1. Jeffrey R. Regester Physics Department High Point University
Telescopes
Jeffrey R. Regester
Physics Department
High Point University

2. How a Telescope Works
An optic (lens or mirror) focuses light to form an image. DEMO: image formation with lens & mirror
That image is recorded on film, CCD chip, or magnified with an eyepiece for direct viewing with the eye.

3. Types of Telescopes Refractor Newtonian Reflector Cassegrain Reflector
different solutions to the “head-in-the-way” problem with reflectors

4. Functions of a Telescope
Magnification (least important)
Collect lots of light
Improved Angular Resolution

5. Functions of a Telescope
Magnification
Collect lots of light
Improved Angular Resolution

6. Light Collecting Area
Most telescopes have a round aperture, so...

7. Example
How much more light does the Keck 10m telescope collect than your eye (pupil diameter 5mm)?

8. A Simpler Way To Do That...

9. Functions of a Telescope
Magnification
Collect lots of light
Improved Angular Resolution

10. Angular Resolution
The smallest angle that can be seen by the telescope, i.e. how much detail.
We can’t see the surfaces of stars — they are too far away for that. The size of the dot of light formed by a telescope (and recorded by film or CCD chip) is determined by
diffraction
“seeing”

11. Diffraction
The spreading of light when it goes through a hole or past a barrier.
DEMOS
light through a slit
light through a round hole
light past a thread

12. Diffraction
Resolution is fundamentally limited by diffraction of the light waves entering the telescope.
where
D=diameter of the telescope
λ= wavelength of the light
Smaller θmin = better!

13. “Seeing”
Caused by refraction of light passing through the turbulent column of air to the telescope.
This limits resolution to no better than about 1arcsec, the diffraction limit of a 4-inch telescope.

14. The Solution to “Seeing”
Simple solution: put the telescope in space.
Another solution: Adaptive optics
(This is only conceptually simple. Very complex to actually do, and expensive.)

15. Adaptive Optics AO system animation
T Tauri system, without and with AO
AO in operation

16. Numerical Resolution Example
What would be the resolution of the Keck 10m telescope, using visible light, if it could be placed in space, or with a working AO system?