1. Midterm!
In 2 weeks Part I (online exam, Mastering Astronomy, 50 pts) will be available, due October 26th, 11:59pm
In 3 weeks, Part II (in class exam, 50 pts.)
Taken in 3rd hour (week of 10/22 to 10/25)
Bring SCANTRON (882 form) and #2 pencil
Based on “Review Questions” handout, available soon!
Also: 10 of the 25 extra credit points are due by October 26th, noon.

2. © Sierra College Astronomy Department
Lecture 5b: Telescopes: Portals of Discovery The Eye: The Everyday Light Sensor
The eye is made of a lens, pupil, and a retina
The pupil allows a certain amount of light to enter the eye.
The pupil is constricted (and lets in less light) when it is bright and dialates (and lets in more light) when it is dim
The lens focuses light to point on the retina
Eye
Demo with blackboard optics.
© Sierra College Astronomy Department

3. © Sierra College Astronomy Department
Lecture 5b: Telescopes: Portals of Discovery Refraction and Image Formation
Focal point (of a converging lens or mirror) is the point at which light from a very distant object converges after being refracted or reflected.
Focal length (F) is the distance from the center of a lens or a mirror to its focal point.
Image is the visual counterpoint of an object, formed by refraction or reflection of light from the object.
Lens
Focusing
Demo
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4. © Sierra College Astronomy Department
Lecture 5b: Telescopes: Portals of Discovery Refraction and Image Formation
Light travels in a straight line as long as it remains in the same medium (i.e., the material that transmits light).
Reflection is the redirecting of light off a surface
Incident angle = reflection angle
Refraction is the bending of light as it crosses the boundary between two materials in which it travels at different speeds. Web Site
Reflection
Demo with blackboard optics.
Refraction1
Refraction2
© Sierra College Astronomy Department

5. © Sierra College Astronomy Department
Lecture 5b: Telescopes: Portals of Discovery Refraction and Image Formation
The amount of refraction is determined by two factors:
Relative speeds of light in the two materials (e.g., air and glass).
The ratio of the speed light in vacuum to its speed in some material is called the index of refraction
The angle between rays of light and the surface (i.e., the smaller the angle between the ray of light and the surface, the more the light bends on passing through the surface).
Demo
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6. Lecture 5b: Telescopes: Portals of Discovery Refraction and Telescopes
Dispersion
Different colored light beams refract at slightly different angles.
Dispersion is the separation of light into its various wavelengths upon refraction (it’s what a prism does).
This effect can seen when light is refracted in water droplets, producing a rainbow
dispersion
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7. Lecture 5b: Telescopes: Portals of Discovery Refraction and Telescopes
Chromatic Aberration (ab-ĕ-RAY-shun)
While dispersion can be useful in examining the colors coming from an object, it also introduces an inherent problem
Chromatic aberration is a defect of optical systems that results in light of different colors being focused at different places. The resulting image will be fuzzy at the edges.
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8. © Sierra College Astronomy Department
Lecture 5b: Telescopes: Portals of Discovery The Camera and Recording Images
The camera works like an eye except it can make a permanent record of what it sees.
A detector is any device that records light: flim, digital-imaging chips
A camera has two way of controlling the amount of light which enters it
Aperture or opening size of the camera (like the pupil of the eye)
Exposure time which is amount of time the camera can collect light.
Eye
Demo with blackboard optics.
© Sierra College Astronomy Department

9. Lecture 5b: Telescopes: Portals of Discovery Large Optical Telescopes
Charge-coupled device (CCD) is an electronic “camera” that serves as a light detector by emitting electrons when struck by incoming photons. The chip which collects the photons is usually divided into pixels (short for picture elements). The data collected is formed into images by a computer.
CCDs have a much wider dynamic range and greater sensitivity than film cameras
90% of photon that strike the chip are recorded (only 10% of photons on film are recorded)
Also digital images may be manipulated after they are taken
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10. Lecture 5b: Telescopes: Portals of Discovery The Refracting Telescope
Objective is the main light-gathering element - lens or mirror - of a telescope. It is also called the primary. It is characterized by its diameter (D).
An eyepiece (which may be a combination of lenses) added just beyond the focal point of the telescope’s objective acts as a magnifier to enlarge the image.
Simple
Telescope
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11. Lecture 5b: Telescopes: Portals of Discovery The Powers of a Telescope
Angular size of an object is the angle between two lines drawn from the viewer to opposite sides of the object.
Magnifying power (or magnification) is the ratio of the angular size of an object when it is seen through the instrument to its angular size when seen with the naked eye.
Magnifying power:
M = Fobjective / Feyepiece
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12. Lecture 5b: Telescopes: Portals of Discovery The Powers of a Telescope
Long focal length eyepieces (>25 mm) produce less magnification; short focal length eyepieces (<10 mm) produce more magnification.
Field of view is the actual angular width of the scene viewed by an optical instrument.
As magnification increases the field of view decreases.
Have class build telescopes with two-lensed magnifying glasses
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13. Lecture 5b: Telescopes: Portals of Discovery The Powers of a Telescope
Light-Gathering Power
Light-gathering power or Light-collecting area is a measure of the amount of light collected by an optical instrument (the area of the objective lens or mirror).
Light-gathering power is related to the size of the objective, which is usually given as a diameter. Remember that the area of a circle is proportional to the (diameter)².
LGP
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14. Lecture 5b: Telescopes: Portals of Discovery The Powers of a Telescope
The angular separation between two points of light depends on the actual separation and their distance from us (See Mathematical Insight 6.1)
Small
angle
If the angles are in arcseconds:
© Sierra College Astronomy Department

15. Lecture 5b: Telescopes: Portals of Discovery The Powers of a Telescope
Resolving Power (Angular Resolution)
Diffraction is the spreading of light upon passing the edge of an object. This also depends on the color of light
Resolving power (or angular resolution) is the smallest angular separation detectable with an instrument. It is a measure of an instrument’s ability to see detail. This is often called the diffraction limit of the telescope and is given by (see Mathematical Insight 6.2):
in arcseconds
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16. Lecture 5b: Telescopes: Portals of Discovery The Powers of a Telescope
A 15-cm (6-inch) telescope has a maximum resolving power of 1 arcsec, or 1/3600°.
Because of atmospheric turbulence (which causes the stars to twinkle), even the largest Earth-based telescopes have a practical resolving power of between 1 and ½ arcsec.
Operating above the atmosphere, Hubble Space Telescope has a resolving power of 0.1 arcsec or better.
Hubble
© Sierra College Astronomy Department

17. Lecture 5b: Telescopes: Portals of Discovery The Reflecting Telescope
An inwardly curved - or concave - mirror can bring incoming light rays to a focus and is used to construct reflecting telescopes.
The reflecting telescope was invented by Isaac Newton, who also used a small flat mirror placed in front of the objective mirror to deflect light rays out to the eyepiece.
Concave
Mirror
Focusing
Telescope
designs
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18. Lecture 5b: Telescopes: Portals of Discovery The Reflecting Telescope
designs
A Newtonian focus reflecting telescope has a plane mirror mounted along the axis of the telescope so that the mirror intercepts the light from the objective mirror and reflects it to the side.
A Cassegrain focus reflecting telescope has a secondary convex mirror that reflects the light back through a hole in the center of the primary mirror.
© Sierra College Astronomy Department
See next slide

19. Newtonian
Prime
Schmidt-Cassegrain
Cassegrain

20. Lecture 5b: Telescopes: Portals of Discovery The Reflecting Telescope
Reflectors can be made larger (and less expensively) than refractors because:
There are fewer surfaces to grind, polish, and configure correctly.
Reflecting mirrors do not exhibit chromatic aberration as do lenses.
Light does not transmit through a mirror so imperfections in the glass are not critical.
Mirrors can be supported on their backs; lenses must be supported along their rims
© Sierra College Astronomy Department

21. © Sierra College Astronomy Department
Lecture 5b: Telescopes: Portals of Discovery What to do with a Telescope?
Comic
So we have a telescope! What do we do with it?
Imagary: Most detectors are monochromatic or are sensitive to a wavelength regime, but by the use of filters one can take pictures that only let in red, green and blue light. Then by combining these, one can get a color picture in the end
© Sierra College Astronomy Department

22. Lecture 5b: Telescopes: Portals of Discovery Large Optical Telescopes
Photometry is the measurement of light intensity from a source, either the total intensity or the intensity at each of various wavelengths. Early photometers were like a camera’s light meter; modern photometers use a CCD for greater speed and accuracy.
Timing: one can measure the time it takes for an object (e.g. star or asteroid) to brighten and dim and produce a light curve, a plot of the intensity vs. time.
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23. Lecture 5b: Telescopes: Portals of Discovery Large Optical Telescopes
Spectral analysis uses a spectrometer - an instrument that separates electromagnetic radiation according to wavelength. A spectrograph is a visual record of the spectrum taken by a spectrometer.
A spectrometer uses a diffraction grating - a device that uses the wave properties of EM radiation to diffract and separate the radiation into its various wavelengths.
© Sierra College Astronomy Department

24. Lecture 5b: Telescopes: Portals of Discovery Large Optical Telescopes
The best viewing conditions for large telescopes is on top of mountains in dry, clear climates and away from artificial lights (i.e. light pollution). This also reduces the amount of atmospheric turbulence.
Active/Adaptive optics is a system that monitors and changes the shape of a telescope’s objective to produce the best image.
© Sierra College Astronomy Department

25. Lecture 5b: Telescopes: Portals of Discovery Radio Telescopes
Radio waves have less intensity than light rays and have much longer wavelengths (which leads to less resolution).
To detect radio waves, very large parabolic dishes are required. However, the dish surfaces do not have to be as smooth as glass mirrors because the longer radio wavelengths diffract more than do light rays.
Windows
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26. Lecture 5b: Telescopes: Portals of Discovery Interferometry
Interferometry is a procedure that allows a number of telescopes to be used as one by taking into account the time at which individual waves from an object strike each telescope.
Interferometry is possible because extremely accurate atomic clocks allow for precise timing between radio telescopes.
Interferometry increases the resolution of the resulting image because the size of the objective is effectively the size of the furthest separated dishes.
© Sierra College Astronomy Department

27. Lecture 5b: Telescopes: Portals of Discovery Interferometry
The Very Large Array (VLA) near Socorro, New Mexico is the most famous example of interferometry
There are twenty-seven 25-m dishes which can be separated by as much as 40 km.
The Very Large Baseline Array (VLBA) comprise of eleven 25-m dishes spread across the United States
A radio antenna has been put in space to extend the resolution further
Optical interferometers have been constructed, but face far more engineering challenges. The Very Large Telescope (VLT) is one example, consists of four 8.2-m telescopes
© Sierra College Astronomy Department

28. The Very Large Array along one Arm

29. VLBA configuration

30. Very Large Telescope (VLT)

31. © Sierra College Astronomy Department
Lecture 5b: Telescopes: Portals of Discovery Detecting Other EM Radiation
Near infrared nm to 40,000 nm - can be detected from high, dry mountain tops such as Mauna Kea in Hawaii.
Far infrared - greater than 40,000 nm - can be detected from aircraft such as the SOFIA, NASA’s Airborne Observatory.
Infrared telescopes must be cooled so heat (IR radiation) from the surroundings does not mask the signals received from space.
Currently, the Spitzer Space Telescope studies objects in the infrared
© Sierra College Astronomy Department

32. © Sierra College Astronomy Department
Lecture 5b: Telescopes: Portals of Discovery Detecting Other EM Radiation
Ozone is the chief absorber of wavelengths shorter than about 300 nm. Ultraviolet telescopes must be located in space.
Likewise, X-ray telescopes and gamma-ray telescopes also must be placed above the atmosphere is orbiting satellites.
Chandra is an X-ray telescope currently in space
© Sierra College Astronomy Department

33. Hubble Space Telescope (HST)
Fig.06.35

34. HST will be upgraded and be in service for a few more years.
Lecture 5b: Telescopes: Portals of Discovery The Hubble Space Telescope (HST)
HST is designed to observe across the spectrum from infrared to ultraviolet.
HST sees light before it encounters the atmospheric turbulence of the Earth
HST underwent successful repair in 1993 and is now functioning at design specifications.
HST will be upgraded and be in service for a few more years.
The James Webb Space Telescope (JWST) will be launched in 2012 and will replace HST

35. © Sierra College Astronomy Department
The End
© Sierra College Astronomy Department