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Announcements

Announcements. Tonight’s “lab”: I ntroduction to the 20” and The Sky software. Also, return your Dobs. If you didn’t bring it in tonight, bring it in tomorrow or Wednesday. Meet at the APSU Farm ~20 minutes after we finish here. Homework: Chapter 6 # 1, 2, 3, 4, 5 & 6

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Announcements

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  1. Announcements • Tonight’s “lab”: Introduction to the 20” and The Sky software. Also, return your Dobs. If you didn’t bring it in tonight, bring it in tomorrow or Wednesday. Meet at the APSU Farm ~20 minutes after we finish here. • Homework: Chapter 6 # 1, 2, 3, 4, 5 & 6 • Due tonight: Telescopic Observations of the Moon

  2. Telescopes Newton’s original telescope with mirror Galileo’s early telescope and lens

  3. Telescope basics Telescopes either use refraction or reflection to focus light to a point. For refraction, the basic law is Snell’s Law: n1sinq1 = n2sinq2. The law of reflection is a much simpler:

  4. If the surfaces of a piece of glass are curved, they will focus light to a point R1 and R2 are the radii of curvature of the two faces, n is the index of refraction of the glass and d is the center thickness of the lens The focal length, f, is the distance from the lens axis to the focal point or focal plane. The focal length of a lens depends on the material of the lens and the curvature of the surfaces

  5. A concave mirror will also focus light to a point For a spherical mirror the focal length is just half the radius of curvature of the mirror. For other shapes the formula is somewhat more complicated

  6. For any telescope, the most important property is the Light Gathering Power (LGP) do is the diameter of the objective in mm. Compares the light gathering power to that of the human eye

  7. Telescopes are often referred to by their f-ratio f is the focal length of the objective and d is its diameter The image scale SI depends on the focal length of the telescope and the size of the image pixels m is the size of a single pixel in micrometers

  8. The ability of a telescope to resolve fine detail is given by the Rayleigh Criterion l is the wavelength of the light being used and d is the diameter of the aperture. q is the smallest resolvable angle of the telescope

  9. Aberrations Spherical aberration can be corrected by using parabolic or hyperbolic surfaces Coma is worse for parabolic surfaces than spherical ones. Correction is to use hyperbolic surfaces Astigmatisms are the result of a non-axially symmetrical lens. All three of these aberrations apply to both lenses and mirrors

  10. The Refracting Telescope

  11. Examples of Refractors

  12. Problems With Refractors:Chromatic Aberration

  13. Achromatic Doublet Provides some color correction but doesn’t completely remove all chromatic aberration.

  14. Apochromatic Design The best apochromats use a three element design with low dispersion glass to reduce chromatic aberration to a minimum. Of course, the more glass the light goes through, the greater the loss.

  15. Newtonian Reflector Still a popular design among amateurs but not widely used by professionals. The focal plane is not large and there are lots of off-axis distortions.

  16. Examples of Newtonian Reflectors

  17. Cassegrain Reflector Classic Cassegrain…Parabolic MirrorsRitchey-Chretien (RC)…Hyperbolic Mirrors

  18. Examples of Cassegrain’s Keck 1 Most professional telescopes use a Cassegrain focus with hyperbolic mirrors: an RC Gemini North

  19. Schmidt-Cassegrain Design Primary mirror is spherical instead of parabolic. The “correcting lens” corrects for spherical aberrations.

  20. Examples of Schmidt-Cassegrain telescopes

  21. Maksutov-Cassegrain Design The correcting lens is a meniscus shape. The Maksutov-Newtonian is also a popular design

  22. Examples of Maksutov Telescopes

  23. Mounts

  24. Alt-Az Mounts

  25. The Dobsonian Mount is an Alt-Az mount

  26. Equatorial Mounts

  27. German Equatorial Mount

  28. Fork Equatorial Mounts

  29. Other Types of Equatorial Mounts Cross-Axis Equatorial English Yoke Equatorial

  30. Example of a Yoke mount Hooker Telescope

  31. Eyepieces come in a variety of different optical designs The magnification of a telescope is just the ratio of the focal length of the objective to the focal length of the eyepiece Since the light is passing through glass, eyepieces suffer from chromatic aberration

  32. My personal favorites are Tele Vue eyepieces

  33. Field of View depends on the eyepiece 60 ° fov 68 ° fov 82 ° fov

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