Telescope Practice Questions

Correct answer: Reflecting telescope

Reflecting telescopes, or reflectors, utilize a concave primary mirror to collect light. This design avoids chromatic aberration, which is common in lens-based systems.

Correct answer: To magnify the image formed by the objective

While the objective lens or mirror gathers light to form a real image, the eyepiece acts as a magnifying glass to enlarge that image for the human eye. The total magnification of the telescope depends on the focal length of both the objective and the eyepiece.

Correct answer: Chromatic aberration

Chromatic aberration occurs because different wavelengths of light bend by different amounts when passing through glass. This results in a 'color fringe' around bright objects in refracting telescopes.

Correct answer: The diameter of the primary lens or mirror

Aperture is the most important specification of a telescope as it determines the light-gathering power and the resolution. A larger aperture allows the telescope to see fainter objects and finer details.

Correct answer: Newtonian reflector

The Newtonian reflector is a simple and popular design that uses a concave primary mirror and a flat diagonal secondary mirror. This allows the observer to look through an eyepiece located at the top side of the telescope.

Correct answer: To avoid the distortion and absorption caused by Earth's atmosphere

The Earth's atmosphere causes 'twinkling' (scintillation) and blocks certain wavelengths like X-rays and ultraviolet light. Space telescopes can capture much clearer images and a broader spectrum of electromagnetic radiation.

Correct answer: Equatorial mount

An equatorial mount has one axis aligned parallel to the Earth's axis of rotation. By rotating the telescope around this axis at the same rate as the Earth's rotation, the telescope can keep a star centered in the field of view.

Correct answer: Magnification = Focal length of objective / Focal length of eyepiece

Magnification is calculated by dividing the focal length of the telescope's objective by the focal length of the eyepiece used. Therefore, using an eyepiece with a shorter focal length results in higher magnification.

Correct answer: Spectroscope

A spectroscope or spectrograph attached to a telescope separates incoming light into its component wavelengths. By analyzing absorption and emission lines in the spectrum, scientists can determine the elements present in distant celestial bodies

Correct answer: Radio waves have much longer wavelengths than visible light

Due to the long wavelengths of radio waves, a very large collecting area is required to achieve a resolution comparable to even a small optical telescope. This is why radio dishes, like the one at Arecibo, are often hundreds of meters wide.

Correct answer: Catadioptric telescope

Catadioptric telescopes combine refraction and reflection. They typically use a corrector lens at the front of the tube and a primary mirror at the back, resulting in a compact design with a long focal length.

Correct answer: Galileo Galilei

While Hans Lippershey is often credited with inventing the telescope, Galileo was the first to point it toward the sky. His observations of the Moon's craters, Jupiter's moons, and the phases of Venus revolutionized our understanding of the universe.

Correct answer: Distinguish between two closely spaced objects as separate

Resolving power determines the level of detail a telescope can reveal. It is limited by the diameter of the aperture; the larger the aperture, the finer the details (and closer the double stars) the telescope can resolve.

Correct answer: At the back of the telescope, behind a hole in the primary mirror

The Cassegrain design uses a convex secondary mirror to reflect light back through a central hole in the primary mirror. This places the focal point and the eyepiece at the rear of the telescope, which is convenient for heavy instruments.

Correct answer: Focal length divided by the aperture

The focal ratio (f/number) is the telescope's focal length divided by its aperture. 'Fast' telescopes (low f/numbers) have wider fields of view and are better for deep-sky photography, while 'slow' telescopes (high f/numbers) are better for high-magnification planetary viewing.

Correct answer: To help locate objects using a wider field of view

Finderscopes are small, low-power auxiliary telescopes with a wide field of view and crosshairs. They are used to center an object before viewing it through the main telescope, which has a much narrower and harder-to-point field of view.

Correct answer: Adaptive Optics

Adaptive optics uses sensors to measure atmospheric distortion and deformable mirrors that change shape hundreds of times per second to cancel it out. This allows ground-based telescopes to achieve resolutions nearly as good as space telescopes.

Correct answer: It becomes smaller

The exit pupil is the diameter of the beam of light that leaves the eyepiece. It is calculated by dividing the aperture by the magnification; as magnification goes up, the exit pupil shrinks, which can eventually make the image appear dim.

Correct answer: Infrared

The JWST is designed to see the very first stars and galaxies, whose light has been 'redshifted' into the infrared spectrum by the expansion of the universe. Infrared light also allows the telescope to peer through dense clouds of cosmic dust.

Correct answer: Large lenses are very heavy and can only be supported by their edges

Large lenses in refracting telescopes are prone to 'lens sag' because they can only be supported at the perimeter. Mirrors in reflecting telescopes can be supported across their entire back surface, allowing for much larger and more stable apertures.