A non-achromatic objective is an
objective lens
In optical engineering, an objective is an optical element that gathers light from an object being observed and focuses the light rays from it to produce a real image of the object. Objectives can be a single lens or mirror, or combinations of ...
which is not corrected for
chromatic aberration
In optics, chromatic aberration (CA), also called chromatic distortion, color aberration, color fringing, or purple fringing, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the ...
. In
telescopes
A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...
they can a be pre-18th century simple single element objective lenses which were used before the invention of
doublet achromatic lens
An achromatic lens or achromat is a lens (optics), lens that is designed to limit the effects of chromatic aberration, chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into ...
es. They can also be specialty monochromatic lenses used in modern research telescopes and other instruments.
Non-achromatic telescope objectives
Early non-achromatic objectives
Early telescope objective, such as those built by
Johannes Hevelius
Johannes Hevelius
Some sources refer to Hevelius as Polish:
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Some sources refer to Hevelius as German:
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*of the Royal Society
* (in German also known as ''Hevel''; ; – 28 January 1687) was a councillor and mayor of Danz ...
and
Christiaan Huygens
Christiaan Huygens, Halen, Lord of Zeelhem, ( , ; ; also spelled Huyghens; ; 14 April 1629 – 8 July 1695) was a Dutch mathematician, physicist, engineer, astronomer, and inventor who is regarded as a key figure in the Scientific Revolution ...
and his brother
Constantijn Huygens, Jr.,
utilized single small (2"-8") positive lenses with enormous focal lengths (up to 150 feet in length in tube telescopes and up to 600 feet in non-tube
aerial telescopes). This allowed the observer to use higher
magnification
Magnification is the process of enlarging the apparent size, not physical size, of something. This enlargement is quantified by a size ratio called optical magnification. When this number is less than one, it refers to a reduction in size, so ...
while limiting the interfering rainbow
halos caused by chromatic aberration (the uncorrected chromatic aberration fell within the large
diffraction
Diffraction is the deviation of waves from straight-line propagation without any change in their energy due to an obstacle or through an aperture. The diffracting object or aperture effectively becomes a secondary source of the Wave propagation ...
pattern at focus).
Modern non-achromatic objectives
Modern instruments may use a non-achromatic objective lens which is well-corrected for
spherical aberration and off-axis
aberrations such as
coma
A coma is a deep state of prolonged unconsciousness in which a person cannot be awakened, fails to Nociception, respond normally to Pain, painful stimuli, light, or sound, lacks a normal Circadian rhythm, sleep-wake cycle and does not initiate ...
and
astigmatism
Astigmatism is a type of refractive error due to rotational asymmetry in the eye's refractive power. The lens and cornea of an eye without astigmatism are nearly spherical, with only a single radius of curvature, and any refractive errors ...
over the desired
field of view
The field of view (FOV) is the angle, angular extent of the observable world that is visual perception, seen at any given moment. In the case of optical instruments or sensors, it is a solid angle through which a detector is sensitive to elec ...
at only one wavelength. Monochromatically corrected objectives can be found in
solar telescope
A solar telescope or a solar observatory is a special-purpose telescope used to observe the Sun. Solar telescopes usually detect light with wavelengths in, or not far outside, the visible spectrum. Obsolete names for Sun telescopes include helio ...
s working with narrow spectral lines such as the
hydrogen alpha spectral line of 0.6562725 micrometres. They are also used in
astrographic telescopes where multiple single narrow wavelength images are used in
stellar classification
In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction gratin ...
.
Other applications
Non-achromatic objectives are also used in monochromatic
laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...
applications such as
collimator
A collimator is a device which narrows a beam of particles or waves. To narrow can mean either to cause the directions of motion to become more aligned in a specific direction (i.e., make collimated light or parallel rays), or to cause the spat ...
s,
beam expander
Beam expanders are optical devices that take a collimated beam of light and expand its width (or, used in reverse, reduce its width).
In laser physics they are used either as intracavity or extracavity elements. They can be telescopic in nature ...
s, and highly corrected
pupil imaging for wavefront error sensors for
adaptive optics
Adaptive optics (AO) is a technique of precisely deforming a mirror in order to compensate for light distortion. It is used in Astronomy, astronomical telescopes and laser communication systems to remove the effects of Astronomical seeing, atmo ...
.
See also
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List of telescope types
References
{{DEFAULTSORT:Non-Achromatic Objective
Lenses
Telescopes