Image of a refracting telescope from the Cincinnati Observatory in 1848
A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens as its objective to form an image (also referred to a dioptric telescope). The refracting telescope design was originally used in spy glasses and astronomical telescopes but is also used for long focus
camera lenses. Although large refracting telescopes were very popular
in the second half of the 19th century, for most research purposes the
refracting telescope has been superseded by the reflecting telescope
which allows larger apertures. A refractor's magnification is
calculated by dividing the focal length of the objective lens by that of
the eyepiece.
Invention
Refractors were the earliest type of optical telescope. The first practical refracting telescopes appeared in the Netherlands about 1608, and were credited to three individuals, Hans Lippershey and Zacharias Janssen, spectacle-makers in Middelburg, and Jacob Metius of Alkmaar. Galileo Galilei, happening to be in Venice
in about the month of May 1609, heard of the invention and constructed a
version of his own. 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.
Refracting telescope designs
All refracting telescopes use the same principles. The combination of an objective lens 1 and some type of eyepiece 2 is used to gather more light than the human eye is able to 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 not parallel converge upon a focal plane.
The telescope converts a bundle of parallel rays to make an angle α,
with the optical axis to a second parallel bundle with angle β. The
ratio β/α is called the angular magnification. It equals the ratio
between the retinal image sizes obtained with and without the telescope.
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.
Galileo's telescope
Optical diagram of Galilean telescope y – Distant object ; y′ – Real image from objective ; y″ – Magnified virtual image from eyepiece ; D – Entrance pupil diameter ; d – Virtual exit pupil diameter ; L1 – Objective lens ; L2 – Eyepiece lens e – Virtual exit pupil – Telescope equals
The design Galileo Galilei used in 1609 is commonly called a Galilean telescope. It used a convergent (plano-convex) objective lens and a divergent (plano-concave) eyepiece lens (Galileo, 1610). A Galilean telescope, because the design has no intermediary focus, results in a non-inverted and upright image.
Galileo’s best telescope magnified
objects about 30 times. Because of flaws in its design, such as the
shape of the lens and the narrow field of view, the images were blurry
and distorted. Despite these flaws, the telescope was still good enough
for Galileo to explore the sky. The Galilean telescope could view the phases of Venus, and was able to see craters on the Moon and four moons orbiting Jupiter.
Parallel rays of light from a distant object (y) would be brought to a focus in the focal plane of the objective lens (F′ L1 / y′). The (diverging) eyepiece (L2)
lens intercepts these rays and renders them parallel once more.
Non-parallel rays of light from the object traveling at an angle α1 to the optical axis travel at a larger angle (α2 > α1)
after they passed through the eyepiece. This leads to an increase in
the apparent angular size and is responsible for the perceived
magnification.
The final image (y″) is a virtual image, located at infinity and is the same way up as the object.
Keplerian telescope
Woodcut illustration of a 46 m (150 ft) focal length Keplerian astronomical refracting telescope built by Johannes Hevelius.
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 that 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 46-metre (150 ft) focal length, and even longer tubeless "aerial telescopes" were constructed). The design also allows for use of a micrometer at the focal plane (used to determine 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 lengths
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. The era of the 'great refractors' in the 19th century saw large achromatic lenses culminating with the largest achromatic refractor ever built, the Great Paris Exhibition Telescope of 1900.
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 (tertiary 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 is virtually free of chromatic
aberration.
Due to the special materials needed in the fabrication, apochromatic
refractors are usually more expensive than telescopes of other types
with a comparable aperture.
Technical considerations
The 1.0-meter (40 in) refractor, at Yerkes Observatory, the largest achromatic refractor ever put into astronomical use
Refractors suffer from residual chromatic and spherical aberration. This affects shorter focal ratios more than longer ones. A 100 mm (4 in) f/6
achromatic refractor is likely to show considerable color fringing
(generally a purple halo around bright objects). A 100 mm (4 in) f/16 has little color fringing.
In very large apertures, there is also a problem of lens sagging, a result of gravity deforming glass.
Since a lens can only be held in place by its edge, the center of a
large lens sags due to gravity, distorting the images it produces. The
largest practical lens size in a refracting telescope is around 1 meter
(39 in).
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 in reflecting telescopes, which can be made in far larger apertures and which have all but replaced refractors for astronomical research.
List of the largest refracting telescopes
A list of the largest refracting telescopes over 60 cm (24 in) diameter.
- Great Paris Exhibition Telescope of 1900 (1.25 m or 49 in) – dismantled after exhibition
- Yerkes Observatory (100 cm or 40 in)
- Swedish 1-m Solar Telescope (98 cm or 39 in)
- Lick Observatory (91 cm or 36 in)
- Paris Observatory (83 cm or 33 in, + 62 cm or 24 in)
- John Wall Telescope a 30" f12 refractor, the largest ever built by an individual maker (John Wall, inventor of the Crayford focuser). It is the equal fifth largest in the world with that at the Paris Observatory. It is now operated by the Hanwell Astronomical Society.
- Nice Observatory (77 cm or 30 in)
- Archenhold Observatory – the longest refracting telescope ever built (68 cm or 27 in × 21 m or 69 ft focal length)
- Lowell Observatory (61 cm or 24 in)
