Unveiling the World: The Power of Telescopes

1. Telescope

2. Telescopes:1) Light gathering power 2) magnify3) resolve

3. See faint objects - Light gathering power See detail on objects - Resolving power Magnify otherwise small objects - Magnification The Job of a Telescope

4. History • Hans Lippershey (1570-1619) of Holland is often credited with the invention in 1608. His claim for the invention was soon challenged by a couple of other men and the Dutch authorities eventually ruled that the situation was confused, and refused to grant a patent to anyone.

5. History • Galileo (1564-1642) was the first one who used the telescope for astronomy (1609) Portrait by Ottavio Leoni.Paris, Musée du Louvre.

6. History • In 1704, Sir Isaac Newton (1642-1727) announced a new concept in telescope design whereby instead of glass lenses, a curved mirror was used to gather in light and reflect it back to a point of focus. Portrait by Sir Godfrey Kneller (Farleigh House, Farleigh Wallop, Hampshire)

7. Telescope properties Light-gathering power: Depends on the surface area A of the primary lens / mirror, A = p (D/2)2 D

8. Galileo's original telescope had a 37mm diameter plano-convex objective lens with a focal length of 980mm. The original eyepiece was lost, but according to Galileo's writings was plano-concave with a diameter of about 22mm and a focal length of about 50 mm. History 37 mm

9. There are two different types of telescopes • A refracting telescope uses a glass lens to concentrate incoming light • A reflecting telescope uses mirrors to concentrate incoming starlight

10. refracting Telescopegeometric definitions D = 2R = aperture, entrance pupil (m) V = vertex f = focal lenght (m) F = focus on axis Q = focus for  (radians) FQ = focal plane f= image distance (mm) reflecting

11. Telescope properties In optics the numerical aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. As to a lens NA= n sinq (n index of refraction)

12. As to Telescopes… the angular acceptance of a lens or mirror is expressed by the f-number, written f/# or N, which is defined as the ratio of the focal length to the diameter of the aperture: N = f / D

13. Both Refracting and Reflecting Telescopes Upside-down image Focal length Focal length

14. Refracting/Reflecting Telescopes Refracting Telescope: Lens focuses light onto the focal plane Focal length Reflecting Telescope: Concave Mirror focuses light onto the focal plane Focal length Almost all modern telescopes are reflecting telescopes.

15. Refracting vs. Reflecting Telescopes Reflecting Telescope Refracting Telescope Advantages: able to see dark or dim objects, powerful, capable of viewing far distances, clearer images Advantages: lenses are more durable than mirrors Disadvantages: produces unclear images sometimes, size of lens affects power so usually are less powerful Disadvantage: telescopes must be large in size in order for the viewer to see the image at the focal point

16. Disadvantages of Refracting Telescopes • Chromatic aberration: Different wavelengths are focused at different focal lengths (prism effect). Can be corrected, but not eliminated by second lens out of different material. Difficult and expensive to produce: All surfaces must be perfectly shaped; glass must be flawless; lens can only be supported at the edges

17. A larger objective lens provides a brighter (not bigger) image

19. Three main functions (Powers) of a Telescope Most important!! • Light Gathering Power: bigger aperture is better making objects appear brighter followed by • Resolving Power:to see fine detail and least important, • Magnifying Power: magnification = M

21. The Focal Plane • If we put our eye at the focal plane, we would only see a bright point • The eye piece straightens out the rays of light so our eye can see the image • If we move the eyepiece out of the focal plane, the image will be distorted

24. Magnification

25. Field of view (FOV) • Angle that the “chief ray” from an object can subtend, given the pupil (entrance aperture) of the imaging system • Recall that the chief ray propagates through the lens un-deviated

26. DIFFRACTION

27. Fraunhofer diffraction from a circular aperture The central Airy disc contains 85% of the light

28. Fraunhofer diffraction and spatial resolution • Suppose two point sources or objects are far away (e.g. two stars) • Imaged with some optical system • Two Airy patterns • If S1, S2 are too close together the Airy patterns will overlap and become indistinguishable S1  S2

29. Reyleigh criterion

30. Aberrations • In optical systems • In atmosphere

31. Spherical Aberration

32. Side view of a fan of rays (No aberrations) “Spot diagram”: Image at different focus positions Shows “spots” where rays would strike a detector 5 2 4 3 1 Different ways to illustrate optical aberrations 5 2 4 3 1

33. Spherical aberration Rays from a spherically aberrated wavefront focus at different planes Through-focus spot diagram for spherical aberration

34. Spherical aberration Rays from a spherically aberrated wavefront focus at different planes

35. Spherical aberration as “the mother of all other aberrations” • Ray bundle on axis shows spherical aberration only • Coma and astigmatism can be thought of as the aberrations from a de-centered bundle of spherically aberrated rays • Ray bundle slightly de-centered shows coma • Ray bundle more de-centered shows astigmatism

36. Coma

37. Coma Rays from a comatic wavefront Through-focus spot diagram for coma

38. Astigmatism Top view of rays Through-focus spot diagram for astigmatism Side view of rays

39. Spherical aberration Rays from a spherically aberrated wavefront focus at different planes

40. Telescope design

41. Imaging with mirrors: spherical and parabolic mirrors

42. Palomar Telescope

43. CassegrainTelescope Parabolic primary mirror • Hyperbolic secondary mirror: 1) reduces off-axis aberrations, 2) shortens physical length of telescope. • Can build mirrors with much shorter focal lengths than lenses. Example: 10-meter primary mirrors of Keck Telescopes have focal lengths of 17.5 meters (f/1.75). About same as Lick 36” refractor. Convex hyperboloidal secondary mirror Focus

44. CassegrainTelescope coincident foci

45. Coma is still a problem

46. Gregory Telescope Fp Fe

47. Ritchey-ChrétienTelescope hyperboloid Eccentricities are free parameters for an aberration free focus

48. Hubble Space Telescope suffered from Spherical Aberration In a Cassegrain telescope, the hyperboloid of the primary mirror must match the specs of the secondary mirror. For HST they didn’t match.