Magnification

A telescope's magnifying power is the ratio of the apparent size of an object seen through the telescope to its size when seen by the eye alone. Telescopes magnify the angular diameter of objects. Thus the image appears to be closer than the object.

For example, to your eye the angular diameter of the full Moon is V2°, the same as an aspirin tablet held at arm's length (Figure 2.11). If the apparent size of the Moon increases 20 times, so that it looks 10° in diameter when you view it through your telescope, then the magnifying power is 20, written 20x.

The value of the magnifying power of a telescope depends on the eyepiece you use. You can figure:

Focal length of telescope

Magnifying power =

Focal length of eyepiece

A telescope usually comes with several eyepieces of different focal lengths so you can vary its magnifying power for viewing different objects.

(a) What is the magnifying power of a 150-mm (6-inch), f/8 telescope when an eyepiece of 12.5-mm (V2-inch) focal length is used?_(b) How could you increase the magnifying power of this telescope?_

Solution: Focal length of telescope

Focal length of eyepiece

1200 mm 48 inches 12.5 mm V2 inch

(b) Use an eyepiece of a shorter focal length.

2.21 MAXIMUM USEFUL MAGNIFICATION

It is a mistake to exaggerate the importance of magnifying power when you buy a telescope. You cannot increase the useful magnifying power indefinitely by changing eyepieces.

Starlight must pass through Earth's atmosphere to reach waiting telescopes on the ground. Disturbances in the air cause blurry images. Seeing refers to atmospheric conditions that affect the sharpness of a telescope's image. If the air is quiet, then the seeing is good, and stars shine with a steady light. If the air is turbulent, then the seeing is bad, and stars twinkle madly.

The practical limit of useful magnification for any telescope is about two times its aperture in millimeters (50 times its aperture in inches). Higher power will just magnify any blurring in the image due to diffraction or bad seeing. It cannot reveal any finer details.

A telescope in space escapes interference from Earth's atmosphere, so it can see farther and image sharper than a telescope on the ground (Figure 2.12). Astronomers operate space observatories by remote control from the ground. Astronauts can maintain, repair, and upgrade a space telescope in orbit around Earth or bring it home for a major overhaul.

The U.S./European Hubble Space Telescope (HST) (1990- ) sends us amazing images of planets, stars, and other objects out to the farthest galaxies. ►www.stsci.edu-M HST orbits a 2.4-m (94-inch) main mirror and eight instruments for visible, ultraviolet, and infrared light observations (Figure 2.13).

Figure 2.12. Effect of atmospheric blurring on resolution. Melnik 34, a very bright star located 163,000 light-years away, imaged with (a) the best available ground-based telescope, European Southern Observatory, Chile; and (b) Hubble Space Telescope.

Figure 2.12. Effect of atmospheric blurring on resolution. Melnik 34, a very bright star located 163,000 light-years away, imaged with (a) the best available ground-based telescope, European Southern Observatory, Chile; and (b) Hubble Space Telescope.

Hubble Space Telescope

Hubble Space Telescope

Tracking and Data Relay

Goddard Space Flight Center Greenbelt, MD

Ground Station^^ White Sands, NM

Space Telescope Science Institute Baltimore, MD

Space Telescope Science Institute Baltimore, MD

Figure 2.13. Hubble Space Telescope data path. HST is a 13-m (43-foot) serviceable space observatory that orbits 600 km (380 miles) above Earth. Instruments are powered by two solar arrays. Communication is via the Tracking and Data Relay Satellite System (TDRSS).

Figure 2.13. Hubble Space Telescope data path. HST is a 13-m (43-foot) serviceable space observatory that orbits 600 km (380 miles) above Earth. Instruments are powered by two solar arrays. Communication is via the Tracking and Data Relay Satellite System (TDRSS).

What is the practical limit of useful magnification for a 150-mm (6-inch) telescope? _

Answer: 300x.

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