General Astronomy

1. General Astronomy Astronomical Instruments

2. The Formation of Images Suppose we want to apply some of the properties of light discussed in the last session? Recall the refraction of light through a prism?

3. Forming Images Let's only consider monochromatic light for now.

4. Forming Images Let's put a couple of prisms together and bring the beams of light to a point…

5. Forming Images If we smooth the rough edges, we can form a lens Parallel light rays are focused into a point by the lens.

6. Simple Lenses Focal Point D Diameter Parallel light from a distant star f Focal Length

7. Simple Lenses • Simple Lenses are characterized by several properties: • Diameter • Focal length • Index of refraction of the material • This relates to how much they will bend the light rays • Shape Concave Convex

8. Simple Lenses A Convex Lens concentrates light A Concave Lens spreads out light

9. Optical Power The inverse of the focal length, measured in meters, is the Diopter For example, a +2 diopter prescription lens has a focal length of ½ meter. Positive is convex; Negative is concave. The eye itself has a refractive power of +60 diopters (1.7 cm focal length) We need a convex lens to reduce the image location to bring it back to the retina, a “+” correction. We need a concave lens to spread the image to reach the retina, a “-” correction

10. Forming Images This image is: Real Inverted Focal Point

11. Forming Images Increasing the focal length, increases the size of the image Old Focal Point New Focal Point

12. Forming Images • Mirrors, using reflection instead of refraction, also form images Real Object The image is: Virtual Erect Same Size

13. Concave Mirror The image is… Real Inverted Smaller Focal Point

14. Illusion

15. Ray TracingFollowing the Photon’s path The ray tracing technique just used to show how mirrors and lenses form images is very powerful. The image below is created without an artist or (clearly) a photo, it is formed by a set of instructions telling where the light source is, the object(s) are located and where the point-of-view is looking at the scene.

16. Mirrors versus Lenses • Mirrors have several advantages over lenses • Generally they are lighter in weight • There is no problems with color • Refraction affects different colored light so that a for a given lens, red light will focus at a different point than blue light • It is easier to produce a large diameter mirror than a large diameter lens

17. Telescopes There are several important considerations in choosing a telescope • Light Gathering Power • Resolving Power • Magnification • Type • Mounting Eyepiece First some definitions: Objective The eyepiece magnifies the image formed by the objective (Main Lens/Mirror)

18. 162 = 64 22 Light Gathering Power The Diameter of the objective determines the amount of light an optical system can gather; It is proportional to the area of the objective Brightness Diameter2 For example, many amateurs have 2" telescopes; Stockton's scope is a 16" An object seen in Stockton's scope is 64 times brighter than through a 2"

19. Light Gathering Power

20. Resolution The resolution (or clarity) of an image also depends on the size of the telescope aperture. The Andromeda galaxy seen through a small telescope… …and through a telescope with a larger aperture.

21. Resolving Power As objects get farther away, it becomes harder and harder to tell them apart. Your eye cannot see all of the craters on the moon – they blend together into the background. In fact the circular shape of the telescope objective produces a circular diffraction pattern as an image. [Actually so does the circular pupil of your eye and the two straight edges of your eyelids] The central disk of this diffraction pattern is what we think of as the "star" when we look at it; it is known as the Airy Disk. It is smeared out – so, two close stars could have their Airy disks overlap. A telescope's Resolving power measures how close together objects can be and still be seen to be separate.

22. Resolving Power 1 2 3 4 Resolving Power is measured in terms of the angular separation Image #3 is "just resolved. The angle of separation when Images are just resolved is the Resolving Power

23. Resolving Power  180 (in degrees) Resolving Power = D Π Notice it depends on both the wavelength and the diameter of the objective. Small numbers are 'good', so redder light (longer wavelengths) are harder to resolve than bluer light. The bigger the objective, the better the resolving power. A Rule of Thumb: ArcSec = 10/D, where D is in cm.

24. Resolving Power Resolving power in arcsec for a given objective diameter In order to subtend the same angle with a dime, you would to be these many miles away:

25. Resolving Power Twinkle, Twinkle Little Star… Because the light from the star follows a winding path through the (sometimes turbulent) atmosphere, the star appears to move around a bit. The motion varies between 1" and 2" (best case is 0.25")

26. Resolving Power What angles do the planets subtend? Mercury 6.4" Venus 16.0" Mars 6.1" Jupiter 37.9" Saturn 17.3" Moon 31' 05" This means that the movement (twinkle) is smaller than the planet – so any motion is invisible; the motion is within the bounds of the planet. This leads to a rule: Stars twinkle, Planets don't

27. fo M = fe Magnification Magnification is the least important of the telescope properties (although the one touted by TV sales shows). Unless you are looking only at planets, the moon or other extended objects, magnifying the point of light that is a star does nothing. Magnification is the ration of the focal lengths of the objective and the eyepiece: The Stockton 16" has a focal length of 4064 mm 32mm eyepiece  M = 4064/32 = 127x 24mm eyepiece  M = 4064/24 = 170x

28. Mounting Systems A telescope mount has two functions • provide a system for smooth controlled movement to point and guide the instrument • support the telescope firmly so that you can view and photograph objects without having the image disturbed by movement. There are two major types of mounts for astronomical telescopes: • Altazimuth • Equatorial

29. Altazimuth • The simplest type of mount with two motions, altitude (up and down/vertical) and azimuth (side-to-side/horizontal). • Good altazimuth mounts will have slow-motion knobs to make precise adjustments, which aid in keeping tracking motion smooth. • These type mounts are good for terrestrial observing and for scanning the sky at lower power, but are not for deep sky photography. • Certain altazimuth mounts are now computer driven and allow a telescope to track the sky accurately enough for visual use, but not for long exposure photography.

30. Superior to non-computerized altazimuth mounts for astronomical observing over long periods of time and absolutely necessary for astrophotography. As the earth rotates around its axis, the stationary stars appear to move across the sky. If you are observing them using an altazimuth mount, they will quickly float out of view in both axes. A telescope on an equatorial mount can be aimed at a celestial object and easily guided either by manual slow-motion controls or by an electric clock drive to follow the object easily across the sky and keep it in the view of the telescope. The equatorial mount is rotated on one axis (polar/right ascension) adjusted to your latitude and that axis is aligned to make it parallel to the Earth's axis, so that if that axis is turned at the same rate of speed as the Earth, but in the opposite direction, objects will appear to sit still when viewed through the telescope. Equatorial

31. German Mount Both reflector and refractor telescopes normally use this type mount. A large counterweight extending on the opposite side of the optical tube is its distinguishing feature. The counterweight is needed to balance the weight of the optical tube. Equatorial

32. Fork Mount Most catadioptric and other shorter optical tubes use this style mount which is generally more convenient to use than the German mount, especially for astrophotography. A more recent state-of-the-art computer controlled telescope allows fully automatic operation making it easy to locate objects while saving the observer considerable time and effort. Equatorial

33. Types of Telescopes Refractor versus Reflector The 'classic' telescope most of us think of when we imagine one is the refractor. In practice, however, there are some significant drawbacks to refractors – especially those of large size. As usual, the telescope is measured by its objective diameter. The largest refractor is 40". Refractors suffer from two main problems • Chromatic Aberration • Weight

34. Chromatic Aberration Recall that refraction bends light differently depending on its wavelength. This means that different colors will have differing focal lengths: Resulting in an image with "color halos"

35. Chromatic Aberration Chromatic aberration Corrected

36. Weight Generally, the bigger the objective diameter, the longer the focal length and therefore the higher in the air the lens of the refractor will be when mounted in the telescope tube. Since large refractors could have an objective lens weighing in the tons, moving it about is a definite problem.

37. Reflectors • Reflectors on the other hand have no chromatic aberration - reflection acts the same no matter what the wavelength of the light. • Second, mirrors are generally placed closer to the ground and with a lower center of gravity are easier to move. • Mirrors are usually spherical rather than parabolic – leading to spherical aberation (because it's cheaper & easier to make a spherical mirror)

38. The 40-inch refractor at Yerkes Observatory: The world’s largest refractor.

39. Yerkes' 40"

40. Spherical Aberration One property of a parabolic shape is the fact that any incoming parallel rays will be focused to a single point: A spherical shape does not have this property:

41. Spherical Aberration

42. Types of Reflectors • Prime Focus • Herschell • Newtonian • Cassegrain • Coudé • Schmidt

43. Observer rides in a 'basket' inside the telescope Brightest image Types of Reflectors: Prime Focus Yes, it blocks some light. No, it doesn't change the image, just dims it a bit

44. The 4-meter reflecting telescope at Kitt Peak National Observatory.

45. The eyepiece is set at the top of the tube and the mirror canted so that the light will be focused into the eyepiece Drawback: You might be very far off the ground on a ladder trying to see some objects – this is especially thrilling when trying to move the scope to follow the motion of a planet Types of Reflectors: Herschell

46. Herschell's Telescope "This wonderful instrument, though gigantic in its size, is moved with great facility in all directions, by means of rollers, ropes, and pullies. The ascent to the uppermost end is by means of steps or rather a ladder; and to this end there is a seat attached, on which the astronomer is placed to make his observations on the starry world. Of course he looks in, and not through the tube; in the lower end of which, near the ground, is placed the mirror which reflects the light through a small tube, upon his eyes. The mirror weighs two thousand five hundred pounds, and is worth, according to the doctor's valuation, ten thousand pounds. While he views the firmament with its glittering orbs, he communicates his observations to his sister, Miss Herschell, who is his amanuensis, and who has her station in a small lodge built in the lower framework of the machinery. This he does by a speaking trumpet, one end of which is applied to his mouth, and the other to her ear; thus they are recorded without either having to leave their seats…" --Description of Herschell's telescope at Slough from Joshua White's Letters on England, written in 1810.

47. A Newtonian reflector allows you to "keep your feet on the ground" It does this by placing a diagonal mirror in the tube so that the eyepiece may be lower. Types of Reflectors: Newtonian The small amount of light blocked by the mirror is minimal in return for the convenience and usefulness of the lower placement of the eyepiece Newton’s 1st Reflector, 1688

48. This clever arrangement puts a small convex mirror in front of the objective and bores a hole in the objective. The small mirror reflects the incoming light through the hole and into the eyepiece. This arrangement allows the focal length to be increased dependent on the placement of the small mirror. Types of Reflectors: Cassegrain

49. Suppose you want to put a camera, or even heavier equipment, in line with the telescope optics. Even the cassegrain focus can be hard-pressed to handle several hundred pounds of analysis equipment hanging on the back of the scope. The arrangement makes use of the fork method of the equatorial mount. Light can be directed into the point where the scope tube is gripped by the fork and then directed down through the mounting (conveniently hollow or with fiber optics) to a room below the telescope where the equipment is located. Types of Reflectors: Coudé

50. Sometimes called a Schmidt Camera, this design allows the use of a spherical mirror and a Corrector Plate These usually have wide fields of vision and "fast optics" allowing for photography. Many of these are also Cassegrain – leading to the designation "Schmidt-Cassegrain" Types of Reflectors: Schmidt