1. 1 Exercise ID Telescopes Begin: Begin: Look at Cameras brought in by students Talk about telescope basics and set up Celestron telescope In-class Objectives: In-class Objectives: To measure the optical properties of lenses and mirrors using an optical bench To measure the optical properties of lenses and mirrors using an optical bench Topics: Topics: Focal length (F), magnification (M), light gathering power (L.G.P.), angular resolution (  ), chromatic aberration Results: Results: Determine F, L.G.P., and resolving power of several lenses and a mirror, plus determine M of three simple telescopes composed of two lenses

2. 2 Exercise ID Telescopes Demonstation: Demonstation: How lenses and mirrors redirect light How lenses and mirrors redirect light Equipment needed (next time): Equipment needed (next time): Optical bench and screen Optical bench and screen Small, Medium and Large convex lenses (may need to share) Small, Medium and Large convex lenses (may need to share) Achromatic lens (please share) Achromatic lens (please share) Concave-convex mirror (please share) Concave-convex mirror (please share) Socket holder, Light bulb, red and blue filters Socket holder, Light bulb, red and blue filters 2 or 3 carriages (depends on the type of socket holder) 2 or 3 carriages (depends on the type of socket holder)

3. 1 Exercise ID Telescopes Telescope basic types Telescope basic types Refractors Refractors Reflectors: Newtonian, Cassegrain Reflectors: Newtonian, Cassegrain Hybrids: Schmidt-Cassegrain Hybrids: Schmidt-Cassegrain Basic Parameters Basic Parameters Diameter of objective lens (D o ) Diameter of objective lens (D o ) Focal Length (F o ) [on white board] Focal Length (F o ) [on white board]

4. Place transparencies here

7. Corrector Plate Schmidt-Cassegrain focal arrangement

8. Exercise ID Telescopes Now set up the Celestron Telescopes Now set up the Celestron Telescopes Objective Diameter = D o = 200 mm Objective Diameter = D o = 200 mm Focal Length of Objective = F o = 2000 mm Focal Length of Objective = F o = 2000 mm Telrad Telrad Three eyepieces characterized by their focal length (Ex: 40 mm) Three eyepieces characterized by their focal length (Ex: 40 mm)

9. Lens O I F The Lens Maker Equation or Eq. #1

10. Procedure: Procedure: Determine the focal length (F ) of the lens or mirror with help from the lens equation: Determine the focal length (F ) of the lens or mirror with help from the lens equation: where I is the image distance and O is the object distance. where I is the image distance and O is the object distance. Both I and O are measured from the lens or mirror. Both I and O are measured from the lens or mirror. When doing the measurement one should also note the brightness and color defect (chromatic aberration) of the images When doing the measurement one should also note the brightness and color defect (chromatic aberration) of the images or Exercise ID Telescopes 3 Eq. #1

11. Telescope magnification Telescope magnification Exercise ID Telescopes Longer Eyepiece Smaller Magnification Example: M = F o /F e = 2000 mm / 40 mm = 50X 50 X – 40 mm eyepiece 50 X – 40 mm eyepiece 100 X – 20 mm eyepiece 250 X – 8 mm eyepiece

12. Procedure: Procedure: Determine the magnification (M ) of a simple telescope composed of the achromat and each of the three other lenses: Determine the magnification (M ) of a simple telescope composed of the achromat and each of the three other lenses: where F o is the focal length of the objective (the achromat) and F e is the focal length of the eyepiece (one of the three other lenses). where F o is the focal length of the objective (the achromat) and F e is the focal length of the eyepiece (one of the three other lenses). Magnification is a unitless number Magnification is a unitless number 4 Exercise ID Telescopes Eq. #2

13. 1 Exercise ID Telescopes Light Gathering Power Light Gathering Power Function of size (diameter) of objective lens (D o ) Function of size (diameter) of objective lens (D o ) Determined by surface area of the lens Determined by surface area of the lens Area of circle =  (radius 2 ) Example: LGP = (200 mm/50 mm) 2 = (4) 2 = 16X

14. Other Things: Other Things: The light gathering power (L.G.P.) is a comparison between two telescopes’ collecting area: The light gathering power (L.G.P.) is a comparison between two telescopes’ collecting area: where D 1 and D 2 are the diameters of the larger and smaller objective, respectively where D 1 and D 2 are the diameters of the larger and smaller objective, respectively The focal ratio (f/ratio) is simply the focal length of the lens or mirror divided by its diameter The focal ratio (f/ratio) is simply the focal length of the lens or mirror divided by its diameter 5 Exercise ID Telescopes Eq. #4 Eq. #3

15. 7b Exercise ID Telescopes Angular Measurement Angular Measurement Units: Units: degrees (°) arc minutes (′) arc seconds (″) degrees (°) arc minutes (′) arc seconds (″) Angular Unit Conversion Angular Unit Conversion 1 degree (°) = 60 arc minutes (′) 1 degree (°) = 60 arc minutes (′) 1 minute (′) = 60 arc seconds(″) 1 minute (′) = 60 arc seconds(″) 1 degree (°) = 60 x 60 = 3600 arc seconds (″) 1 degree (°) = 60 x 60 = 3600 arc seconds (″)

16. Other Things: Other Things: The angular resolution (  ) or resolving power of a telescope (observing at visible wavelengths): The angular resolution (  ) or resolving power of a telescope (observing at visible wavelengths): where D is the diameter of the objective measured in millimeters and  is measured in arc seconds. where D is the diameter of the objective measured in millimeters and  is measured in arc seconds. 6 Exercise ID Telescopes Eq. #5