1. Telescopes & Detectors
(Power Point 06)
Images: NASA

2. Student Learning Objectives
Classify and compare optical systems used in astronomy
Differentiate between reflection and refraction
Identify types and properties of telescopes
Name basic detectors used in astronomy

3. What are the main features of optical telescopes?
Reflection is a bounce of energy from a surface (mirrors)
Refraction is a change in velocity as energy passes from one type of matter to another (lenses)
Image: cnx.org
Image: wordpress.com

4. Reflecting Telescope
Reflecting telescopes utilize a mirror to focus light.
Objective/Primary

5. Reflector Issues
Reflectors may produce a fuzzy image due to spherical aberration (curvature distortion).
Hubble Images

6. Refracting Telescope
Refracting telescopes utilize a lens to focus light.
Objective/Primary
Eyepiece Lens
Creative Commons Image

7. Refractor Issues
All refractors produce chromatic aberration (“color deviation or distortion”).
A very long focal length will minimize chromatic aberration. This is why the early refracting telescopes (like Galileo’s) were made very long.
Image Credit: Wikimedia Commons

8. Practice
What are the advantages of using segments instead of one large mirror?

9. More Practice
Telescopes focus light and magnify the light. Which is more important?
Where are the telescopes that astronomers use?
Are there telescopes that can be used during the day?

10. Mauna Kea, Hawaii (Elevation 14,000 Feet)
Image Credit: Institute for Astronomy, University of Hawaii

11. Light Gathering Power LGP = (Diameter)2
Ability of the telescope to collect photons depends on the diameter of the objective/primary lens or mirror
LGP = (Diameter)2

12. Larger D and Smaller l  Better Resolution
Resolving Power
Ability of the telescope to “see” fine detail and to distinguish individual objects, depends on both the telescope and the wavelength of light being gathered.
a = (2.06 x 105) l
Diameter
Larger D and Smaller l  Better Resolution

13. An image of two stars through a telescope with low angular resolution
An image of two stars through a telescope with high angular resolution.
Image Credit: Richard Bloch

14. Images: NASA & Las Cumbres Observatory
Magnifying Power
The eyepiece enlarges the image, and makes light rays parallel.
Images: NASA & Las Cumbres Observatory

15. Practice Which telescope would allow you to see fainter objects?
Small diameter telescope
Large diameter telescope
Either as long as there is good magnification
What are some issues astronomers may encounter when trying to observe objects from Earth?

16. Credit: Stellarium.org

17. Atmospheric Windows
Some l’s are blocked by our atmosphere. We say there are “atmospheric windows” for the l’s that pass through air.
Image Credit: Australia Telescope

18. Practice Describe what an atmospheric window is.
Would a full moon effect observations? Explain.
What are the advantages of using a space telescope? What are the disadvantages?

19. Hubble
Image Credit: NASA

20. What is a radio telescope?
A large parabolic dish gathers radio waves and reflects this energy to a central focus.
A single radio telescope has very poor resolution. Some l’s are larger than the dish!

21. Star formation regions
Radio Astronomy
Radio waves are used to detect large structures and to “listen”.
Star formation regions
SETI program
Radio telescopes are added together to increase resolution. (Interferometers)

22. NRAO Interferometer

23. Light Interference
An interferometer is a set of radio telescopes connected together.
Light waves are added together (constructive interference)
Angular resolution is based on diameter of system
Ability to change effective diameter

24. Practice
Radio telescopes can be used during the day, and when it is cloudy. What proof do you have from your experiences that this is true?

25. Telescopes vs Detectors
Telescopes are used to gather and focus light.
Detectors are used to find the detail in the light.

26. An X-ray Space Telescope

27. Photometers Photometers count photons. (measure intensity)
The Double Helix Nebula captured by Spitzer's MIPS Credit: NASA/JPL-Caltech/M. Morris

28. CCD’s
Charged Coupled Device (CCD) produces electronic images with millions of light detectors, pixels on a small silicon wafer.
Measure Intensity per Pixel
Kepler’s 94.6 million active pixels
(NASA)

29. Spectrometers
Spectrometers divide the light into individual wavelengths for analysis.
Spitzer Space Telescope detects water deep within the core of a forming star system
(NASA)

30. What do adaptive optics do?
Image Credit: Canada-France-Hawaii Telescope

31. Adaptive Optics
A computer compensates for atmospheric turbulence in real time by quickly adjusting light path in the optics.
Deformable mirrors controlled by computers
Measures blurring of reference star and changes mirrors
Corrects distortions caused by turbulence in atmosphere

32. UCLA Galactic Center Group / W. M. Keck Observatory Laser Team

33. Practice What do people use everyday that has a CCD camera?
Optics are lenses and mirrors. How do adaptive optics differ from regular optics?
What contributes to “bad-seeing” for an astronomer using a ground based telescope?