1. Announcements No lab tonight due to Dark Sky Observing Night last night Homework: Chapter 6 # 1, 2, 3, 4, 5 & 6 First Quarter Observing Night next Wednesday. Set-up starts around 6:45pm Due in two weeks: Telescopic Observations of the Moon. If you want to check out a Dobs for the weekend, you can get it tonight or tomorrow before 4:30pm.

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

3. Telescope basics Telescopes either use refraction or reflection to focus light to a point. For refraction, the basic law is Snell’s Law: n 1 sin  1 = n 2 sin  2. 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 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 R 1 and R 2 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

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) d o 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

8. Magnification is determined by the focal lengths When using an eyepiece When using a CCD camera, the image scale determines the magnification  is the size of a single pixel in micrometers

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

10. 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

11. The Refracting Telescope

12. Examples of Refractors

13. Problems With Refractors: Chromatic Aberration

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

15. 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.

16. 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.

17. Examples of Newtonian Reflectors Invented by Isaac Newton in 1668

18. Cassegrain Reflector Classic Cassegrain…Parabolic Mirrors Ritchey-Chretien (RC)…Hyperbolic Mirrors Invented by Laurent Cassegrain in 1672

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

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

21. Examples of Schmidt-Cassegrain telescopes

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

23. Examples of Maksutov Telescopes

24. Mounts

25. Alt-Az Mounts

26. The Dobsonian Mount is an Alt-Az mount

27. Equatorial Mounts

28. German Equatorial Mount

29. Fork Equatorial Mounts

30. Other Types of Equatorial Mounts Cross-Axis EquatorialEnglish Yoke Equatorial

31. Example of a Yoke mount Hooker Telescope

32. 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

33. My personal favorites are Tele Vue eyepieces

34. Field of View depends on the eyepiece 82 ° fov68 ° fov60 ° fov