in both hemispheres.
With a telescope, we can see billions of stars.

8. Optical Telescopes
Optical telescopes observe the universe in the visible portion of the electromagnetic spectrum.
An optical telescope is essentially a “light bucket”:
It gathers light
Resolves the object
And magnifies the object

9. Optical Telescopes
The telescope’s primary function is to capture as many photons as possible from a given region of the sky.
Use of telescopes to observe space started with Galileo and the refracting telescope.

10. Refracting Telescopes
Parts of a refracting telescope:
Objective – main lens used to bring light to the focus. Mounted on end of telescope.
Focus – a point in a telescope that all light rays pass through.
Focal length = distance from the lens to the focus. It determines the angular field of view. Longer focal length = narrower field of view and vice versa.

11. Refracting Telescopes
Eyepiece - magnifies the image.
The magnification = the telescope focal length divided by the eyepiece focal length

12. Refracting Telescopes

13. Reflecting Telescopes
Parts of a reflecting telescope
Curved mirror = objective.
Called the primary mirror because there is usually more than one
The primary mirror reflects the light rays to the focus (called the prime focus)
Focal length = distance from the primary mirror to the prime focus.

14. Reflecting Telescopes

15. Reflecting Telescopes
Most telescopes in use today are reflecting telescopes because there are disadvantages to refracting telescopes:
Light must pass through the lens on a refractor. Some of this light does not pass through uniformly – called chromatic aberration – which causes a colored halo around the object being observed.

16. Reflecting Telescopes
2. Some light is absorbed by the glass as it passes through the lens. This is severe for IR and UV observations.
3. Large lenses can be very heavy. In a telescope, they can only be supported by their sides and can deform under their own weight. Mirrors, on the other hand, can be supported along their back.
4. Lenses have two surfaces, which must be accurately machine polished. A mirror has only one surface.

17. Telescopes in Use Today
Largest refracting telescope is 1 meter
in diameter.
Located at Yerkes Observatory in Wisconsin.
Built in 1897.

18. Telescopes in Use Today
A few of the largest reflecting telescopes:
3.8 m – Kitt Peak, Arizona
5 m - Hale telescope at Mt. Palomar in California
10 m – Keck and Keck II, Mauna Kea, Hawaii
Mirror is composed of 36 segment
First light in 1993 & 1996, respectively
10.4 m – Gran Telescopio Canarias, La Palma, Canary Islands
Segmented mirror based on Keck
First light in 2007

19. Kitt Peak
Hale Telescope

20. Twin Keck Telescopes

21. Gran Telescopio Canarias

22. Telescopes in Use Today
Hubble Space Telescope
2.4 meter
Above Earth’s atmosphere, so has good resolution

23. Telescopes in Use Today
Use CCD’s (charge coupled devices) instead of photographic plates.
These are electronic detectors hooked up to computers to make digital images
Advantage of CCD’s: they’re more efficient so can show objects 10 to 20 times fainter than the same telescope using a photographic plate.

24. Telescopes in Use Today
Adaptive optics (developed because of Star Wars) - deform the shape of mirrors to undo effects of atmospheric turbulence.
Starfire Optical Range
Kirtland Air Force base:
laser beams probe the
atmosphere to allow a
computer to adjust for
turbulence

25. Why Build Bigger and Bigger Telescopes?
Bigger telescopes increase light gathering power.
Can see objects further away
A 5 meter telescope will produce an image 25 times brighter than a 1 meter telescope
Andromeda Galaxy: bottom image take with telescope
twice the size of one used to make top image

26. Why Build Bigger and Bigger Telescopes?
Bigger telescopes have better resolving power
Resolution – ability of a device to form distinct, separate images of objects lying close together in the field of view
Better resolution = better detail

27. Resolving Power Andromeda Galaxy as resolution is improved 600 times
Same objects as resolution improves

28. Why Build Bigger and Bigger Telescopes?
Making mirrors for reflecting telescopes:
Use materials that can be polished and withstand large temperature changes with little expansion or contraction, such as quartz, glass or ceramics
Molten material is poured into a cast and cooled over several years. Then it is ground and polished
Surface is coated with thin film of aluminum to provide a reflecting surface

29. Earth’s Turbulent Atmosphere
Reflects and refracts incoming energy (light), so images are blurred
Light from a star is refracted – this causes it to twinkle
Hubble Space Telescope is above atmosphere, so it allows astronomers to view the universe 20 times sharper than from ground based telescopes