1. Eyes & Cameras
Telescopes

2. Class viewing at the observatory Tuesday, October 17 (tomorrow!)
Starting at 7:30
Dress warmly
Bring a flashlight
(for walking to and from)
Walk in groups
Bring paper/pencil

3. Reflection and Refraction
faster
slower
Reflection and Refraction
When light goes from air to glass, it slows down.
This causes it to bend
We call that bending “refraction”
There is also some reflection from the surface.
Inbound light
Refracted light
Reflected light

4. Refraction (bending) occurs when light enters any medium in which it slows down or speeds up
Air
Water

5. Rods cannot detect color, but they are very sensitive to faint light.
Parts of the eye
cornea
iris
cones
rods
ciliary muscles
Cones can detect color (red, green, and blue), but they are not sensitive to faint light.
Rods cannot detect color, but they are very sensitive to faint light.
 “Off-center viewing”

6. Parts of the eye
(From the outside)
cornea
iris
pupil

7. A lens is shaped to bring light to a focus
“focal length”
Light from very far away
(Rays are parallel)
The cornea and lens of the eye also bring light to a focus.
The ciliary muscles are relaxed when looking at things very far away.

8. The muscles change the focal length of the lens!
The lens creates an image where the light from the object is brought to a focus
“Image”
“Object”
For an object that is not very far away, the ciliary muscles have to stretch and reshape the lens to put the image on the retina.
The muscles change the focal length of the lens!

9. The lens on a camera does the same thing, but the focal length of the lens stays fixed.
Instead, the lens is moved back and forth so the image is formed on the detector.

10. The colored part of the eye is the
A) Retina
B) Lens
C) Cornea
D) Iris
E) Pupil

11. The part of the eye that plays the role of the camera’s detector is the
A) Retina
B) Lens
C) Cornea
D) Iris
E) Pupil

12. A telescope that uses a lens here is called a refracting telescope or
simply a refractor
This is called the “objective” lens
The objective lens creates an image here
The eyepiece is used as a magnifier to examine the image

13. Mirrors can also bring light to a focus
“focal length”

14. And so mirrors are also used to make telescopes
You should know these telescope types!
Telescopes that use mirrors are called “reflectors”

15. Reflectors can be made MUCH larger than refractors
Segmented mirror
Each segment 1.8 m wide
Reflectors can be made MUCH larger than refractors
One solid mirror, 8.5 m wide
One of 7 that will make up the GMT

16. The most important “powers of a telescope” are…
Resolving
Light gathering

17. Resolving power
Angular resolution is the smallest angle over which we can tell that two points – stars, craters on the Moon, etc. – are distinct

18. Resolving power It’s limited by “diffraction” Diffraction
A diffraction pattern is formed whenever light goes through an opening, like when it enters a telescope!
Points of light form bulls-eye patterns

19. Resolving power Overlapping diffraction patterns
It’s limited by “diffraction”
Diffraction
A diffraction pattern is formed whenever light goes through an opening, like when it enters a telescope!
Points of light form bulls-eye patterns
Overlapping diffraction patterns
The diffraction patterns from nearby points of light overlap each other
resolved
borderline
NOT resolved

20. Resolving power It’s limited by “diffraction” Diffraction
A diffraction pattern is formed whenever light goes through an opening, like when it enters a telescope!
Points of light form bulls-eye patterns
Actual diffraction patterns from the Hubble Space Telescope

21. The most important “powers of a telescope” are…
Resolution
Light gathering
Formally,
Small q = “Good angular resolution”
1) Larger telescope opening (bigger D)
 Smaller diffraction pattern
 Better resolution
2) Shorter wavelength (l) observations