3. Telescopes
Chapter 3

4. Objectives
To know the general types of telescopes and the advantages and disadvantages of each one.
To know the primary parts and functions of each part of a telescope.
To know the importance of the diameter of the objective and to know how the magnification of a telescope is related to the focal lengths of the objective and eyepiece.
To know the advantages and disadvantages of earth and space-based telescopes.

5. Galileo Galilei ( )

6. History invented by Dutch lens maker in 1608 Galileo: small 30X scope
Observed the moon and “began” the modern age of Astronomy where measurement was more important than philosophy

7. Galileo noticed moons orbiting Jupiter phases of Venus
craters on the moon
sunspots
This was strong evidence that Copernicus was right although Galileo wasn’t willing to die for it.

8. How a telescope works
gathers light through the objective (mirror or lens)
bigger is better because it gathers more light
ability to see faint objects increases proportionally with the square of the radius of the objective
focuses light
viewed through an eyepiece (changing the eyepiece changes the magnification)
magnification is the ratio of the focal length of the objective to the focal length of the eyepiece

9. General types of telescopes
Refracting (objective is a lens)
Reflecting (objective is a mirror)
Newtonian
Cassegrain
Catadioptrics
uses mirrors and lenses
Schmidt-Cassegrain
Maksutov-Cassegrain

10. Refractors (glass lens)

11. Advantages and Disadvantages
Easy to use and reliable
Excellent for lunar, planetary and binary star observing especially in larger apertures.
High contrast images with no secondary mirror or diagonal obstruction.
Sealed optical tube reduces image degrading air currents and protects optics.
More expensive per inch of aperture
Heavier, longer and bulkier than equivalent aperture Newtonians and catadioptrics.
Small apertures
Less suited for viewing small and faint deep sky objects.
Color aberration due to colors of light bending different amounts.

12. Reflectors (mirror)

13. Advantages and disadvantages
Lowest cost per inch of aperture
Reasonably compact and portable up to focal lengths of 1000mm.
Excellent for faint deep sky objects such as remote galaxies, nebulae and star clusters.
Reasonably good for lunar and planetary work.
Low in optical aberrations.
Open optical tube design allows image-degrading air currents and air contaminants
More fragil
Large apertures (over 8") are bulky, heavy and tend to be expensive.
Slight light loss due to secondary obstruction when compared with refractors.

14. Cassegrain reflector

15. Catadioptric telescopes
Best all-around, all-purpose telescope design. Combines the optical advantages of both lenses and mirrors while canceling their disadvantages.
Sharp images over a wide field.
Excellent for deep sky observing or astrophotography with fast films or CCD’s.
Very good for lunar, planetary and binary star observing or photography.
Closed tube design reduces image degrading air currents.
Most are extremely compact and portable.
Large apertures at reasonable prices and less expensive than equivalent aperture refractors.

16. Schmidtt-Cassegrain

17. Problems with earth-based telescopes
Earth’s atmosphere reflects certain wavelengths
x-rays, gamma rays and most UV light is not transmitted by our atmosphere
Earth’s atmosphere blurs images
the bending of light by the atmosphere depends on the temperature of the “air”
“twinkling” (shimmering) effect
“Light pollution”
Solution? Put the telescope in space.

19. Disadvantages of space-based telescopes
Expensive to launch and maintain
Difficult to repair
Low lifetime

20. Examples of space-based telescopes
Hubble Space Telescope
3 times better resolution
can see fainter objects
Chandra X-ray Observatory
Compton Gamma-Ray Observatory

21. Light Gathering Power
The ability of a telescope to collect a lot more light than the human eye, its light-gathering power, is probably its most important feature. The telescope acts as a ``light bucket'', collecting all of the photons that come down on it from a far away object. Just as a bigger bucket catches more rain water, a bigger objective collects more light in a given time interval.

22. Light Gathering Power
This makes faint images brighter. This is why the pupils of your eyes enlarge at night so that more light reaches the retina

23. Focal Length
Generally expressed, focal length is the distance (given in millimeters) between the telescope's primary lens or mirror and the point where the light rays come together in focus.

24. Focal Length Why is this important?
Focal length is the major determining factor of any given telescope's magnifying power. You might think that using a lot of magnification would be a bonus when looking at a small object, but all telescopes have a practical limit.

25. Focal Length
At lower magnifications, the image is small, bright and well-resolved, but too much magnification makes for nothing more than a big, blurry image.
Why? Telescopes can only gather so much light and high magnification means you're just spreading that same amount of light over a larger area - resulting in useless or "empty" magnification. 

26. Resolving Power
The smallest angular separation that can be unambiguously distinguished is called the resolving power of the telescope and is proportional to the ratio of the wavelength of light being observed to the diameter of the telescope.
The ability to reveal fine detail.

27. Abberation
Chromatic –
Spherical-

28. Chromatic Aberration

29. Spitzer Space Telescope
 Formerly the Space Infrared Telescope Facility, is an infrared space observatory launched in It is the fourth and final of the NASA Great Observatories program.
Has to be far from Earth