Telescopes & Detectors (Power Point 07) Images: NASA

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

What are the main features of optical telescopes? Reflection is a bounce (mirrors) Refraction is a change in velocity (lenses) Image: cnx.org Image: wordpress.com

Reflecting Telescope Reflecting telescopes utilize a mirror to collect light.

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

Refracting Telescope Refracting telescopes utilize a lens to collect light. Objective or Primary Lens Eyepiece Lens Creative Commons Image

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

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

More Practice Telescopes collect 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?

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

Light Gathering Power LGP = (Diameter)2 Ability of the telescope to collect photons depends on the diameter of the main lens/mirror LGP = (Diameter)2

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

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

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

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?

Credit: Stellarium.org

Atmospheric Windows Some l’s are blocked by our atmosphere. Image Credit: Australia Telescope

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?

Hubble Image Credit: NASA

What is a radio telescope? A large parabolic dish gathers radio waves and reflects this energy to a central focus. Star formation regions SETI program

NRAO Interferometer

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

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? Explain why interferometers are needed to increase resolution.

How do astronomers analyze the light gathered by telescopes? Telescopes are used to gather the light. Detectors are used to find the detail in the light.

Photometers Photometers count photons. (measure intensity) http://www.spitzer.caltech.edu/mission/396-The-Multiband-Imaging-Photometer-MIPS- The Double Helix Nebula captured by Spitzer's MIPS Credit: NASA/JPL-Caltech/M. Morris

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)

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)

What do adaptive optics do? http://lot.astro.utoronto.ca/ao_new.html Image Credit: Canada-France-Hawaii Telescope

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

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

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?