An astronomical filter is a
telescope
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 observe ...
accessory consisting of an
optical filter
An optical filter is a device that selectively transmits light of different wavelengths, usually implemented as a glass plane or plastic device in the optical path, which are either dyed in the bulk or have interference coatings. The optical ...
used by
amateur astronomers
Amateur astronomy is a hobby where participants enjoy observing or imaging celestial objects in the sky using the unaided eye, binoculars, or telescopes. Even though scientific research may not be their primary goal, some amateur astronomer ...
to simply improve the details and contrast of
celestial objects
An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often us ...
, either for viewing or for photography.
Research astronomers, on the other hand, use
various
Various may refer to:
* Various (band), an English dubstep/electronic music duo
* Various artists, a term for a compilation album containing pieces by various musicians
* Various authors, a book containing works by several writers
* '' The Vario ...
band-pass
filters
Filter, filtering or filters may refer to:
Science and technology
Computing
* Filter (higher-order function), in functional programming
* Filter (software), a computer program to process a data stream
* Filter (video), a software component tha ...
for
photometry Photometry can refer to:
* Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision
* Photometry (astronomy), the measurement of the flux or intensity of an astronomical object's electro ...
on telescopes, in order to obtain measurements which reveal objects'
astrophysical properties, such as
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 grati ...
and placement of a
celestial body on its
Wien curve.
Most astronomical filters work by blocking a specific part of the color spectrum above and below a ''bandpass'', significantly increasing the
signal-to-noise ratio
Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in deci ...
of the interesting wavelengths, and so making the object gain detail and contrast. While the color filters transmit certain colors from the spectrum and are usually used for observation of the
planets
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a young ...
and the
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
, the polarizing filters work by adjusting the brightness, and are usually used for the Moon. The broad-band and narrow-band filters transmit the wavelengths that are emitted by the
nebulae
A nebula ('cloud' or 'fog' in Latin; pl. nebulae, nebulæ or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral or molecular hydrogen and also cosmic dust. Nebulae are often star-forming region ...
(by the
Hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
and
Oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
atoms), and are frequently used for reducing the effects of
light pollution
Light pollution is the presence of unwanted, inappropriate, or excessive use of artificial Visible spectrum, lighting. In a descriptive sense, the term ''light pollution'' refers to the effects of any poorly implemented lighting, during the day ...
.
Filters have been used in astronomy at least since the
solar eclipse of May 12, 1706
The solar eclipse on May 12, 1706 was a total eclipse.
The astronomic event, part of the Saros 133 cycle, took place during the Spanish War of Succession, crossing Spain, France and Northern Italy: for this reason it was seen at the time as a me ...
.
Solar filters
White light filters
Solar filters block most of the
sunlight
Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when t ...
to avoid any damage to the eyes. Proper filters are usually made from a durable glass or
polymer
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
film that transmits only 0.00001% of the light. For safety, solar filters must be securely fitted over the
objective
Objective may refer to:
* Objective (optics), an element in a camera or microscope
* ''The Objective'', a 2008 science fiction horror film
* Objective pronoun, a personal pronoun that is used as a grammatical object
* Objective Productions, a Brit ...
of a
refracting telescope
A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens (optics), lens as its objective (optics), objective to form an image (also referred to a dioptrics, dioptric telescope). The refracting telescope d ...
or
aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane.
An opt ...
of a
reflecting telescope
A reflecting telescope (also called a reflector) is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century by Isaac Newton as an alternati ...
so that the body does not heat up significantly.
Small solar filters threaded behind
eyepiece
An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is named because it is usually the lens that is closest to the eye when someone looks through the device. The ...
s do not block the radiation entering the scope body, causing the
telescope
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 observe ...
to heat up greatly, and it’s not unknown for them to shatter from
thermal shock
Thermal shock is a type of rapidly transient mechanical load.
By definition, it is a mechanical load caused by a rapid change of temperature of a certain point.
It can be also extended to the case of a thermal gradient, which makes different pa ...
. Therefore, most experts do not recommend such solar filters for eyepieces, and some stockists refuse to sell them or remove them from telescope packages. According to
NASA
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeeding t ...
: "Solar filters designed to thread into eyepieces that are often provided with inexpensive
telescope
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 observe ...
s are also unsafe. These glass filters can crack unexpectedly from overheating when the telescope is pointed at the Sun, and
retina
The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...
l damage can occur faster than the observer can move the eye from the eyepiece."
Solar filters are used to safely observe and photograph the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
, which despite being white, may appear as a yellow-orange disk. A
telescope
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 observe ...
with these filters attached can directly and properly view details of solar features, especially
sunspots and
granulation
Granulation is the process of forming grains or granules from a powdery or solid substance, producing a granular material. It is applied in several technological processes in the chemical and pharmaceutical industries. Typically, granulation invo ...
on the
surface
A surface, as the term is most generally used, is the outermost or uppermost layer of a physical object or space. It is the portion or region of the object that can first be perceived by an observer using the senses of sight and touch, and is ...
,
as well as
solar eclipse
A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby obscuring the view of the Sun from a small part of the Earth, totally or partially. Such an alignment occurs during an eclipse season, approximately every six month ...
s and
transits of the
inferior planets Mercury and
Venus
Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...
across the solar disk.
Narrowband filters
The
Herschel Wedge is a prism-based device combined with a
neutral-density filter
In photography and optics, a neutral-density filter, or ND filter, is a filter that reduces or modifies the intensity of all wavelengths, or colors, of light equally, giving no changes in hue of color rendition. It can be a colorless (clear) or ...
that directs most of the heat and
ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
rays out of the telescope, generally giving better results than most filter types. The
H-alpha
H-alpha (Hα) is a specific deep-red visible spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared wit ...
filter transmits the H-alpha
spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...
for viewing
solar flare
A solar flare is an intense localized eruption of electromagnetic radiation in the Sun's atmosphere. Flares occur in active regions and are often, but not always, accompanied by coronal mass ejections, solar particle events, and other solar phe ...
s and
prominences invisible through common filters. These H-alpha filters are much narrower than those use for night H-alpha observing (see Nebular filters below), passing only 0.05 nm (0.5
angstrom
The angstromEntry "angstrom" in the Oxford online dictionary. Retrieved on 2019-03-02 from https://en.oxforddictionaries.com/definition/angstrom.Entry "angstrom" in the Merriam-Webster online dictionary. Retrieved on 2019-03-02 from https://www.m ...
) for one common model,
compared with 3 nm-12 nm or more for night filters. Due to the narrow bandpass and temperature shifts often telescopes like that are tunable within about a ±0.05 nm.
NASA included the following filters on the
Solar Dynamics Observatory
The Solar Dynamics Observatory (SDO) is a NASA mission which has been observing the Sun since 2010. Launched on 11 February 2010, the observatory is part of the Living With a Star (LWS) program.
The goal of the LWS program is to develop the ...
, of which only one is visible to human eyes (450.0 nm):
450.0 nm, 170.0 nm, 160.0 nm, 33.5 nm, 30.4 nm, 19.3 nm, 21.1 nm, 17.1 nm, 13.1 nm, and 9.4 nm. These were chosen for temperature, instead of particular emission lines, as are many narrowband filters such as the H-alpha line mentioned above.
Color filters
Color filters work by absorption/transmission, and can tell which part of the spectrum they are reflecting and transmitting. Filters can be used to increase contrast and enhance the details of the Moon and planets. All of the visible spectrum colors each have a filter, and every color filter is used to bring a certain lunar and planetary feature; for example, the #8 yellow filter is used to show
Mars's maria and
Jupiter's belts.
The Wratten system is the standard number system used to refer to the color filter types. It was first manufactured by
Kodak
The Eastman Kodak Company (referred to simply as Kodak ) is an American public company that produces various products related to its historic basis in analogue photography. The company is headquartered in Rochester, New York, and is incorpor ...
in 1909.
Professional filters are also colored, but their bandpass centers are placed around other midpoints (such as in the
UBVRI and Cousins systems).
Some of common color filters and their uses are:
* Chromatic aberration filters: Used for reduction of the purplish
halo
Halo, halos or haloes usually refer to:
* Halo (optical phenomenon)
* Halo (religious iconography), a ring of light around the image of a head
HALO, halo, halos or haloes may also refer to:
Arts and entertainment Video games
* ''Halo'' (franch ...
, caused by
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 ...
of
refracting telescope
A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens (optics), lens as its objective (optics), objective to form an image (also referred to a dioptrics, dioptric telescope). The refracting telescope d ...
s. Such halo can obscure features of bright objects, especially Moon and planets. These filters have no effect on observing faint objects.
* Red: Reduces
sky brightness
Sky brightness refers to the visual perception of the sky and how it scatters and diffuses light. The fact that the sky is not completely dark at night is easily visible. If light sources (e.g. the Moon and light pollution) were removed fro ...
, particularly during daylight and twilight observations. Improves definition of
maria
Maria may refer to:
People
* Mary, mother of Jesus
* Maria (given name), a popular given name in many languages
Place names Extraterrestrial
* 170 Maria, a Main belt S-type asteroid discovered in 1877
* Lunar maria (plural of ''mare''), large, ...
, ice, and polar areas of Mars. Improves contrast of blue clouds against background of Jupiter and Saturn.
* Deep yellow: Improves resolution of
atmospheric features of Venus, Jupiter (especially in polar regions), and Saturn. Increases contrast of polar caps, clouds, ice and dust storms on Mars. Enhances comet tails.
* Dark green: Improves cloud patterns on Venus. Reduces sky brightness during daylight observation of Venus. Increases contrast of ice and polar caps on Mars. Improves visibility of the
Great Red Spot on Jupiter and other features in Jupiter atmosphere. Enhances white clouds and polar regions on Saturn.
* Medium blue: Enhances contrast of Moon. Increases contrast of faint shading of Venus clouds. Enhances surface features, clouds, ice and dust storms on Mars. Enhances definition of boundaries between features in atmospheres of Jupiter and Saturn. Improves definition of
comet gas tails.
Moon filters
Neutral density filter
In photography and optics, a neutral-density filter, or ND filter, is a filter that reduces or modifies the intensity of all wavelengths, or colors, of light equally, giving no changes in hue of color rendition. It can be a colorless (clear) ...
s, also known in astronomy as Moon filters, are another approach for contrast enhancement and
glare reduction. They work simply by blocking some of the object's light to enhance the contrast. Neutral density filters are mainly used in traditional photography, but are used in astronomy to enhance lunar and planetary observations.
Polarizing filters
Polarizing filters adjust the brightness of images to a better level for observing, but much less so than solar filters. With these types of filter, the range of transmission varies from 3% to 40%. They are usually used for the observation of the Moon,
but may also be used for planetary observation. They consist of two polarizing layers in a rotating
aluminum
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has ...
cell,
which changes the amount of transmission of the filter by rotating them. This reduction in brightness and improvement in contrast can reveal the lunar surface features and details, especially when it is near full. Polarizing filters should not be used in place of solar filters designed specially for observing the sun.
Nebular filters
Narrowband
Narrow-band filters are astronomical filters which transmit only a narrow band of
spectral lines
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...
from the spectrum (usually 22 nm bandwidth, or less). They are mainly used for
nebulae
A nebula ('cloud' or 'fog' in Latin; pl. nebulae, nebulæ or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral or molecular hydrogen and also cosmic dust. Nebulae are often star-forming region ...
observation.
Emission nebulae mainly radiate the doubly
ionized
Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule ...
oxygen in the
visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visual perception, visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' or simply light. A typical human eye wil ...
, which emits near 500 nm wavelength. These nebulae also radiate weakly at 486 nm, the
Hydrogen-beta line.
There are two main types of Narrowband filters: Ultra-high contrast (UHC), and specific emission line(s) filters.
Specific Emission line filters
Specific emission line (or lines) filters are used to isolate line or lines of specific elements or molecules to allow for being able to see the distribution within Nebula. This is a common method to produce
false color
False color (or pseudo color) refers to a group of color rendering methods used to display images in color which were recorded in the visible or non-visible parts of the electromagnetic spectrum. A false-color image is an image that depicts ...
images. Common filters are often use for the
Hubble Space Telescope
The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...
, forming the so-called HST-pallet, with colors assigned as such: Red = S-II; Green = H-alpha; Blue = O-III. These filters will commonly be specified with a second figure in
nm, which refers to how wide a band is passed, which may cause it to exclude or include other lines. For example, H-alpha at 656 nm, may pick up N-II (at 658–654 nm), some filters will block most of the N-II if they are 3 nm wide.
Commonly used lines / filters are:
* H-Alpha ''Hα / Ha'' (656 nm) from the
Balmer series
The Balmer series, or Balmer lines in atomic physics, is one of a set of six named series describing the spectral line emissions of the hydrogen atom. The Balmer series is calculated using the Balmer formula, an empirical equation discovered b ...
is emitted by
HII Regions
An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundred ...
and is one of the stronger sources.
* H-Beta ''Hβ / Hb'' (486 nm) from the Balmer series is visible from stronger sources.
* O-III (496 nm and 501 nm) filters allow for both of the Oxygen-III lines to pass through. This is strong in many Emission nebulae.
* S-II (672 nm) filters show the Sulfur-II line.
Less common lines/filters:
* He-II (468 nm)
* He-I: (587 nm)
* O-I: (630 nm)
* Ar-III: (713 nm)
* CA-II ''Ca-K/Ca-H'': (393 and 396 nm)
For solar observing, shows the sun with the K and H
Fraunhofer lines
In physics and optics, the Fraunhofer lines are a set of spectral absorption lines named after the German physicist Joseph von Fraunhofer (1787–1826). The lines were originally observed as dark features (absorption lines) in the optical spectru ...
* N-II (658 nm and 654 nm) Often included in wider H-alpha filters
* Methane (889 nm)
allowing clouds to be seen on the gas giants, Venus and (with filter) the Sun.
Ultra-High Contrast filters
Known commonly as ''UHC filters'', these filters consist of things which allow multiple strong common emission lines to pass through, which also has the effect of the similar ''Light Pollution Reduction'' filters (see below) of blocking most light sources.
The UHC filters range from 484 to 506 nm.
It transmits both the O-III and H-beta spectral lines, blocks a large fraction of light pollution, and brings the details of
planetary nebula
A planetary nebula (PN, plural PNe) is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives.
The term "planetary nebula" is a misnomer because they are unrelate ...
and most of emission nebulae under a dark sky.
Broadband
The broadband, or light pollution reduction (LPR), filters are designed to block the
Sodium
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable iso ...
and
Mercury vapor light, and also block natural
skyglow
Skyglow (or sky glow) is the diffuse luminance of the night sky, apart from discrete light sources such as the Moon and visible individual stars. It is a commonly noticed aspect of light pollution. While usually referring to luminance arising ...
such as the
aurora
An aurora (plural: auroras or aurorae), also commonly known as the polar lights, is a natural light display in Earth's sky, predominantly seen in high-latitude regions (around the Arctic and Antarctic). Auroras display dynamic patterns of bri ...
l light.
This allows observing nebulae from the city and light polluted skies.
Broadband filters differ from narrowband with the range of wavelengths transmission.
LED lighting is more broadband so it is not blocked, although white LEDs have a considerably lower output around 480 nm, which is close to O III and H-beta wavelength. Broadband filters have a wider range because a narrow transmission range causes a fainter image of sky objects, and since the work of these filters is revealing the details of nebulae from light polluted skies, it has a wider transmission for more brightness.
These filters are particularly designed for galaxy observation and photography, and not useful with other
deep sky objects
A deep-sky object (DSO) is any astronomical object that is not an individual star or Solar System object (such as Sun, Moon, planet, comet, etc.). The classification is used for the most part by amateur astronomers to denote visually observed f ...
such as emission nebulae. However, they can still improve the contrast between the DSOs and the background sky, which may clarify the image.
See also
*
Infrared cut-off filter
*
List of telescope parts and construction Hardware Accessories
*Finderscope
* Iron sight
* Reflector (reflex) sight
* Cheshire collimator: A simple tool to collimate a telescope
Control
*Clock drive
* GoTo
Mechanical construction
*Mirror support cell
*Serrurier truss
*Silvering
Mounts
* ...
*
Optical filter
An optical filter is a device that selectively transmits light of different wavelengths, usually implemented as a glass plane or plastic device in the optical path, which are either dyed in the bulk or have interference coatings. The optical ...
*
Photographic filter
In photography and cinematography, a filter is a camera accessory consisting of an optical filter that can be inserted into the optical path. The filter can be of a square or oblong shape and mounted in a holder accessory, or, more commonly, a ...
*
Photometric system
In astronomy, a photometric system is a set of well-defined passbands (or optical filters), with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used. For each photometric sy ...
*
UBV photometric system
The UBV photometric system (from ''Ultraviolet, Blue, Visual''), also called the Johnson system (or Johnson-Morgan system), is a photometric system usually employed for classifying stars according to their colors.
It was the first standardized p ...
References
{{reflist, 25em
Optical telescope components
Astrophotography
Astronomical imaging
Optical filters