1. Telescope

2. Telescopes: 1) Light gathering power 2) magnify 3) resolve

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

4. History
Hans Lippershey ( ) of Holland is often credited with the invention in 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 ( ) 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 ( ) 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 A = p (D/2)2
Light-gathering power: Depends on the surface area A of the primary lens / mirror, D
A = p (D/2)2

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. Telescope geometric definitions
refracting
Telescope geometric 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. In optics Telescope properties
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
Refracting Telescope
Reflecting 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
“d” is the diameter of the objective or primary mirror

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

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

34. Rays from a spherically aberrated wavefront focus at different planes
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. Rays from a comatic wavefront Through-focus spot diagram for coma

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

39. Rays from a spherically aberrated wavefront focus at different planes
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. Parabolic primary mirror
Cassegrain Telescope
Parabolic primary mirror
Convex hyperboloidal secondary mirror
Focus
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.

44. Cassegrain Telescope
coincident foci

45. Coma is still a problem

46. Gregory Telescope Fp Fe

47. Ritchey-Chrétien Telescope
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.

53. Schmidt di Asiago primary 90 cm secondary 60 cm

54. Cassegrain Telescope
coincident foci

55. Gregory Telescope Fp Fe

56. Ritchey-Chrétien Telescope
hyperboloid

57. The main, or primary, mirror
This is the large mirror in the bottom of the tube, with a concave, aluminised face figured to an extremely accurate parabolodal surface. It concentrates the light from a star into a sharp image - not really a point, but a diffraction pattern with a small circle of light surrounded by small, faint rings.
It is held in some kind of mirror cell, fancy or simple, that rests on 3 set screws. By adjusting these screws, you can finely adjust the tilt of the main mirror, this is an important part of collimation (you only need to adjust 2 of them - it might be wise to leave the third in a middle position). Often there are 3 extra screws for locking the mirror cell in place, once it is adjusted. It may look something like this:
The secondary, or diagonal, mirror.
This is a smaller mirror with an elliptical face (its size is given as the length of its minor axis, i.e. its "width"). It is suspended by a spider with one or several vanes inside the tube near its opening, and the face is at 45 degrees to the tube. It is used to deflect the light from the main mirror sideways, so that you can see the image without having your head in the way of the incoming starlight.
The secondary mirror holder, and often the spider itself, is adjustable. It can be (more or less easily) moved sideways and along the tube, and it can be tilted (or rotated) slightly. Commonly, the mirror holder has a centre bolt and three screws for adjustment.
The eyepiece
This is a more or less fancy magnifying glass, used to see the image of the star or whatever else you look at. It has a certain focal length, and with several eyepieces of different focal lengths, you can select the magnification (often called "power") that you want.
The focuser
is where you put the eyepiece, it has a drawtube that holds the eyepiece and can be moved a little bit in and out, as needed to "focus" to get the sharpest view
Finally
These optical parts are held in mechanical alignment by a tube of sorts. The tube, in turn, is supported by some mounting that lets you aim it wherever you want in the sky, and perhaps track its apparent motion as the Earth rotates.
What is important to note here is that there are two axis
The primary axis - formed by the primary and the secondary…………
The secondary axis – formed by the secondary and the eyepiece. I shall have more to say about the significance of this shortly.