Everything about Refracting Telescope totally explained
A
refracting or
refractor telescope is a
dioptric telescope that uses a
lens as its
objective to form an image. The refracting telescope design was originally used in
spy glasses and
astronomical telescopes but is also used in other devices such as
binoculars and long or
telephoto camera lenses.
Invention
Refractors were the earliest type of
optical telescope. The first practical refracting telescopes appeared in the
Netherlands in about 1608, and were credited to three individuals,
Hans Lippershey and
Zacharias Janssen, spectacle-makers in
Middelburg, and
Jacob Metius of
Alkmaar also known as Jacob Adriaanszoon.
Galileo, happening to be in
Venice in about the month of May 1609, heard of the invention and constructed a much improved version of his own based on his understanding of the effects of refraction. Galileo then communicated the details of his invention to the public, and presented the instrument itself to the
doge Leonardo Donato, sitting in full council. Galileo may thus claim to have invented the refracting telescope independently, but not until he'd heard that others had done so.
Refracting telescope designs
All refracting telescopes use the same principles. The combination of an
objective lenses (1) and some type of
eyepiece (2) is used to gathered more light than the human eye could collect on its own, focus it (5), and present the viewer with a
brighter,
clearer, and
magnified virtual image (6). The objective in a refracting telescope
refracts or bends
light. This refraction causes
parallel light rays to converge at a
focal point; while those which were not parallel converge upon a
focal plane. Refracting telescopes can come in many different configurations to correct for image orientation and types of aberration. Because the image was formed by the bending of light, or refraction, these telescopes are called
refracting telescopes or
refractors.
Galilean telescope
The original design Galileo came up with in 1608 is commonly called a
Galilean telescope. It uses a convex objective lens and a concave eyepiece lens.it also can be used to see distant objects.
Galileo’s best telescope magnified objects about 30 times. Because of flaws in its design, such as the shape of the lens, the images were blurry and distorted. Despite these flaws, the telescope was still good enough for Galileo to explore the sky.
Keplerian Telescope
The
Keplerian Telescope, invented by
Johannes Kepler in 1611, is an improvement on Galileo's design. It uses a convex lens as the eyepiece instead of Galileo's concave one. The advantage of this arrangement is the rays of light emerging from the eyepiece are converging. This allows for a much wider field of view and greater eye relief but the image for the viewer is inverted. Considerably higher magnifications can be reached with this design but to overcome aberrations the simple objective lens needs to have a very high
f-ratio (
Johannes Hevelius built one with a 45 m (150 ft)
focal length). The design also allows for use of a
micrometer at the focal plane (used to determining the angular size and/or distance between objects observed).
Achromatic refractors
The
Achromatic refracting lens was invented in 1733 by an English barrister named
Chester Moore Hall although it was independently invented and patented by
John Dollond around 1758. The design overcame the need for very long focal legths in refracting telescopes by using an objective made of two pieces of
glass with different
dispersion, "
crown" and "
flint glass", to limit the effects of
chromatic and
spherical aberration. Each side of each piece is ground and
polished, and then the two pieces are assembled together. Achromatic lenses are corrected to bring two
wavelengths (typically red and blue) into focus in the same plane.
Apochromatic refractors
Apochromatic refractors have objectives built with special, extra-low dispersion materials. They are designed to bring three wavelengths (typically red, green, and blue) into focus in the same plane. The residual color error (secondary spectrum) can be up to an order of magnitude less than that of an achromatic lens. Such telescopes contain elements of
fluorite or special, extra-low dispersion (ED) glass in the objective and produce a very crisp image that's virtually free of chromatic aberration. Such telescopes are sold in the high-end amateur telescope market. Apochromatic refractors are available with objectives of up to 553 mm in diameter, but most are between 80 and 152 mm.
Technical considerations
Refractors have been criticized for their relatively high-degree of residual
chromatic and
spherical aberration. This affects shorter
focal lengths more than longer ones. A 4" achromatic refractor is likely to show considerable color fringing (generally a purple halo around bright objects). A 4" 16 will have little color fringing.
In very large apertures, there's also a problem of
lens sagging, a result of
gravity deforming
glass. There is a further problem of glass defects, striae or small
air bubbles trapped within the glass. In addition, glass is
opaque to certain
wavelengths, and even
visible light is dimmed by reflection and absorption when it crosses the air-glass interfaces and passes through the glass itself. Most of these problems are avoided or diminished by using
reflecting telescopes, that can be made in far larger apertures.
Notable refracting telescopes
Further Information
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