“Telescopes”For Physics 101

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Chapter 4, Telescopes

Ability to Focus Bending of Light Index of Refraction

( Dependent)

Collecting Power How Bright! Depends on Collector Area

Resolving Power Two Objects Close Depends on Quality (Ability to Discern) of Collector Area

Magnification Image Size/Object Size

Related ConceptsAtmospheric Refraction The Moon Illusion (page 122, text)

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How your perception may be fooled.

From Explorations An Introduction to Astronomy 3rd ed, Thomas Arny p 123

Both circles in the sky and the bottom circle look smaller than the circle on the horizon.

Indeed all the circles are the same size!

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A classical Newtonian reflecting telescope.(Image by Duncan Kopernicki.)

Small reflectors are often in a Newtonian configuration (shown above). They have a paraboloid primary mirror which brings the light of any object in the field of the telescope to a focus near the top end of the tube, called the prime focus. A flat mirror is placed at 45 to the axis of the tube and reflects the light out to an eyepiece at the secondary focus.

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A classical Cassegrain reflecting telescope.(Image by Duncan Kopernicki.)

In the classical Cassegrain telescope the primary mirror takes a paraboloid shape. This brings the light of any object in the field of the telescope to a focus near the top end of the tube, called the prime focus. This is used on big telescopes to take pictures of small areas of the sky. This used to be done using photographic plates but these have largely been replaced by more efficient digital detectors, called Charge Coupled Devices (CCDs).

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Basic Type of Telescopes

Basic Diagram of Schmidt-Cassegrain Technology

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The Horsehead Nebula in Orion. This image, approximately 1.5° across, was obtained with the UK Schmidt telescope at the Anglo-Australian Observatory.(Image Credit: David Malin, Anglo Australian Observatory/Royal Observatory

Edinburgh.)

For photography of large areas of the sky the primary mirror is made with spherical curvature and an aspheric `corrector plate' is placed at the top end of the telescope tube. There are three large Schmidt telescopes in the world with fields about 6° across (the Moon's apparent diameter in the sky is half a degree). The oldest of these is the Palomar Schmidt (not to be confused with the Palomar 200-inch) and the other two are the ESO Schmidt in Chile and the United Kingdom Schmidt in Australia. These have been used to produce photographic charts of the whole sky.

The Schmidt Telescope

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Magnifying Power (not discussed in detail in text)

“The ability of a telescope to enlarge images is the best-known feature of a telescope. Though it is so well-known, the magnifying power is the least important power of a telescope because it enlarges any distortions due to the telescope and atmosphere. A small, fuzzy faint blob becomes only a big, fuzzy blob. Also, the light becomes more spread out under higher magnification so the image appears fainter! The magnifying power = (focal length of objective) / (focal length of eyepiece); both focal lengths must be in the same length units. A rough rule for the maximum magnification to use on your telescope is 20 × D to 24 × D, where the objective diameter D is measured in centimeters. So an observer with a 15-centimeter telescope should not use magnification higher than about 24 × 15 = 360-power. “

Figure and Text from http://www.astronomynotes.com/ Nick Strobel’s Astronomy Notes

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Why Reflecting Telescopes are Preferred over Refracting

• A large mirror can be thin but a large lens must be thicker thus heavier.

• A lens has two surfaces that must be cleaned and polished; a mirror only has one;.

• Glass absorbs light! The thicker the light the more absorption.

• Lenses need to be supported only around the outside; mirrors can be supported by the back

• For large lenses, glass deforms under its own weight; thus changing the lenses’ properties.

• In a lens, different colors are refracted by different amounts. (Chromatic Aberrations). Lenses are corrected for chromatic aberrations and are called achromats.

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http://oposite.stsci.edu/pubinfo/pictures.html20

10-meter Keck Telescope at the W.M. Keck Observatory.

1. This page was copied from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version.

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“The Hubble Space Telescope orbits far above the distorting effects of the atmosphere, about 600 kilometers above the Earth. This perch gives astronomers with their clearest view ever, but it also prevents them from looking directly through the telescope. Instead, astronomers use Hubble's scientific instruments as their electronic eyes.” Upper Left: Closer View

Photo and text courtesy of http://hubble.nasa.gov/

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“This color image of Saturn was taken with the HST's Wide Field and Planetary Camera (WF/PC) in the wide field mode at 8:25 A.M. EDT, August 26, 1990, when the planet was at a distance of 1.39 billion kilometers (860 million miles) from Earth.”

Credit for picture and text: NASA29

“This enlargement of the Saturn image reveals unprecedented detail in atmospheric features at the northern polar hood. Saturn's north pole is presently tilted toward Earth by 24 degrees”

Courtesy for picture and text: NASA

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View of a colliding galaxy dubbed the "Tadpole" (UGC10214): Photo Courtesy NASA Hubble

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End of Presentation