Achromatic telescope
Encyclopedia
The achromatic telescope is a refracting telescope
that uses an achromatic lens
to correct for chromatic aberration
.
s. This produces focal length
s that are dependent on the color of the light. So, for example, at the focal plane an image may be focused at the red end of the spectrum, but blurred at the blue end. This effect is particularly noticeable the further an object lies from the central axis of the telescope. The image of a star can appear blue on one side and orange on the other. Early refracting telescopes with non-achromatic objective
s were constructed with very long focal length
s to mask the chromatic aberration. An Achromatic telescope uses an achromatic lens to correct for this. An achromatic lens is a compound lenses made with two types of glass with different dispersion
. One element, a concave lens made out of Flint glass
, has relatively high dispersion, while the other, a convex element made of Crown glass
, has a lower dispersion. The crown lens is usually placed at the front due to the higher susceptibility of flint glass to atmospheric attack (exception: Steinheil doublet). The lens elements are mounted next to each other and shaped so that the chromatic aberration of one is counter-balanced by the chromatic aberration of the other, while the positive power
of the crown lens element is not quite equaled by the negative power of the flint lens element. Together they form a weak positive lens that will bring two different wavelengths of light to a common focus.
Refracting telescope
A refracting or refractor telescope is a type of optical 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 for long focus camera lenses...
that uses an achromatic lens
Achromatic lens
An achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths into focus in the same plane....
to correct for chromatic aberration
Chromatic aberration
In optics, chromatic aberration is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have a different refractive index for different wavelengths of light...
.
How it works
When an image passes through a lens, the light is refracted at different angles for different wavelengthWavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s. This produces focal length
Focal length
The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...
s that are dependent on the color of the light. So, for example, at the focal plane an image may be focused at the red end of the spectrum, but blurred at the blue end. This effect is particularly noticeable the further an object lies from the central axis of the telescope. The image of a star can appear blue on one side and orange on the other. Early refracting telescopes with non-achromatic objective
Non-achromatic objective
A non-achromatic objective is an objective lens which is not corrected for chromatic aberration. In telescopes they can a be pre-18th century simple single element objective lenses which were used before the invention of doublet achromatic lenses...
s were constructed with very long focal length
Focal length
The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...
s to mask the chromatic aberration. An Achromatic telescope uses an achromatic lens to correct for this. An achromatic lens is a compound lenses made with two types of glass with different dispersion
Dispersion (optics)
In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency.Media having such a property are termed dispersive media...
. One element, a concave lens made out of Flint glass
Flint glass
Flint glass is optical glass that has relatively high refractive index and low Abbe number. Flint glasses are arbitrarily defined as having an Abbe number of 50 to 55 or less. The currently known flint glasses have refractive indices ranging between 1.45 and 2.00...
, has relatively high dispersion, while the other, a convex element made of Crown glass
Crown glass (optics)
Crown glass is type of optical glass used in lenses and other optical components. It has relatively low refractive index and low dispersion...
, has a lower dispersion. The crown lens is usually placed at the front due to the higher susceptibility of flint glass to atmospheric attack (exception: Steinheil doublet). The lens elements are mounted next to each other and shaped so that the chromatic aberration of one is counter-balanced by the chromatic aberration of the other, while the positive power
Optical power
Optical power is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the reciprocal of the focal length of the device. The dioptre is the most common unit of measurement of optical power...
of the crown lens element is not quite equaled by the negative power of the flint lens element. Together they form a weak positive lens that will bring two different wavelengths of light to a common focus.
Littrow doublet
Uses an equiconvex crown with R1=R2, and a flint with R3=-R2 and a flat back. Can produce a ghost image between R2 and R3 because they have the same radii. May also produce a ghost image between the flat R4 and rear of the telescope tube.Fraunhofer
R1 is set greater than R2, and R2 is set close to, but not equal, R3. R4 is usually greater than R3.Clark doublet
Uses an equiconvex crown with R1=R2, and a flint with R3~R2 and R4>>R3. R3 is set slightly shorter than R2 to create a focus mismatch between R2 and R3, thereby reducing ghosting between the crown and flint.Oil spaced doublet
The use of oil between the crown and flint eliminates the effect of ghosting, particularly where R2=R3. It can also increase light transmission slightly and reduce the impact of errors in R2 and R3.Steinheil doublet
Is a flint-first doublet in need of stronger curvature than, e.g., a Fraunhofer doubletSee also
- ApochromatApochromatAn apochromat, or apochromatic lens , is a photographic or other lens that has better correction of chromatic and spherical aberration than the much more common achromat lenses.-Explanation:...
- Achromatic lensAchromatic lensAn achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths into focus in the same plane....
- History of telescopesHistory of telescopesThe earliest known working telescopes appeared in 1608 and are credited to Hans Lippershey. Among many others who claimed to have made the discovery were Zacharias Janssen, a spectacle-maker in Middelburg, and Jacob Metius of Alkmaar. The design of these early refracting telescopes consisted of a...
- List of telescope types