A non-achromatic objective is an objective lens which is not corrected 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...

. In telescopes

Optical telescope

An optical telescope is a telescope which is used to gather and focus light mainly from the visible part of the electromagnetic spectrum for directly viewing a magnified image for making a photograph, or collecting data through electronic image sensors....

 they can a be pre-18th century simple single element objective lenses which were used before the invention of doublet

Doublet (lens)

In optics, a doublet is a type of lens made up of two simple lenses paired together. Such an arrangement allows more optical surfaces, thicknesses, and formulations, especially as the space between lenses may be considered an "element." With additional degrees of freedom, optical designers have...

 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....

es. They can also be specialty monochromatic lenses used in modern research telescopes and other instruments.

Early non-achromatic objectives

Early telescope objective, such as those built by Johannes Hevelius

Johannes Hevelius

Johannes Hevelius Some sources refer to Hevelius as Polish:Some sources refer to Hevelius as German:*Encyplopedia Britannica * of the Royal Society was a councilor and mayor of Danzig , Pomeranian Voivodeship, in the Polish-Lithuanian Commonwealth...

 and Christiaan Huygens and his brother Constantijn Huygens, Jr.

Constantijn Huygens, Jr.

Constantijn Huygens Jr. was a Dutch statesman also known for his work on scientific instruments and as a chronicler of his times...

, 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 telescope

Aerial telescope

An aerial telescope is a type of very-long-focal-length refracting telescope built in the second half of the 17th century that did not use a tube. Instead, the objective was mounted on a pole, tree, tower, building or other structure on a swivel ball-joint. The observer stood on the ground and held...

s). This allowed the observer to use higher magnification

Magnification

Magnification is the process of enlarging something only in appearance, not in physical size. This enlargement is quantified by a calculated number also called "magnification"...

 while limiting the interfering rainbow halos

Halo (optical phenomenon)

A halo from Greek ἅλως; also known as a nimbus, icebow or gloriole) is an optical phenomenon produced by ice crystals creating colored or white arcs and spots in the sky. Many are near the sun or moon but others are elsewhere and even in the opposite part of the sky...

 caused by chromatic aberration (the uncorrected chromatic aberration fell within the large diffraction

Diffraction

Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...

 pattern at focus).

Modern non-achromatic objectives

Modern instruments may use a non-achromatic objective lens which is well-corrected for spherical aberration

Spherical aberration

thumb|right|Spherical aberration. A perfect lens focuses all incoming rays to a point on the [[Optical axis|optic axis]]. A real lens with spherical surfaces suffers from spherical aberration: it focuses rays more tightly if they enter it far from the optic axis than if they enter closer to the...

 and off-axis aberration

Aberration in optical systems

Aberrations are departures of the performance of an optical system from the predictions of paraxial optics. Aberration leads to blurring of the image produced by an image-forming optical system. It occurs when light from one point of an object after transmission through the system does not converge...

s such as coma

Coma (optics)

In optics , the coma in an optical system refers to aberration inherent to certain optical designs or due to imperfection in the lens or other components which results in off-axis point sources such as stars appearing distorted, appearing to have a tail like a comet...

 and astigmatism

Astigmatism

An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances...

 over the desired field of view

Field of view

The field of view is the extent of the observable world that is seen at any given moment....

 at only one wavelength. Monochromatically corrected objectives can be found in solar telescope

Solar telescope

A solar telescope 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.-Professional solar telescopes:...

s working with narrow spectral lines such as the hydrogen alpha spectral line of 0.6562725 micrometres. They are also used in astrographic telescopes

Astrograph

An astrograph is a telescope designed for the sole purpose of astrophotography. Astrographs are usually used in wide field surveys of the night sky as well as detection of objects such as asteroids, meteors, and comets.-Design:...

 where multiple single narrow wavelength images are used in stellar classification

Stellar classification

In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...

 .

Other applications

Non-achromatic objectives are also used in monochromatic laser

Laser

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

 applications such as collimators, beam expander

Beam expander

Beam expanders are used in laser physics either as intracavity or extracavity elements. They can be telescopic in nature or prismatic. Generally prismatic beam expanders use several prisms and are known as multiple-prism beam expanders....

s, and highly corrected pupil imaging for wavefront error sensors for adaptive optics

Adaptive optics

Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...

.