A non-achromatic objective is an
objective lens which is not corrected for
chromatic aberration
In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the w ...
. In
telescopes
A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observ ...
they can a be pre-18th century simple single element objective lenses which were used before the invention of
doublet achromatic lenses. 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 and
Christiaan Huygens
Christiaan Huygens, Lord of Zeelhem, ( , , ; also spelled Huyghens; la, Hugenius; 14 April 1629 – 8 July 1695) was a Dutch mathematician, physicist, engineer, astronomer, and inventor, who is regarded as one of the greatest scientists o ...
and his brother
Constantijn Huygens, Jr.
Constantijn Huygens Jr., Lord of Zuilichem (10 March 1628 – October 1697), was a Dutch statesman and poet, mostly known for his work on scientific instruments (sometimes together with his younger brother Christiaan Huygens). But, he was also a c ...
,
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 calculated number also called "magnification". When this number is less than one, it refers to a reduction in si ...
while limiting the interfering rainbow
halos caused by chromatic aberration (the uncorrected chromatic aberration fell within the large
diffraction 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 respond normally to painful stimuli, light, or sound, lacks a normal wake-sleep cycle and does not initiate voluntary actions. Coma patients exhi ...
and
astigmatism over the desired
field of view at only one wavelength. Monochromatically corrected objectives can be found in
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. Obsolete names for Sun telescopes include heliograph and photoheliograp ...
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 .
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" is an acronym for "light amplification by stimulated emission of radiation". The ...
applications such as
collimators,
beam expanders, and highly corrected
pupil imaging for wavefront error sensors for
adaptive optics.
See also
*
List of telescope types
The following are lists of devices categorized as types of telescopes or devices associated with telescopes. They are broken into major classifications with many variations due to professional, amateur, and commercial sub-types. Telescopes can be ...
References
{{DEFAULTSORT:Non-Achromatic Objective
Lenses
Telescopes