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Optics and Telescopes
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  1. Ohio University - Lancaster Campus slide 1 of 71Spring 2009 PSC 100 Optics and Telescopes Credit: www.sherwoods-photo.com Credit: www.telescopeguides.net

  2. Ohio University - Lancaster Campus slide 2 of 71Spring 2009 PSC 100 This evening we will investigate: • how lenses and mirrors can be used to focus light and form an image. • the 3 basic telescope designs and the advantages and disadvantages of each. • some numbers that characterize a telescope: f-ratio, light gathering power, resolution, magnification

  3. Ohio University - Lancaster Campus slide 3 of 71Spring 2009 PSC 100 This evening we will investigate: • recording the images produced by a telescope. • telescopes that use the other wavelengths of light.

  4. Ohio University - Lancaster Campus slide 4 of 71Spring 2009 PSC 100 • Optics – The science of reflecting and/or refracting (bending) light so as to produce an image of an object. The image is usually recorded so that it can be studied more extensively.

  5. Ohio University - Lancaster Campus slide 5 of 71Spring 2009 PSC 100 • Regarding Mirrors • The Law of Reflection: When a ray of light strikes a shiny or “specular” surface, the ray reflects away at the same angle at which it struck the surface. The angle of incidence equals the angle of reflection, as measured from a ‘normal’ to the surface.

  6. Ohio University - Lancaster Campus slide 6 of 71Spring 2009 PSC 100 i = r a “shiny” or reflective surface

  7. Ohio University - Lancaster Campus slide 7 of 71Spring 2009 PSC 100 • If the reflecting surface is curved correctly, the light can be focused to a point, called the focal point. An image forms near the focal point. Credit: www.antonine-education.co.uk

  8. Ohio University - Lancaster Campus slide 8 of 71Spring 2009 PSC 100 • Regarding Lenses • The Law of Refraction: When light moves from a less dense medium (empty space or air) to a denser medium (glass), the light slows down and bends INTO the denser medium.

  9. Ohio University - Lancaster Campus slide 9 of 71Spring 2009 PSC 100 speed of light in air = 3 x 108 m/s speed of light in glass = 2 x 108 m/s

  10. Ohio University - Lancaster Campus slide 10 of 71Spring 2009 PSC 100 • Glass can be formed into a convex lens which will also focus light. An image forms near the focal point. The focal length is the distance from the centerline of the lens to the focal point. focal length

  11. Ohio University - Lancaster Campus slide 11 of 71Spring 2009 PSC 100 • The f-ratio is a way to compare or rate convex (converging) lenses. • The f-ratio is the focal length of the lens divided by the lens’ diameter.

  12. Ohio University - Lancaster Campus slide 12 of 71Spring 2009 PSC 100 Thicker lenses tend to focus closer to the lens and give brighter images. These are “fast” lenses. Do these lenses have low or high f-ratios? But these lenses have other problems.

  13. Ohio University - Lancaster Campus slide 13 of 71Spring 2009 PSC 100 Thinner lenses focus farther from the lens, give less-bright images, and are described as “slow” lenses.

  14. Ohio University - Lancaster Campus slide 14 of 71Spring 2009 PSC 100 • When taking photographs of space objects, using a “fast” lens with a low f-ratio means less time is needed for the photograph. This results in less blurring due to vibration of the telescope and the motion of the stars. Credit: Gemini Observatory/AURA

  15. Ohio University - Lancaster Campus slide 15 of 71Spring 2009 PSC 100 • Chromatic Aberration – a problem with lenses • The edges of lenses act like prisms. They split “white” light into all the colors of the rainbow. • Problem: the different colors focus at different focal points. This means that if you focus the blue color of an object, the red is fuzzy, and vice versa.

  16. Ohio University - Lancaster Campus slide 16 of 71Spring 2009 PSC 100 Chromatic Aberration

  17. Ohio University - Lancaster Campus slide 17 of 71Spring 2009 PSC 100 • There’s always a trade-off in optics. The problem of chromatic aberration is worst with “fast” or low f-ratio lenses. These are the lenses we’d like to use most! • The problem is fixed by making compound lenses out of 2 or more different kinds of glass. • Mirror-based telescopes don’t have this problem – a definite advantage!

  18. Ohio University - Lancaster Campus slide 18 of 71Spring 2009 PSC 100 • 3 Types of Telescopes • Refractors (gathers light with a lens) • Reflectors (gathers light with a mirror) • Mixed (uses a combination of lenses and mirrors) • Schmidt-Cassegrain Telescopes • Maksutov-Cassegrain Telescopes

  19. Ohio University - Lancaster Campus slide 19 of 71Spring 2009 PSC 100 • Refracting Telescopes • The “original” type, invented in the 1500’s and first used by Galileo to explore space. • Sharpest, brightest images. • Lenses are heavy and expensive! • Prone to chromatic aberration. • Give an inverted (upside-down) image. • Can only be made up to about 40 inches in diameter.

  20. Ohio University - Lancaster Campus slide 20 of 71Spring 2009 PSC 100 Credit: library.thinkquest.org

  21. Ohio University - Lancaster Campus slide 21 of 71Spring 2009 PSC 100 • Reflecting Telescopes…Advantages • Mirrors are much cheaper to make than lenses, and are very light-weight, easy to carry. • Mirrors can be VERY large. Multiple mirrors can be combined to simulate a single gigantic mirror. • No chromatic aberration.

  22. Ohio University - Lancaster Campus slide 22 of 71Spring 2009 PSC 100 • Reflecting Telescopes…Disadvantages • Not quite as sharp or bright an image as the same size refractor. • Large scopes get currents of different temperature air inside their tubes. This can make images blurry. • Mirrors will oxidize (corrode) over time.

  23. Ohio University - Lancaster Campus slide 23 of 71Spring 2009 PSC 100

  24. Ohio University - Lancaster Campus slide 24 of 71Spring 2009 PSC 100 • Combination ‘scopes…the Cassegrains • Very short tube length, because the light gets “folded” back on itself twice. This makes the scope easy to handle & transport. • Moderately expensive. • Best choice for amateur astrophotography, because the tube doesn’t vibrate or shake very much.

  25. Ohio University - Lancaster Campus slide 25 of 71Spring 2009 PSC 100 The corrector plate is a type of lens. A secondary mirror is glued to its inner surface.

  26. Ohio University - Lancaster Campus slide 26 of 71Spring 2009 PSC 100 • The telescope mount is as important as the optics! There are two types… • Altitude-Azimuth. Like aiming a tank. Point it in the compass direction (azimuth) you want, then point it up to the angle (altitude) you want. • Easy to use, but image rotates over time.

  27. Ohio University - Lancaster Campus slide 27 of 71Spring 2009 PSC 100 • Equatorial. Part of the mount is aimed at the north celestial pole. The mount then swivels east-west to follow an object through the sky. • Disadvantage: a real bear to use! • Advantage: the picture in the telescope doesn’t appear to rotate over time.

  28. Ohio University - Lancaster Campus slide 28 of 71Spring 2009 PSC 100 • What is the function of a telescope? It’s not just to make the image bigger! • Gathering light • Resolving details • Magnifying the image

  29. Ohio University - Lancaster Campus slide 29 of 71Spring 2009 PSC 100 • A Telescope is a Light Funnel • Gathering light from dim objects is the MOST important function of a telescope. • Which would you rather see, a large but very dim image or a smaller, but very bright image?

  30. Ohio University - Lancaster Campus slide 30 of 71Spring 2009 PSC 100 • Light-gathering power (LGP) • How much light can the human eye gather? A “typical” human eye has a pupil that is about 0.5 cm in diameter when fully dilated at night. • Area of the pupil =  r2 =  (0.25 cm)2 = about 0.2 cm2. • The main purpose of the telescope is to take light from a much larger area and “funnel” it into your pupil.

  31. Ohio University - Lancaster Campus slide 31 of 71Spring 2009 PSC 100 • How much light can a telescope gather? • A 10 inch diameter scope (25 cm diameter) gathers (12.5cm)2 = 490 cm2. • This is 490 cm2 / 0.2cm2 = almost 2500 times more light than the naked eye.

  32. Ohio University - Lancaster Campus slide 32 of 71Spring 2009 PSC 100 • To compare a telescope’s LGP to that of a “typical” eye, use the formula LGP = 4D2 where D is the telescope’s lens/mirror diameter in centimeters. (2.54 cm/inch) • What is the LGP of a 6 inch telescope?

  33. Ohio University - Lancaster Campus slide 33 of 71Spring 2009 PSC 100 • Seeing Small Details – Resolution • Resolution is defined as the minimum angle between 2 objects, that will allow you to see them as 2 separate objects and not one big blob. • Units are arcseconds (1/3600th of a degree) • The smaller the theoretical resolution number is, the smaller the details you can see.

  34. Ohio University - Lancaster Campus slide 34 of 71Spring 2009 PSC 100 • Theoretical Resolution ()= (2.1x105)(wavelength in m) (diameter of objective mirror or lens) • The diameter is in meters, not inches! • What is the resolution of a 10 inch scope for blue light (450 nm or 4.5 x 10-7 meters)? • Calculate the resolution again for red light (7.0 x 10-7 meters)

  35. Ohio University - Lancaster Campus slide 35 of 71Spring 2009 PSC 100 • Resolution – not the same for all light! • What color of visible light would have the poorest resolution? The best? • What “color” of all the types of light would have the poorest resolution? How is this limitation overcome?

  36. Ohio University - Lancaster Campus slide 36 of 71Spring 2009 PSC 100 • There’s a practical limit to resolution for a ground-based telescope…the Atmosphere! • Air currents in the atmosphere will make the image blurry. Think twinkling stars! • The best time for viewing is in the hours before dawn, since the air currents are least. • Are there any other accommodations that could be made?

  37. Ohio University - Lancaster Campus slide 37 of 71Spring 2009 PSC 100 • Magnification – the least important function of a telescope • M = focal length of the objective lens or mirror focal length of eyepiece lens • What is the magnification factor (power) of a telescope with a 1000 mm focal length, using an eyepiece with a 2.5 cm focal length?

  38. Ohio University - Lancaster Campus slide 38 of 71Spring 2009 PSC 100 • My 10 inch (25 cm) Schmidt-Cassegrain telescope has a 250 cm focal length. If I use an eyepiece with a 1.25 cm focal length, what is the magnification? • If I want to increase the magnification, should I use a 2.5 cm focal length eyepiece, or a 0.75 cm focal length eyepiece?

  39. Ohio University - Lancaster Campus slide 39 of 71Spring 2009 PSC 100 • A bit of review: • If you doubled the size of a telescope’s objective mirror without making any other changes, how would the telescope’s properties change?

  40. Ohio University - Lancaster Campus slide 40 of 71Spring 2009 PSC 100 • Why do astronomers no longer use film in their cameras? • Film has been replaced by CCD chips (Charge-Coupled Device).

  41. Ohio University - Lancaster Campus slide 41 of 71Spring 2009 PSC 100 Credit: rst.gsfc.nasa.gov/Intro/ccd.jpg

  42. Ohio University - Lancaster Campus slide 42 of 71Spring 2009 PSC 100 The surface of a CCD chip is divided up into rows of rectangular light-sensitive pixels (picture elements). Films have irregularly shaped and distributed grains of light-sensitive chemicals. The pixels are usually much more sensitive than the chemical grains.Advantage???

  43. Ohio University - Lancaster Campus slide 43 of 71Spring 2009 PSC 100 Film Emulsions Credit: www.imx.nl/photosite/technical/Filmbasics/grainshapes.jpg

  44. Ohio University - Lancaster Campus slide 44 of 71Spring 2009 PSC 100 Individual pixel Light-sensitive layer (gives off electrons when struck by light) Semi-conductor layer (acts as an electron filter) Collector layer (holds the electrons until counted) This stack of 3 layers is one pixel.

  45. CCD Detector 70% efficient Shorter exposures Resolution can be higher (8 Mpixels or higher) Film 5% to 10% efficient 7 to 14 times longer exposures Resolution is limited by grain size Ohio University - Lancaster Campus slide 45 of 71Spring 2009 PSC 100 Why use CCD’s instead of film?

  46. Pictures are available in seconds. Pictures can be digitally added together. Initial cost is similar to film but operating costs are much lower. Pictures must be developed (hours to days) Digital techniques are possible, but more difficult. Operating costs higher. Ohio University - Lancaster Campus slide 46 of 71Spring 2009 PSC 100

  47. Ohio University - Lancaster Campus slide 47 of 71Spring 2009 PSC 100 A typical, high-res image produced by a CCD. Credit: solarsystem.nasa.gov/multimedia/gallery/PIA02888.jpg

  48. Ohio University - Lancaster Campus slide 48 of 71Spring 2009 PSC 100 • All astrophotographs are black & white. • Photographs can be taken in color, but you lose resolution. • 4 pixels must be “binned” or clustered for color photographs (1 B&W, 1 red, 1 green, 1 blue) This makes the overall pixel size 4 times bigger = lower resolution.

  49. Ohio University - Lancaster Campus slide 49 of 71Spring 2009 PSC 100 1 big pixel if the photo is taken in color 4 smaller pixels if the photo is taken in B/W. Better resolution.

  50. Ohio University - Lancaster Campus slide 50 of 71Spring 2009 PSC 100 • So how can we see all those beautiful “color” photographs? NGC 2393 – The “Eskimo” Nebula Credit: Andrew Fruchter (STScI) et al., WFPC2, HST, NASA