![]() ![]() ![]() We'll start by exploring the basic features common to all telescopes, and then look at some specific designs. Whether you're seriously considering buying your first telescope or just daydreaming about it, this guide will help you narrow your options. Naturally, this brings the burden of choice: the bewildering variety makes it hard for an uninformed consumer to make the right decision on what type of telescope to buy. Never before have novice stargazers been presented with such a vast array of telescopes and accessories to pursue their hobby. So the much longer waves overcome the benefit of a larger aperture and radio telescopes give a blurry view of the universe.This is an exciting time to become an amateur astronomer. A large radio telescope has a diameter of 100 meters, so if it imaged radio waves with a wavelength of 1 cm or 0.01 meter, the resolution would be 2.5 × 10 5 (0.01 / 100) = 25 arc seconds. At the opposite end of the electromagnetic spectrum radio waves are gathered and focused in a similar way to light waves. In fact, these very short waves are absorbed by glass so other techniques must be used to focus the waves and the simple equation for angular resolution does not apply. So all ultraviolet, X-ray, and gamma-ray astronomy must be done from space. No waves beyond the blue end of the visible spectrum - shorter than 3 × 10 -7 meters - can penetrate the Earth's atmosphere. What are the diffraction limits in other parts of the electromagnetic spectrum. Also, information is stored and read from compact disks using lasers and engineers devote much effort to developing blue or ultraviolet lasers that can pack more data or music or video onto a CD. Integrated circuits are etched onto silicon wafers using chemicals and light, and the semiconductor industry strives to use shorter or bluer wavelengths for this process so that more components can be packed onto a single wafer. Interestingly this principle also controls the miniaturization of electronics. The same telescope will make sharper images in blue light than in red light. ![]() You can also see that the angular diameter varies in proportion to the wavelength of radiation - images made with short waves have better resolution than images made with long waves. Equivalently, the Hubble Space Telescope can distinguish a pair of objects that is ten times further away than the resolution limit of a ground-based telescope.Īngular resolution varies inversely with the telescope diameter - the larger the telescope, the smaller (or better) the resolution. The Hubble Space Telescope has an aperture of 2.2 meters and an angular resolution of 2.5 × 10 5 (5 × 10 -7 / 2.2) = 0.05 arc seconds, which is about ten times better than can be done from the ground. This is excellent - the angular diameter of a dime seen at a distance of 3 miles! Larger telescopes than this gain in light gathering power but have effectively no gain in resolution because turbulence in the atmosphere jumbles all images to an angular size of about 1 arc second. A telescope of aperture 0.2 meters (about 8 inches) gives a resolution of 2.5 × 10 5 (5 × 10 -7 / 0.2) = 0.6 arc seconds. This accounts for the limited accuracy of the measurement of star and planet positions before the invention of the telescope.Ĭonsider a modest-sized telescope that a hobbyist might use. Visible light has a mean wavelength of about 5 × 10 -7 meters, so the resolution of the eye is 2.5 × 10 5 (5 × 10 -7 / 0.002) = 63 arc seconds or about one arc minute. What is the angular resolution of the eye? The pupil has a diameter of about 2 millimeters or 0.002 meters. ![]() In this equation, the angular resolution is in seconds of arc and the wavelength of radiation and diameter of the telescope must be in the same units. The angular resolution is:Īngular Resolution = 2.5 × 10 5 × (Wavelength / Diameter) The fundamental limit to a telescope's ability to resolve small angles is due to diffraction - the slight bending of light as it passes through a telescope. In either case, good resolution is required. Or we might want to discern details on the surface of a planet. We might want to distinguish two stars with a small angular separation. The other important attribute is resolution, or the ability to discriminate light arriving at the telescope from slightly different angles on the sky. Light-collecting area is not the entire story of a telescope. Each Keck telescope on Mauna Kea has an aperture of 10 meters, so the Keck telescope has (10/2.2) 2 = 20.7 times the collecting area of the Hubble Space Telescope. Hubble Space Telescope as seen from the Space Shuttle Atlantisįor another example, the Hubble Space Telescope has a mirror 2.2 meters across – this was the largest structure that could fit in the cargo bay of the Space Shuttle. ![]()
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