1. Jeffrey R. Regester Greensboro Day School Greensboro NC
Telescopes
Jeffrey R. Regester
Greensboro Day School
Greensboro NC

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. Angular Magnification
Angular magnification definition: how many times larger the image looks, compared to the object seen without the telescope. In symbolic form...
For a telescope with an eyepiece, the angular magnification is

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

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

8. Example
How much more light does one of the Keck 10m telescopes collect than your eye (pupil diameter 5mm)?

9. The Keck Telescopes Mauna Kea, Hawaii

10. A Simpler Way To Do That...

11. Continued...
So the Keck telescope collects roughly 4,000,000 times more light than your eye. All other things being equal, it can see objects 4,000,000 times fainter than your eye. But, all other things are NOT equal...
CCD detector are about 90% efficient; your eye is about 5% efficient. That is, CCDs are about 18 times more sensitive than your eye.
Your eye cannot store or record the light. In other words, your eye can’t do a time exposure. The human eye is like a video camera that can’t record, taking “exposures” about 1/10th of a second long. (That’s why you don’t notice the flickering of a movie screen, which shows 24 frames per second.) So, if a CCD camera takes a 1-hour time exposure (3600 seconds), that’s about 36,000 times more than one “frame” of human vision.
So really, the Keck can see things that are 4,000,000 times 18 times 36,000 times fainter than the human eye. That’s about 3 trillion times fainter!

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

13. 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”

14. 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

15. 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!

16. “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.

17. 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.)

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

19. 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?