Radiation astronomy/Violets

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This image of Venus is taken through a violet filter by the Galileo spacecraft on February 14, 1990. Credit: NASA/JPL-Caltech.

Violet astronomy is the astronomy of emissions (and absorptions), reflections or fluorescences, and transmissions over the wavelength band 380–450 nm.


Main source: Astronomy

When any effort to acquire a system of laws or knowledge focusing on an astr, aster, or astro, that is, any natural body in the sky especially at night,[1] discovers an entity emitting, reflecting, or fluorescing violet, succeeds even in its smallest measurement, violet astronomy is the name of the effort and the result. Once an entity, source, or object has been detected as emitting, reflecting, or fluorescing violet, it may be necessary to determine what the mechanism is. Usually this information provides understanding of the same entity, source, or object. Violet is a color that suggests the sense of sight. Most people associate astronomy with the sense of seeing, what can be termed visual astronomy. As telescope optics transmit violet well, violet astronomy is also a field within optical astronomy.

For violet astronomy, the proof of concept is demonstrated by unique or novel astronomy in the violet band of the electromagnetic spectrum.


Main source: Radiation

Violet is a bright bluish purple color that takes its name from the violet flower.[2] ... Violet is at the lower end of spectrum of light, with a wavelength between approximately 380-450 nanometers.[3]


Various shades of violet are shown. Credit: Mizunoryu, Badseed, Jacobolus.
Variations of violet are shown. Credit: Badseed.
The diagram shows various shades of purple. Credit: Mizunoryu, Badseed, Jacobolus.

Def. a bluish-purple colour is called violet.

Def. a colour/color that is a dark blend of red and blue; dark magenta is called purple.

Purple is a range of hues of color occurring between red and blue.[4] The Oxford English Dictionary describes it as a deep, rich shade between crimson and violet.[5]


Main source: Minerals
Axinite is a calcium aluminum borosilicate mineral that can occur in violet. Credit: Didier Descouens.
This fluorapatite specimen is primarily violet. Credit: Vassil.
The color of the purple apatites (which are to almost 1 cm in size) leaps out at you. Credit: Rob Lavinsky.
The tanzanite shown is a rough stone and a cut stone. Credit: Didier Descouens.
A rough sample of tanzanite is pictured. Credit: Wela49.
This raw sapphire is from Madagascar. Credit: Kluka.
Lavender lepidolite has been found in the Himalaya Mine, Mesa Grande District, San Diego County, California, USA. Credit: Rob Lavinsky.

Axinite-(Mg) or magnesioaxinite, Ca2MgAl2BOSi4O15(OH) magnesium rich, [can be] pale blue to pale violet[6]

Fluorapatite [a sample of which is shown at right] ... is a mineral with the formula Ca5(PO4)3F (calcium fluorophosphate). ... Fluorapatite as a mineral is the most common phosphate mineral. It occurs widely as an accessory mineral in igneous rocks and in calcium rich metamorphic rocks. It commonly occurs as a detrital or diagenic mineral in sedimentary rocks and is an essential component of phosphorite ore deposits. It occurs as a residual mineral in lateritic soils.[7]

At lower left is another fluorapatite example that is violet in color on quartz crystals.

Lower right shows both a rough stone and a cut stone of tanzanite. "Tanzanite is the blue/purple variety of the mineral zoisite (a calcium aluminium hydroxy silicate) with the formula (Ca2Al3(SiO4)(Si2O7)O(OH))]. Tanzanite is noted for its remarkably strong trichroism, appearing alternately sapphire blue, violet and burgundy depending on crystal orientation.[8] Tanzanite can also appear differently when viewed under alternate lighting conditions. The blues appear more evident when subjected to fluorescent light and the violet hues can be seen readily when viewed under incandescent illumination. A rough violet sample of tanzanite is third down at left.

Tanzanite in its rough state is usually a reddish brown color. It requires artificial heat treatment to 600 °C in a gemological oven to bring out the blue violet of the stone.[9]

Tanzanite is found only in the foothills of Mount Kilimanjaro.

Tanzanite is universally heat treated in a furnace, with a temperature between 550 and 700 degrees Celsius, to produce a range of hues between bluish-violet to violetish-blue. Some stones found close to the surface in the early days of the discovery were gem-quality blue without the need for heat treatment.

Perhaps the most common violet mineral is sapphire. A sample of uncut natural sapphire is at lowest right. "Sapphires may be found naturally, by searching through certain sediments (due to their resistance to being eroded compared to softer stones) or rock formations.

Lepidolite (KLi2Al(Al,Si)3O10(F,OH)2 is a lilac-gray or rose-colored member of the mica group that is a secondary source of lithium. It is a phyllosilicate mineral[10] and a member of the polylithionite-trilithionite series.[11]

It is associated with other lithium-bearing minerals like spodumene in pegmatite bodies. It is one of the major sources of the rare alkali metals rubidium and caesium.[12]

It occurs in granite pegmatites, in some high-temperature quartz veins, greisens and granites. Associated minerals include quartz, feldspar, spodumene, amblygonite, tourmaline, columbite, cassiterite, topaz and beryl.[7]

Human color visions[edit]

Spectral absorption curves of the short (S), medium (M) and long (L) wavelength pigments in human cone and rod (R) cells are shown. Credit: TAKASUGI Shinji.

At right are normalized absorption spectra of the three human photopsins and of human rhodopsin.

Iodopsins are the photoreceptor proteins found in the cone cells of the [human] retina that are the basis of color vision. Iodopsins are very close analogs of the visual purple rhodopsin [R] that is used in night vision. Iodopsins consist of a protein called photopsin and a bound chromophore, retinal.

In humans there are three different iodopsins (rhodopsin analogs) that form the protein-pigment complexes photopsin I, II, and III. They are called erythrolabe, chlorolabe, and cyanolabe, respectively.[13] These photopsins have absorption maxima for yellowish-green (photopsin I), green (photopsin II), and bluish-violet light (photopsin III).

Cyanolabe, or tritan, has an absorption range of 400 to 500 nm and a peak absorption wavelength of 420-440 nm.[14][15]

In humans the OPN1SW gene (GeneID: 611) encodes cyanolabe.[16][17][18]

Planetary sciences[edit]

Mars Pathfinder imaged the Martian sky with water ice clouds. Credit: NASA/JPL.
The image shows a carpet of purple flowers of Melampyrum. Credit: Joadl.
A jacaranda tree of purple flowers produces a large canopy. Credit: Patricio.lorente.
This is an image of Clements Mountain in Glacier National Park, Montana, from the Going-to-the-Sun Road. Credit: Acroterion.
Depending on lighting for photography or digital imaging, the rocks that compose a mountain such as Clements Mountain in Glacier National Park may appear violet in color. Credit: unknown US National Park Service employee.
Red Eagle Mountain in Glacier National Park appears violet in this National Park Service image. Credit: unknown US National Park Service employee.

The clouds of Mars shown in the image at left "from Sol 15 have a new look. As water ice clouds cover the sky, the sky takes on a more bluish cast. This is because small particles (perhaps a tenth the size of the martian dust, or one-thousandth the thickness of a human hair) are bright in blue light, but almost invisible in red light. Thus, scientists expect that the ice particles in the clouds are very small. The clouds were imaged by the Imager for Mars Pathfinder (IMP)."[19] The usually pinkish-red color of Mars's sky turns a violet color on occasion, due to the presence of water ice clouds with very small ice particles.

While the plant whose violet or purple flowers covers the greatest surface area of the Earth at any given time may not be known, many violet or purple flowers as shown at left and right occur on Earth.

At right is a close up, well lit exposure of Clements Mountain in Glacier National Park showing a more accurate rock coloration. Compare this image with another on the right below it from the United States National Park Service of Clements Mountain which has a different orientation and apparently higher contrast.

A similar photographic or imaging effect is shown in the image at left of Red Eagle Mountain on Earth in the United States of America Glacier National Park.

Theoretical violet astronomy[edit]

As of 1977, "model calculations cannot reproduce the observed breadth of the Ca II λ3933 line in Da,F stars like Ross 627 without appealing to an unknown line-broadening mechanism".[20]

Radiation astronomy sources[edit]

White phosphorus and resulting allotropes, including violet phosphorus, are indicated. Credit: UserXresu.

Elemental phosphorus can exist in several allotropes; the most common of which are white and red solids. Solid violet and black allotropes are also known.

It would appear that violet phosphorus is a polymer of high relative molecular mass, which on heating breaks down into P2 molecules. On cooling, these would normally dimerize to give P4 molecules (i.e. white phosphorus) but, in vacuo, they link up again to form the polymeric violet allotrope.


"[T]he blue-violet continuum of some S and C-S stars is greatly depressed similar to that observed for N stars."[21] "Depending upon the polytype, temperature, and concentration of various possible impurities, SiC exhibits a fundamental absorption edge lying between about 2.2 and 3.0 eV."[21]


"Carbon stars are enormously fainter in the violet region than expected from appropriate blackbody spectra."[22]

"The spectra of six carbon stars increase in brightness shortward of 3900 Å, indicating that the violet opacity in these stars is dominated by C3, not SiC."[22]


Main sources: Chemicals/Hydrogens and Hydrogens
The spectrum shows the lines in the visible due to emission from elemental hydrogen. Credit:Teravolt.

The Balmer series of emission lines from hydrogen occur in the visible spectrum of the Sun at: 397, 410, 434, 486, and 656 nm.

Hydrogen has two emission lines that occur in an electron cyclotron resonance (ECR) heated plasmas at 397.007 nm of the Balmer series (Hε) and 434.05 nm Hγ.[23]


Main sources: Chemicals/Heliums and Heliums
The spectrum shows the lines in the visible due to emission from elemental helium. Credit:Teravolt.

As shown in the above spectrum, helium has at least one emission line in the violet.

The He I emission lines are at 414.3 nm and 447.1 nm.[24]


Main sources: Chemicals/Lithiums and Lithiums
This spectrograph shows the visual spectral lines of lithium. Credit: T c951.

"Violet satellite bands are caused by those lithium atoms which undergo an optical transition while a helium atom is nearby."[25]


This spectrograph shows the visual spectral lines of beryllium. Credit: Penyulap.

Above is a light spectrum of the emission and absorption lines of neutral, atomic beryllium. Important for violet astronomy is the apparent absence of strong lines well within the violet range and one strong line on the fringe of the violet and blue portions of the visual spectrum.


Main sources: Chemicals/Borons and Borons
This image shows the emission lines for boron and their approximate locations in the visible spectrum. Absorption lines occur at the same locations but with subtraction of light from the continuum. Credit: Penyulap.

Above is a light spectrum of the emission and absorption lines of neutral, atomic boron. Important for violet astronomy is the two strong lines well within the violet range and one weaker line on the fringe of the violet and blue portions of the visual spectrum.

The emission and absorption spectra for boron contain lines on the border between violet and blue.


Main sources: Chemicals/Carbons and Carbons
The spectrum shows the lines in the visible due to emission from elemental carbon. Credit:Teravolt.

Carbon has an emission line that occurs in plasmas at 449.881 nm from C VI.[23]

From the spectrum above, carbon has at least two lines in the violet.


Main sources: Chemicals/Nitrogens and Nitrogens
The spectrum shows the lines in the visible due to emission from elemental nitrogen. Credit:Kurgus.

Nitrogen has an emission line that occurs in plasmas at 388.678 nm from N VII.[23]

As seen in its spectrum above, nitrogen has many emission lines in the violet.


Main sources: Chemicals/Oxygens and Oxygens
The spectrum shows the lines in the visible due to emission from elemental oxygen. Credit:Teravolt.

Oxygen has several emission lines that occur in an electron cyclotron resonance (ECR) heated plasmas: 406.963, 406.99, 407.22, 407.59, 407.89, 408.51, 435.12, 441.489, and 441.697 nm from O II, and 434.045 nm from O VIII.[23]

"Electron temperatures are generally derived from the ratio of auroral to nebular lines in [O III] or [N II]."[26] "[B]ecause of the proximity of strong night-sky lines at λ4358 and λλ5770, 5791, the auroral lines of [O III] λ4363 and [N II] λ5755 are often contaminated."[26]


Main sources: Chemicals/Fluorines and Fluorines
This diagram contains the emission and absorption lines for the element fluorine. Credit: Alex Petty.

Fluorine has an emission line that occurs in plasmas at 429.92 nm from F II.[23]

The emission and absorption spectra of fluorine contains at least eight lines or bands from the cyan to the ultraviolet.[27]


Main sources: Chemicals/Neons and Neons
This image shows the emission lines for atomic neon. absorption lines occur at the same locations by subtraction of light from the continuum. Credit: Teravolt.

Like fluorine, neon has at least fourteen emission and absorption lines or bands from the cyan to the violet.[28]


Magnesium (Mg I) has an absorption band at 416.727±2.9 nm with an excitation potential of 4.33 eV.[29]

Magnesium (Mg II) has an absorption band at 439.059±6.6 nm with an excitation potential of 9.96 eV.[29]


Main sources: Chemicals/Aluminums and Aluminums

"The aluminium abundance was derived from the resonance line at 394.4nm, and Al is underabundant by ∼ −0.7 dex with respect to iron."[30] "These abundances are the LTE values; no NLTE corrections, as prescribed by Baumüller and Gehren (1997) and Baumüller et al. (1998), have been applied. The prescribed NLTE corrections for Teff = 6500K, log g = 4.0, [Fe/H] = –3.0 are –0.11 ... for ... Al .... If we assume these values to apply for our lower-gravity star [CS 29497-030], then Al follows iron"[30]. The elemental abundance ratios for CS 29497-030 of aluminum are [Al/H] = -3.37, [Al/Fe] = -0.67.[30]

Both Al I absorption lines at 394.401±8.5 and 396.152±6.5 have been measured for Sirius.[29]


Main sources: Chemicals/Silicons and Silicons

Silicon (Si II) has two absorption bands at 412.805±10.8 nm and 413.088±13.0 nm with excitation potentials of 9.79 eV and 9.80 eV, respectively.[29]

Silicon has an absorption line (Si IV) at 408.9 nm.[24]


Main sources: Chemicals/Argons and Argons

Argon has several emission lines that occur in an electron cyclotron resonance (ECR) heated plasmas: 426.653, 428.29, 433.12, 434.8064, 437.075, 437.967, 442.60, and 443.019 nm from Ar II.[23]


Main sources: Chemicals/Calciums and Calciums

As of 1977, "model calculations cannot reproduce the observed breadth of the Ca II λ3933 line in Da,F stars like Ross 627 without appealing to an unknown line-broadening mechanism".[20]

Calcium has a line occurring in the solar corona at 408.63 nm of Ca XIII.[31]

Calcium (Ca I) has two absorption bands, 422.673±4.5 nm and 430.253±0.6.[29] The second has an excitation potential of 1.89 eV.[29]

Calcium (Ca II) has an absorption band, 393.366±55.0.[29]


Main sources: Chemicals/Scandiums and Scandiums

Scandium (Sc II) has an absorption band, 424.683±1.0 nm, with an excitation potential of 0.31 eV.[29]


Main sources: Chemicals/Titaniums and Titaniums

Titanium (Ti II) has an absorption band, 391.346-441.108 nm, with an excitation potential range of 0.60-3.08 eV.[29]


Main sources: Chemicals/Vanadiums and Vanadiums
Oxidation states of vanadium are shown from left +2 (lilac), +3 (green), +4 (blue) and +5 (yellow). Credit: Steffen Kristensen.

The chemistry of vanadium is noteworthy for the accessibility of the four adjacent oxidation states 2-5. In aqueous solution the colours are lilac V2+(aq), green V3+(aq), blue VO2+(aq) and, at high pH, yellow VO42-.

Vanadium (V II) has an absorption band, 392.973-403.678 nm, with an excitation potential range of 1.07-1.81 eV.[29]


Main sources: Chemicals/Chromiums and Chromiums

Chromium (Cr) has emission lines that occur in plasmas at 425.435, 427.48, and 428.972 nm from Cr I.[23]

Chromium has absorption lines that occur at 425.435-428.972 nm from Cr I and 400.333-428.421 nm, 3.09-6.46 eV from Cr II near Sirius.[29]


Manganese (Mn I) has two absorption bands at 403.449±1.4 nm and 405.554±0.8 nm, where the second has an excitation potential of 2.13 eV.[29]

Manganese (Mn II) has an absorption band at 420.638±0.8 nm with an excitation potential of 5.37 eV.[29]


Main sources: Chemicals/Irons and Irons
This digitally altered image shows the Fraunhofer lines. Credit: Saperaud.

Iron has a line occurring in the solar corona in the violet at 398.69 nm of Fe XI.[31]

Iron (Fe I) has an absorption band at 392.291-440.475 nm with an excitation potential range of 0.05-2.46 eV.[29]

Iron (Fe II) has an absorption band at 393.829-438.538 nm with an excitation potential range of 1.66-2.77 eV.[29]

Iron (Fe II) has an emission line at 489.1 nm.[32]

The Fraunhofer G line at 430.790 nm is from iron.


Main sources: Chemicals/Nickels and Nickels

Nickel has three emission lines occurring in the solar corona at 380.08 nm of Ni XIII and 423.14 nm and 431.1 of Ni XII.[31]

Nickel has an absorption band at 401.550-436.210 nm with an excitation potential of 4.01 eV.[29]


Strontium (Sr II) has two absorption bands: 407.771±11.3 nm and 421.552±10.4 nm.[29]


Main sources: Chemicals/Yttriums and Yttriums

Yttrium (Y II) has an absorption band from 395.035 to 439.802 nm, with an excitation potential range of 0.10-0.13 eV.[29]


Zirconium (Zr II) has an absorption band, 395.824-415.624 nm, with an excitation potential of 0.52-0.75 eV.[29]


Main sources: Chemicals/Mercuries and Mercuries
This is a visible emission spectrum of mercury (Hg). Credit: teravolt.

As the spectrum of mercury (Hg) indicates, it has lines in the violet.


"The CN radical through emission in its Violet system bands has a long-established presence in comets."[33]


Blue is between violet and green in the spectrum of visible light. Credit: .

Def. of the higher-frequency region of the part of the electromagnetic spectrum which is relevant in the specific observation is called blue.

Def. the colour of the clear sky or the deep sea, between green and violet in the visible spectrum, and one of the primary additive colours for transmitted light; the colour obtained by subtracting red and green from white light using magenta and cyan filters; or any colour resembling this is called blue.


Cherenkov radiation glows in the core of the Advanced Test Reactor. Credit: Matt Howard.

The frequency spectrum of Cherenkov radiation by a particle is given by the Frank–Tamm formula. Unlike fluorescence or emission spectra that have characteristic spectral peaks, Cherenkov radiation is continuous. Around the visible spectrum, the relative intensity per unit frequency is approximately proportional to the frequency. That is, higher frequencies (shorter wavelengths) are more intense in Cherenkov radiation. This is why visible Cherenkov radiation is observed to be brilliant blue. In fact, most Cherenkov radiation is in the ultraviolet spectrum—it is only with sufficiently accelerated charges that it even becomes visible; the sensitivity of the human eye peaks at green, and is very low in the violet portion of the spectrum.

Plasma objects[edit]

Representation of upper-atmospheric lightning and electrical-discharge phenomena are displayed. Credit: .
The surface of a MEMS device is cleaned with bright, blue oxygen plasma in a plasma etcher to rid it of carbon contaminants. (100mTorr, 50W RF) Credit: .
This image of the Northern Lights shows the very rare blue light. Credit: Varjisakka.

Blue jets differ from sprites in that they project from the top of the cumulonimbus above a thunderstorm, typically in a narrow cone, to the lowest levels of the ionosphere 40 to 50 km (25 to 30 miles) above the earth. In addition, whereas red sprites tend to be associated with significant lightning strikes, blue jets do not appear to be directly triggered by lightning (they do, however, appear to relate to strong hail activity in thunderstorms).[34] They are also brighter than sprites and, as implied by their name, are blue in color. The color is believed to be due to a set of blue and near-ultraviolet emission lines from neutral and ionized molecular nitrogen.

"Blue starters were discovered on video from a night time research flight around thunderstorms [35] and appear to be "an upward moving luminous phenomenon closely related to blue jets."[36] They appear to be shorter and brighter than blue jets, reaching altitudes of only up to 20 km.[37]

"Blue starters appear to be blue jets that never quite make it".[38]

"Plasma cleaning involves the removal of impurities and contaminants from surfaces through the use of an energetic plasma created from gaseous species. Gases such as argon and oxygen, as well as mixtures such as air and hydrogen/nitrogen are used. The plasma is created by using high frequency voltages (typically kHz to >MHz) to ionise the low pressure gas (typically around 1/1000 atmospheric pressure), although atmospheric pressure plasmas are now also common.

At left is an image of the Northern Lights on Earth showing the very rare blue lights.


Main sources: Stars/Sun and Sun (star)

As an astronomical object sets or rises in relation to the horizon, the light it emits travels through Earth's atmosphere, which works as a prism separating the light into different colors. The color of the upper rim of an astronomical object could go from green to blue to violet depending on the decrease in concentration of pollutants, as they spread throughout an increasing volume of atmosphere.[39]


Main source: Mercury
The color image was generated by combining the mosaics taken through the MESSENGER WAC filters that transmit light at wavelengths of 1000 nanometers (infrared), 700 nanometers (far red), and 430 nanometers (violet). Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

"MESSENGER's Wide Angle Camera (WAC), part of the Mercury Dual Imaging System (MDIS), is equipped with 11 narrow-band color filters. As the spacecraft receded from Mercury after making its closest approach on January 14, 2008, the WAC recorded a 3x3 mosaic covering part of the planet not previously seen by spacecraft. The color image shown here was generated by combining the mosaics taken through the WAC filters that transmit light at wavelengths of 1000 nanometers (infrared), 700 nanometers (far red), and 430 nanometers (violet). These three images were placed in the red, green, and blue channels, respectively, to create the visualization presented here. The human eye is sensitive only across the wavelength range from about 400 to 700 nanometers. Creating a false-color image in this way accentuates color differences on Mercury's surface that cannot be seen in black-and-white (single-color) images."[40]


Main source: Venus
This violet light image at right was taken in February 1990 by Galileo's Solid State Imaging System at range of about 2 million miles. Credit: NASA/JPL.

Violet photographs of the planet Venus taken in 1927 “recorded two nebulous bright streaks, or bands, running ... approximately at right angles to the terminator” that may be from the upper atmosphere.[41]

In 1959 "observations of the spectrum of the planet Venus, with spectrographs of low and high dispersion at the Georgetown College Observatory, show that a wide, continuous absorption band is present in the violet and near-ultraviolet."[42]

The image at the page top right is from the Galileo spacecraft solid state imaging system taken on February 14, 1990. The satellite was about 2.7 million km from the planet. The highpass violet filter (418 nm) has been applied to emphasize the smaller scale cloud features. This rendition has been colorized bluish to emphasize subtle contrasts in the cloud markings. The sulfuric acid clouds indicate considerable convective activity. The filamentary dark features are composed of several dark nodules, like beads on a string, each about 96 km across.

The image at right is from a "series of pictures [that show] four views of the planet Venus obtained by Galileo's Solid State Imaging System at ranges of 1.4 to 2 million miles as the spacecraft receded from Venus. The pictures [the first two] were taken about 4 and 5 days after closest approach; those ... were taken about 6 days out, 2 hours apart [of which the image at right is the last]. In these violet-light images, north is at the top and the evening terminator to the left. The cloud features high in the planet's atmosphere rotate from right to left, from the limb through the noon meridian toward the terminator, traveling all the way around the planet once every four days. The motion can be seen by comparing the last two pictures, taken two hours apart. The other views show entirely different faces of Venus. These photographs are part of the 'Venus global circulation' sequence planned by the imaging team."[43]


Main source: Earth
This image contains a comparison of emission lines from a light polluted site (Asiago Astrophysical Observatory – Italy, upper panel) and a dark site (ESO-Paranal – Chile, lower panel). Credit: Ferdinando Patat, ESO.
The figure contains tracings of night sky spectra at a polluted (Asiago Astrophysical Observatory – Italy, red tracing) and at a dark site (ESO-Paranal – Chile, blue tracing). Credit: Ferdinando Patat, ESO.

The spectra at right show a "[c]omparison between a night sky spectrum obtained in a light polluted site (Asiago Astrophysical Observatory – Italy, upper panel) and a dark site (ESO-Paranal – Chile, lower panel). Spectral line identifications for the main features are traced in red for artificial sources and in blue for the natural ones. The emissions generated by street lighting are clearly visible, mainly in the form of strong lines of Mercury and Sodium, which fall not only in the visible range (500-600nm), but also in the in blue and violet parts of the spectrum."[44]

The second figure at right shows "[t]racings of night sky spectra at a polluted (red) and at a dark site (blue). Main line identifications for artificial features are marked. The colored curves on the top of the figure are the transmission functions of UBVR standard astronomical passbands. The light pollution appears to be maximum in the V passband, which is very close to the human eye sensitivity region. The B band is also severely contaminated, making astrophotographer’s life quite difficult."[44]

"[T]he broad Sodium feature at about 590nm is a clear imprint of high pressure lamps, which are the most disturbing sources of light pollution."[44]

"The 1.8m Copernicus telescope is placed on the top of Mount Ekar, at about 1300 meters on the sea level. In spite of its relatively good location, the brightest feature in its night sky spectrum is the Mercury line at 436nm (the only red line in [the first figure]), which is much brighter than the natural Oxygen line at 558nm, the most intense feature emitted by the night sky."[44]

"The presence of these lines causes the sky to become artificially brighter, and it turns into a disaster for broad band imaging. In a relatively protected site like Mount Ekar, in the B and V filters the enhancement is more than a magnitude. But when one goes close to a relatively big town, this degradation can reach two or more magnitudes, causing many astronomical objects to be lost, not only for the visual observers, but also for the evolved amateurs equipped with modern CCD cameras."[44]


Main sources: Wanderers/Moon and Moon
During Galileo's December 1992 flyby of Earth's Moon, controllers took this dramatically illuminated picture through the violet filter. Credit: Galileo Project, JPL, NASA.

"Checking out the Galileo spacecraft's cameras during its December 1992 flyby of Earth's Moon, controllers took this dramatically illuminated picture [at right] through a violet filter. The view looks down on the Moon's north polar region with the Sun shining from the left at a low angle and the direction toward the moon's North pole toward the lower right. Across the image upper left stretches the smooth volcanic plain of the Mare Imbrium. Pythagoras crater, 65 miles wide, is near the center of the image -- mostly in shadow, its central peak just catches the sunlight. Yesterday, the Moon made its closest approach to Earth and was full for the second time in July (as reckoned by UT dates). The closest point in the Moon's orbit is referred to as Lunar Perigee, a mere 221,797 miles at 8 hours UT. The second full moon in a month is known as a "Blue Moon"."[45]


Main source: Mars
This image shows a polygonal pattern in the ground near NASA's Phoenix Mars Lander, similar in appearance to icy ground in the arctic regions of Earth. Credit: NASA/JPL-Caltech/University of Arizona.
This color composite image, reconstructed through violet, green, and orange filters, vividly shows the distribution of clouds against the rust colored background of this Martian desert. Credit: NASA/JPL-Caltech.
This view of the Martian atmosphere and surface is taken through the Viking Orbiter violet filter. Credit: NASA.

"Phoenix touched down on the Red Planet at 4:53 p.m. Pacific Time (7:53 p.m. Eastern Time), May 25, 2008, in an arctic region called Vastitas Borealis, at 68 degrees north latitude, 234 degrees east longitude."[46]

In the image at right "is an approximate-color image taken shortly after landing by the spacecraft's Surface Stereo Imager, inferred from two color filters, a violet, 450-nanometer filter and an infrared, 750-nanometer filter."[46]

The "image shows a polygonal pattern in the ground near NASA's Phoenix Mars Lander, similar in appearance to icy ground in the arctic regions of Earth."[46] Noteworthy is the blue to violet faces of many of the small stones shown.

"As the sun rises over Noctis Labyrinthus (the labyrinth of the night), bright clouds of water ice can be observed in and around the tributary canyons of this high plateau region of Mars. This color composite image, reconstructed through violet, green, and orange filters, vividly shows the distribution of clouds against the rust colored background of this Martian desert."[47]

"Scientists have puzzled why the clouds cling to the canyon areas and, only in certain areas, spill over onto the plateau surface. One possibility is that water which condensed during the previous afternoon in shaded eastern facing slopes of the canyon floor is vaporized as the early morning sun falls on those same slopes. The area covered is about 10,000 square kilometers (4000 square miles), centered at 9 degrees South, 95 degrees West, and the large partial crater at lower right is Oudemans. The picture was taken on Viking Orbiter 1's 40th orbit."[47]

At lower right is a view of the Martian atmosphere and surface taken through the Viking Orbiter violet filter to illustrate the dramatic clarity "of the atmosphere in the region east and northeast of the Argyre basin during winter in the southern hemisphere. [The image is] taken just after the winter solstice when solar heating is minimal."[48]


Main sources: Wanderers/Jupiter and Jupiter
This movie of changes in Jupiter's cloud patterns is from Voyager 2 acquired in the Violet filter around May 6, 1979. Credit: NASA/JPL.
This is a Voyager 1 image through the violet filter showing Jupiter with its satellite Io visible at lower left. Credit: NASA.
These images show the apparent edge (limb) of the planet Jupiter. Credit: NASA/JPL Galileo spacecraft.

"This movie [at right] records an eruptive event in the southern hemisphere of Jupiter over a period of 8 Jupiter days. Prior to the event, an undistinguished oval cloud mass cruised through the turbulent atmosphere. The eruption occurs over a very short time at the very center of the cloud. The white eruptive material is swirled about by the internal wind patterns of the cloud. As a result of the eruption, the cloud then becomes a type of feature seen elsewhere on Jupiter known as "spaghetti bowls.""[49]

"As Voyager 2 approached Jupiter in 1979, it took images of the planet at regular intervals. This sequence is made from 8 images taken once every Jupiter rotation period (about 10 hours). These images were acquired in the Violet filter around May 6, 1979. The spacecraft was about 50 million kilometers from Jupiter at that time."[49]

At left is a "Voyager 1 image showing Jupiter with its satellite Io visible at lower left. Jupiter is 140,000 km in diameter and Io is 3600 km across. This image was taken with the narrow angle camera using the violet filter from a distance of 25 million km on 9 February 1979. North is at about 11:00 (Voyager 1, 15672.37)".[50]

"These images [at lower right] show the apparent edge (limb) of the planet Jupiter as seen through both the violet filter (top frame) and an infrared filter (756 nanometers, bottom frame) of the Solid State Imaging (CCD) system aboard NASA's Galileo spacecraft. North is to the top of the picture. A separate haze layer is clearly visible above the northern part of the limb."[51]

"This haze layer becomes less well defined to the south (bottom left). Such a detached haze layer has been seen previously on only one other body with a thick atmosphere: Saturn's satellite Titan. The haze layer cannot be lower in the atmosphere than a pressure of about 10 millibars (mbar), or about 40 kilometers (km) above the tropopause. (The tropopause, where the temperature stops decreasing with height, is at about 100 mbar, 20 km above the tops of the ammonia clouds.) There is some indication of streaks of slightly brighter and darker material running roughly north-south (parallel to the limb) on Jupiter's crescent."[51]

"The images, which show the limb between 60.5 degrees and 61.8 degrees North latitude (planetographic) and near 315 degrees West longitude, were obtained on December 20, 1996 Universal Time. The spacecraft was about 1,286,000 km (18.0 Jovian radii) from the limb of Jupiter and the resolution is about 13 kilometers per picture element."[51]

Satellites of Jupiter[edit]

"A definite color gradient is observed [in the small inner satellites of Jupiter], with the satellites closer to Jupiter being redder: the mean violet/green ratio (0.42/0.56 μm) decreases from Thebe to Metis. This ratio also is lower for the trailing sides of Thebe and Amalthea than for their leading sides."[52]


Main source: Ganymede
In this image of Ganymede's trailing side, the colors are enhanced to emphasize color differences. Credit: NASA/JPL/DLR.

"In this global view of Ganymede's trailing side, the colors are enhanced to emphasize color differences. The enhancement reveals frosty polar caps in addition to the two predominant terrains on Ganymede, bright, grooved terrain and older, dark furrowed areas. Many craters with diameters up to several dozen kilometers are visible. The violet hues at the poles may be the result of small particles of frost which would scatter more light at shorter wavelengths (the violet end of the spectrum). Ganymede's magnetic field, which was detected by the magnetometer on NASA's Galileo spacecraft in 1996, may be partly responsible for the appearance of the polar terrain. Compared to Earth's polar caps, Ganymede's polar terrain is relatively vast. The frost on Ganymede reaches latitudes as low as 40 degrees on average and 25 degrees at some locations. For comparison with Earth, Miami, Florida lies at 26 degrees north latitude, and Berlin, Germany is located at 52 degrees north."[53]

"North is to the top of the picture. The composite, which combines images taken with green, violet, and 1 micrometer filters, is centered at 306 degrees west longitude. The resolution is 9 kilometers (6 miles) per picture element. The images were taken on 29 March 1998 at a range of 918000 kilometers (570,000 miles) by the Solid State Imaging (SSI) system on NASA's Galileo spacecraft."[53]


Main source: Io
In this image of the rocky object Io are violet or purple patches. Credit: NASA/JPL/University of Arizona.
This violet filter image of the Masubi volcano on Jupiter's moon Io is acquired on March 5, 1979 at 14:37:29 UTC. Credit: NASA/JPL/UA/Jason Perry.

"Io, the most volcanic body in the solar system is seen in the highest resolution obtained to date by NASA's Galileo spacecraft. The smallest features that can be discerned are 2.5 kilometers in size. There are rugged mountains several kilometers high, layered materials forming plateaus, and many irregular depressions called volcanic calderas. Several of the dark, flow-like features correspond to hot spots, and may be active lava flows. There are no landforms resembling impact craters, as the volcanism covers the surface with new deposits much more rapidly than the flux of comets and asteroids can create large impact craters. The picture is centered on the side of Io that always faces away from Jupiter; north is to the top."[54]

"Color images acquired on September 7, 1996 have been merged with higher resolution images acquired on November 6, 1996 by the Solid State Imaging (CCD) system aboard NASA's Galileo spacecraft. The color is composed of data taken, at a range of 487,000 kilometers, in the near-infrared, green, and violet filters and has been enhanced to emphasize the extraordinary variations in color and brightness that characterize Io's face. The high resolution images were obtained at ranges which varied from 245,719 kilometers to 403,100 kilometers."

The image at left is through the violet filter of the Voyager 1 Imaging Science Sub-system Wide-Angle Camera of the Masubi volcano on Jupiter's moon Io, acquired on March 5, 1979 at 14:37:29 UTC.


Main source: Saturn
This view from Voyager 2 is of Saturn's north polar region through the orange and violet filters. Credit: NASA/JPL.
The image shows a subtle northward gradation from gold to azure on Saturn. Credit: NASA/JPL.

"The north polar region of Saturn is pictured in great detail in this Voyager 2 image obtained Aug. 25 from a range of 633,000 kilometers (393,000 miles)."[55]

"Two oval cloud systems some 250 km (150 mi) across are visible at about 72 degrees north latitude. The bright spot in the center of the leftmost cloud is a convective cloud storm about 60 km. (37 mi.)across. The outer ring of material rotates in an anti-cyclonic sense(counterclockwise in the northern hemisphere). A similar cloud structure of comparable dimension appears at 55 degrees north (bottom center of this picture). These northern latitudes contain many bright, small-scale cloud spots--only a few tens of kilometers across--representative of convective cloud systems. Across the top of this image stretch several long, linear, wavelike features that may mark the northernmost east-flowing jet in Saturn's atmosphere."[55]

"In this orange-and-violet-image composite, the smallest features visible are about 16 km. (10 mi.) across."[55]

In the second image at right, "[t]he gas planet's subtle northward gradation from gold to azure is a striking visual effect that scientists don't fully understand. Current thinking says that it may be related to seasonal influences, tied to the cold temperatures in the northern (winter) hemisphere. Despite Cassini's revelations, Saturn remains a world of mystery."[56]


Main source: Dione
Enhanced color composite of Saturn's moon Dione is based on infrared, green, ultraviolet, and clear-filter images taken by the Cassini spacecraft December 14, 2004. Credit: Matt McIrvin, Cassini/NASA.
Dione is shown here in a composite of images from Cassini. Credit: NASA, JPL, SSI, ESA.

This at right is an "[e]nhanced color composite of Saturn's moon Dione, based on infrared, green, ultraviolet, and clear-filter images [is] taken by the Cassini spacecraft December 14, 2004."[57]

It shows "the darker, fractured terrain of the trailing hemisphere. The Padua Chasmata trace an arc on the left, interrupted near the top by central peak crater Ascanius. The Janiculum Dorsa extend along the upper right terminator. Near the lower left limb is the small crater Cassandra with its prominent ray system."[57]

At left is another image of Dione partially rotated from the one at right and showing a violet cast on the apparent higher elevation portion toward the terminator. This image is from Cassini "taken 1 August 2005 from 243,000 km away."[58]


Main source: Titan
With its thick, distended atmosphere, Titan's orange globe shines softly, encircled by a thin halo of purple light-scattering haze. Credit: NASA/JPL/Space Science Institute.

At right is an image of Titan showing a purple haze.

"With its thick, distended atmosphere, Titan's orange globe shines softly, encircled by a thin halo of purple light-scattering haze."[59]

"Small particles that populate high hazes in Titan's atmosphere scatter short wavelengths more efficiently than longer visible or infrared wavelengths, so the best possible observations of the detached layer are made in ultraviolet light."[59]

"The images in this view were taken by the Cassini narrow-angle camera on May 5, 2005, at a distance of approximately 1.4 million kilometers (900,000 miles) from Titan and at a sun-Titan-spacecraft, or phase, angle of 137 degrees. Image scale is 8 kilometers (5 miles) per pixel."[59]


Main source: Comets

"The interpretation of the peculiar structure of the CN violet band observed in cometary spectra became clear when Swings (1941) realized the essential role played by the absorption lines present in the exciting solar radiation."[60]

The R-branch of the CN violet (0,0) band has been observed in the spectrum of Comet Mrkos with the Hale telescope in August 1957.[60]

Comet Seki-Lines has also produced the CN violet (0,0) band.[60]


Main source: Uranus
Six 15-second narrow-angle images were used to extract color information from the extremely dark and faint rings. Two images each in the green, clear and violet filters were added together and averaged to find the proper color differences between the rings. Credit: NASA/JPL.

"This false-color view of the rings of Uranus was made from images taken by Voyager 2 on Jan. 21, 1986, from a distance of 4.17 million kilometers (2.59 million miles). All nine known rings are visible here; the somewhat fainter, pastel lines seen between them are contributed by the computer enhancement. Six 15-second narrow-angle images were used to extract color information from the extremely dark and faint rings. Two images each in the green, clear and violet filters were added together and averaged to find the proper color differences between the rings. The final image was made from these three color averages and represents an enhanced, false-color view. The image shows that the brightest, or epsilon, ring at top is neutral in color, with the fainter eight other rings showing color differences between them. Moving down, toward Uranus, we see the delta, gamma and eta rings in shades of blue and green; the beta and alpha rings in somewhat lighter tones; and then a final set of three, known simply as the 4, 5 and 6 rings, in faint off-white tones. Scientists will use this color information to try to understand the nature and origin of the ring material. The resolution of this image is approximately 40 km (25 mi). The Voyager project is managed for NASA by the Jet Propulsion Laboratory."[61]


The complex terrain of Ariel is viewed in this image, the best Voyager 2 color picture of the Uranian moon. Credit: NASA/JPL.

"The complex terrain of Ariel is viewed in [the image at right], the best Voyager 2 color picture of the Uranian moon. The individual photos used to construct this composite were taken Jan. 24, 1986, from a distance of 170,000 kilometers (105,000 miles. Voyager captured this view of Ariel's southern hemisphere through the green, blue and violet filters of the narrow-angle camera; the resolution is about 3 km (2 mi). Most of the visible surface consists of relatively intensely cratered terrain transected by fault scarps and fault-bounded valleys (graben). Some of the largest valleys, which can be seen near the terminator (at right), are partly filled with younger deposits that are less heavily cratered. Bright spots near the limb and toward the left are chiefly the rims of small craters. Most of the brightly rimmed craters are too small to be resolved here, although one about 30 km (20 mi) in diameter can be easily distinguished near the center. These bright-rim craters, though the youngest features on Ariel, probably have formed over a long span of geological time. Although Ariel has a diameter of only about 1,200 km (750 mi), it has clearly experienced a great deal of geological activity in the past."[62]


This color composite of the Uranian satellite Miranda was taken by Voyager 2 on January 24, 1986, from a distance of 147,000 kilometers (91,000 miles). Credit: NASA.

"This color composite [at lower right] of the Uranian satellite Miranda was taken by Voyager 2 on Jan. 24, 1986, from a distance of 147,000 kilometers (91,000 miles). This picture was constructed from images taken through the narrow-angle camera's green, violet and ultraviolet filters. It is the best color view of Miranda returned by Voyager."[63]

"Miranda, just 480 km (300 mi) across, is the smallest of Uranus' five major satellites. Miranda's regional geologic provinces show very well in this view of the southern hemisphere, imaged at a resolution of 2.7 km (1.7 mi). The dark- and bright-banded region with its curvilinear traces covers about half of the image. Higher-resolution pictures taken later show many fault valleys and ridges parallel to these bands. Near the terminator (at right), another system of ridges and valleys abuts the banded terrain; many impact craters pockmark the surface in this region. The largest of these are about 30 km (20 mi) in diameter; many more lie in the range of 5 to 10 km (3 to 6 mi) in diameter."[63]


This false color image of Triton is a composite of images taken through the violet, green and ultraviolet filters. Credit: NASA.

"This false color image of Triton is a composite of images taken through the violet, green and ultraviolet filters. The image was taken early on Aug. 25, 1989 when Voyager 2 was about 190,000 kilometers (118,000 miles) from Triton's surface. The smallest visible features are about 4 kilometers (2.5 miles) across. The image shows a geologic boundary between completely dark materials and patchy light/dark materials. A layer of pinkish material stretches across the center of the image. The pinkish layer must be thin because underlying albedo patterns show through. Several features appear to be affected by the thin atmosphere; the elongated dark streaks may represent particulate materials blown in the same direction by prevailing winds, and the white material may be frost deposits. Other features appear to be volcanic deposits including the smooth, dark materials alongside the long, narrow canyons. The streaks themselves appear to originate from very small circular sources, some of which are white, like the source of the prominent streak near the center of the image. The sources may be small volcanic vents with fumarolic-like activity. The colors may be due to irradiated methane, which is pink to red, and nitrogen, which is white."[64]


Main sources: Stars/Sirius and Sirius

A variety of elemental violet lines occur in the star Sirius. These include calcium (Ca I & II), iron (Fe I), magnesium (Mg I & II), manganese (Mn I & II), nickel (Ni II), scandium (Sc II), silicon (Si II), strontium (Sr II), titanium (Ti II), vanadium (V II), yttrium (Y II), and zirconium (Zr II).[29]

Eta Carinae[edit]

Main sources: Stars/Eta Carinae and Eta Carinae
This Hubble Space Telescope image shows excess violet light escaping along the equatorial plane between the bipolar lobes of the Eta Carinae Homunculus. Credit: Jon Morse (University of Colorado) & NASA Hubble Space Telescope.

"The “Purple Haze” is a diffuse blueish/purple glow within a few arcseconds of the central star in HST images of the Homunculus (Morse et al. 1998; Smith et al. 2000, 2004). This emission is seen in excess of violet starlight scattered by dust, and the strength of the excess increases into the far UV (Smith et al. 2004; hereafter Paper I)."[65]

Notation: let the symbol LH stand for the Little Homunculus.

"The LH has no outstanding correspondence with any of the clumps and filaments seen in scattered light in normal UV or visual-wavelength images of η Car, although it does match the spatial extent of the "Purple Haze"".[32] Bold added.

The Fe II emission line at 489.1 nm occurs in the Little Homunculus (Eta Carinae)[32]

Mass "loss at the η Carinae rate produces considerable changes of Teff on a human timescale. As an example, for the 120 Mʘ model, a change from about 20,000 K to 34,000 K was obtained over a time of 50 yr as a result of the secular bluewards evolution following the rapid ejection."[66]

RY Sagittarii[edit]

"Near minimum light, the spectra [of RY Sgr] display the same main features as those observed at a preceding minimum (Alexander et al. 1972) [including] three broad emission lines in the violet (Ca II and He I λ3888)".[67]

"The "broad bright lines" are displaced towards the red during the 1977 minimum, and they were displaced towards the violet during the 1967 minimum."[67]

"A possible interpretation of these differences is an unsymmetrical ejection of carbon clouds. An ejection mainly directed towards the observer would be linked with a deeper minimum and violet-shifted broad bright lines. An ejection mainly directed backwards would be linked with a less deep minimum and red-shifted lines."[67]

W UMa-type systems[edit]

A W Ursae Majoris variable is a type of eclipsing binary variable star. These stars are close binaries of spectral types F, G, or K that share a common envelope of material and are thus in contact with one another. They are termed contact binaries because the two stars touch and transfer mass and energy through the connecting neck. W Ursae Majoris variables are the most common variable stars in the present day Universe. About 1 percent of all stars belong to this group.

“[T]he violet and ultraviolet parts of the energy distributions in spectra of W UMa-type systems are abnormal with elevated ultraviolet fluxes for systems having the shortest periods at a given blue-visual colour.”[68]

NGC 5584[edit]

This is a colour-composite of the barred spiral galaxy NGC 5584. Credit: ESO: observations by Susana Randall, Claudio Melo, Swetlana Hubrig; day astronomer Dominique Naef; Henri Boffin (ESO) processed the data and made the colour-composite, and Haennes Heyer (ESO) made the final adjustments.

"This image is a colour-composite of the barred spiral galaxy NGC 5584. It is based on data collected by the Paranal Science Team with the FORS1 instrument on Kueyen, the second 8.2-m Unit Telescope of ESO's Very Large Telescope. The supernova SN 2007af is the bright object seen slightly below and to the right of the galaxy's centre. The galaxy and its bright supernova were observed on the nights of 16, 19 and 22 March 2007 through a B, V, R, H-alpha and OII filter."[69]

The B filter is centered at 440 nm and the OII filter is centered at 372 nm.[69]

"Located about 75 million light years away towards the constellation Virgo ('the Virgin'), NGC 5584 is a galaxy slightly smaller than the Milky Way. It belongs, however, to the same category: both are barred spirals."[70]

"Spiral galaxies are composed of a 'bulge' and a flat disc. The bulge hosts old stars and usually a central supermassive black hole. Younger stars reside in the disc, forming the characteristic spiral structures from which the galaxies get their name. Barred spirals are crossed by a bright band of stars."[70]

"In this amazing new image of NGC 5584 two dominant spiral arms are clearly visible, while the others are deformed, probably due to interactions with other galaxies. Luminous patches are spread all over the disc, indicating that stars are being formed in this gigantic rose at a frantic pace."[70]


Main sources: Stars/Quasars and Quasars

As of 1979, "[m]ost astrophysicists seem to consider redshifts of quasars as being completely cosmological. ... In local models which involve expulsion of "quasars" from parent galaxies at distances of up to ~100 Mpc there must be objects with large violet shifts (Burbidge and Burbidge, 1967). ... [I]n the catalogue of quasars by Burbidge et al. (1977) among 633 quasars there are 57 objects which have only two lines in their spectra identified with C III 1909 and Mg II 2798. ... If local "quasars" do exist ... then objects with large violet shifts ... could be among those 57 quasars".[71]


Main source: Physics

"Spectra obtained during the intense auroral storm of August 18 and 19, 1950, showed that the emission observed at the magnetic zenith is asymmetrically displaced to the violet. The violet wing is shifted approximately 71 A [7.1 nm], indicating a velocity of impact of 3300 km/sec. The emission observed at the magnetic horizon is symmetrical and undisplaced but considerably broadened."[72]

A violet wing of an emission peak (indicating increases in velocity ranging from about +200 km/sec to +600 km/sec) or absorption trough of a star may show an anomalous structure that is either a nonsymmetrical outflow, or a fluctuation in the flow velocity.[24]


Main source: Technology

Most spacecraft designed for optical astronomy or visual astronomy carried aboard a violet or blue filter covering the wavelength range from 350-430 nm.

"ZnSe appears as an attractive material to blue and near UV optoelectronics."[73]

Galileo spacecraft[edit]

Engineers at NASA's Jet Propulsion Laboratory pose with the completed Galileo orbiter and probe. Credit: NASA/JPL.
This is an image of the Solid-State Imaging camera of the Galileo spacecraft. Credit: NASA/JPL.

"[T]he Solid State Imaging (SSI) camera ... [at right of the Galileo spacecraft] uses a high-resolution, 800 x 800 charge-coupled device (CCD) array with a field of view of 0.46 degrees. Multi-spectral coverage is provided by an eight-position filter wheel on the camera, consisting of three broad-band filters: violet (404 nm), green (559 nm), and red (671 nm); four near-infrared filters: 727 nm, 756 nm, 889 nm, and 986 nm; and one clear filter (611 nm) with a very broad (440 nm) passband."[74]

Hubble Space Telescope[edit]

The Hubble Space Telescope is seen from the departing Space Shuttle Atlantis, flying Servicing Mission 4 (STS-125), the fifth and final human spaceflight to visit the observatory. Credit: Ruffnax (Crew of STS-125).

The Hubble Space Telescope (HST) is an excellent example of a radiation astronomy satellite designed for more than one purpose: the various astronomies of optical astronomy.

The HST is an optical astronomy telescope that incorporated a set of 48 filters isolating spectral lines of particular astrophysical interest.

The Wide Field Planetary Camera (PC-1) was in use from about 1990 through 1993. It carried 48 filters on 12 filter wheels of four each.

The violet band filters are F330W, F336W, F344N, F368M, F375N, F413M, F435W, F437N, F439W, and F469N.[75]

The Wide Field Planetary Camera (PC-2) replaced PC-1 and carried the following filters on the same filter wheels: F300W, F336W, F343N, F375N, F380W, F390N, F410M, F437N, F439W, F450W, F467M and F469N.[75] In December 1993 PC-1 was replaced with PC-2 and the HST was declared operational on January 13, 1994.

Onboard the HST is the Faint Object Camera (FOC) which carries filters for violet astronomy: F320W, F342W, F346M, F370LP, F372M, F410M, F430W, F437M, F470M.[75]

The Wide Field Camera 3 (WFC3) is the Hubble Space Telescope's last and most technologically advanced instrument to take images in the visible spectrum. It was installed as a replacement for the Wide Field and Planetary Camera 2 during the first spacewalk of Space Shuttle mission STS-125 on May 14, 2009.


Viking 1 launched aboard a Titan IIIE rocket August 20, 1975 and arrived at Mars on June 19, 1976. Credit: NASA.

"Each Viking Orbiter was equipped with two identical vidicon cameras. The camera system is commonly called the Visual Imaging Subsystem (VIS). Each camera consists of a telescope, a slow scan vidicon, a filter wheel, and associated electronics. The filter wheel contains blue, minus blue, violet, clear [no filter], green, and red filters."[76]

"The radiometric calibration converts the digitized signal received from the camera (DN value) into a quantity that is proportional to the radiance reaching the sensor. The sensitivity of the vidicon varies across the field of view. The sensitivity of the vidicon also varies with time. Each Viking Orbiter imaging instrument was calibrated before flight. In addition, changes in the calibration over time have been estimated from analyses of images of deep space and dust storms. The radiometric calibration applies additive and multiplicative corrections that account for the varying sensitivity of the vidicon. The resulting values are proportional to radiance factor, which is defined as the ratio of the observed radiance to the radiance of a normally illuminated lambertian reflector of unit reflectance at the same heliocentric distance. The geometric calibration removes electronic distortions and transforms the point perspective geometry of the original image into a map projection. The electronic distortions are barrel-shaped distortions from the electron beam readout and complex distortions from interactions between the charge on the vidicon face plate and the electron beam. The electronic distortions are modeled by comparing the undistorted locations of reseau marks with the actual locations in an image."[76]

"The VIS detector is a Westinghouse 5166 selenium vidicon. It is about 3.7 cm (1-1/2 in.) in diameter."[76]

"The saturation current from the vidicon is 43 nA. The residual dark current is 0.2 nA. The response of the visual imaging subsystem is linear to first order. Analyses of imaging data acquired inflight indicate that the system is linear to within 3% over its dynamic range."[76]

The blue filter is centered at 470 nm with a range from 350-530 nm.[76]

The violet filter is centered at 440 nm with a range of 350-470 nm.[76]

"The filter transmittance was measured using a Cary 14 spectrophotometer with a spare set of filters."[76]


This is a NASA photograph of one of the two identical Voyager space probes Voyager 1 and Voyager 2 launched in 1977. Credit: NASA.
The image shows the spectral range for the violet filter of Voyager 1 and Voyager 2. Credit: Xession.

The scan platform of Voyager 1 and 2 comprises: the Infrared Interferometer Spectrometer (IRIS) (largest camera at right) in the image at left; the Ultraviolet Spectrometer (UVS) to the right of the UVS; the two Imaging Science Subsystem (ISS) vidicon cameras to the left of the UVS; and the Photopolarimeter System (PPS) barely visible under the ISS.

At right is an image of the spectral range of the Violet filter (50 to 400 nm)[77] on the Imaging Science System aboard the Voyager 1 and Voyager 2 Spacecraft, as defined by the instrument descriptions of the Narrow Angle Camera and Wide Angle Camera.


Main source: Hypotheses
  1. Half of all the sources that appear violet are actually hotter.

See also[edit]


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Further reading[edit]

  • S. M. Rucinski (October 1983). "Violet and ultraviolet continua of W UMa systems on the basis of UVBY photometry observations". Astronomy and Astrophysics 127 (1): 84-92. 

External links[edit]

{{Astronomy resources}}{{Principles of radiation astronomy}}