Telescopes

Chapter 3

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.

Galileo Galilei (1564-1642)

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

• • • • 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.

Galileo noticed

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

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

Refractors (glass lens)

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.

Reflectors (mirror)

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.

Cassegrain reflector

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.

Schmidtt-Cassegrain

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.

QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html

Disadvantages of space-based telescopes

• • • Expensive to launch and maintain Difficult to repair Low lifetime

Examples of space-based telescopes

• • • Hubble Space Telescope – 3 times better resolution – can see fainter objects Chandra X-ray Observatory Compton Gamma-Ray Observatory