1. TRIVIA QUESTION!
How big (diameter of largest reflective mirror) is the LARGEST telescope in the world?
(a)
(b)
(c )
(d) 34 feet
(e)
Telescope, Location, Date Built
Aperture Size
Gran Telescopio Canarias, Canary Islands, Spain, 2009
409 inches

2. Diffraction through a Lens

3. Diffraction through a Lens
do=-(z-L) di
VIRTUAL object
ANGULAR width of diffraction pattern at z approaching infinity and at f IS THE SAME

4. Diffraction through a telescope
NOTE: Ray THROUGH MIDDLE of lens is NOT refracted in the thin lens approximation
Images of stars no longer resolved when diffraction patterns overlap

5. Diffraction limits through a lens
Diffraction limits your ability to resolve two sources
Diffraction Limitations on Resolving Two Sources
Changing an Aperture size (or size of Lens) limits the resolution of two sources. BIG telescopes are BETTER at resolving objects which are close together
Diffraction Limits and Aperture Size
The larger the wavelength, the lower the resolution in resolving objects – eg. Blue lasers for CD readers
Diffraction Limits and Wavelength

6. Diffraction through a telescope
Use x=1.22π for first zero of First Order Bessel Function J1 to determine half width of diffraction peak

7. Rayleigh Criteria ρ
Why is it desirable to have LARGE telescopes with LARGE diameter mirrors?

8. Examples of Diffraction
Focusing light with a lens wo
Geometric Optics – Wavelength small compared to other dimensions. No Diffraction. Light focuses to a point.
Wave Optics – Wavelength can be comparable to other dimensions. Diffraction MUST be considered. Light focuses to a minimum spot size.

9. Gaussian Beams Polar coordinates
wo is the radius at which the ELECTRIC field is reduced by 1/e or the INTENSITY is reduces by 1/e2

10. Gaussian Beams

11. Gaussian Beams Beam size ROUGHLY constant
As beam size increases, intensity decreases

12. Gaussian Beams
Max intenxity at CENTER of beam profile

13. Gaussian Beams Phase of CURVED wavefronts
At LARGE distances (z >> zo ), radius of curvature =z. Spherical-like wave front.
At MINIMUM beam waist (z=0), Radius of curvature of wavefronts goes to infinity (planar wave front)

14. Gaussian Beams – Gouy Phase Shift
Phase shift as light goes through focus
Gouy phase shift not that important at OPTICAL frequencies, but important at lower (microwave to Terahertz) since in the lower frequency ranges one can DIRECTLY measure electric