(Redirected from Optical astronomy)
Mt.Palomar's 200-inch Telescope, pointing to the zenith, is seen from the east side. Note the person standing below the telescope (center-right at the bottom of the image). Credit: NASA.

In popular culture optical astronomy encompasses a wide variety of observations via telescopes that are sensitive in the range of visible light. Scientists would call this visible-light astronomy. It includes imaging, where a picture of some sort is made of the object; photometry, where the amount of light coming from an object is measured, spectroscopy, where the distribution of that light with respect to its wavelength is measured, and polarimetry where the polarisation state of that light is measured.

Scientists use the term optical astronomy to mean astronomy at infrared, visible and ultraviolet wavelengths (i.e. observations using either infrared, visible or ultraviolet wavelengths of light). Observations at these wavelengths generally use optical components (mirrors, lenses and solid state digital detectors).

## Astronomy

Def. astronomy using observations using telescopes and recording media that capture visible light is called optical astronomy.

Def. astronomy using infrared, visible and/or ultraviolet wavelengths is called optical astronomy.

"The radio quasar 3C 454.3 underwent an exceptional optical outburst lasting more than 1 year and culminating in spring 2005. The maximum brightness detected was R = 12.0, which represents the most luminous quasar state thus far observed (MB ~ −31.4). In order to follow the emission behaviour of the source in detail, a large multiwavelength campaign was organized by the Whole Earth Blazar Telescope (WEBT). Continuous optical, near-IR and radio monitoring was performed in several bands. ToO pointings by the Chandra and INTEGRAL satellites provided additional information at high energies in May 2005."[1]

Radiation may affect materials and devices in deleterious ways:

1. By causing the materials to become radioactive (mainly by neutron activation, or in [the] presence of high-energy gamma radiation by photodisintegration).
2. By nuclear transmutation of the elements within the material including, for example, the production of Hydrogen and Helium which can in turn alter the mechanical properties of the materials and cause swelling and embrittlement.
3. By radiolysis (breaking chemical bonds) within the material, which can weaken it, cause it to swell, polymerize, promote corrosion, cause belittlements, promote cracking or otherwise change its desirable mechanical, optical, or electronic properties.
4. By formation of reactive compounds, affecting other materials (e.g. ozone cracking by ozone formed by ionization of air).
5. By ionization, causing electrical breakdown, particularly in semiconductors employed in electronic equipment, with subsequent currents introducing operation errors or even permanently damaging the devices.

## Rays

Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it.[2] Optics usually describes the behavior of visible, ultraviolet, and infrared light. Geometric optics treats light as a collection of rays that travel in straight lines and bend when they pass through or reflect from surfaces. Physical optics is a more comprehensive model of light, which includes wave effects such as diffraction and interference that cannot be accounted for in geometric optics.

Although visible light itself extends from approximately 4000 Å to 7000 Å (400 nm to 700 nm),[3] the same equipment used at these wavelengths is also used to observe some near-ultraviolet and near-infrared radiation.

## Quartzes

Optical astronomy includes those portions of ultraviolet, visual, and infrared astronomy that benefit from the use of quartz crystal or silica glass telescope components. Fused quartz is manufactured by fusing (melting) naturally occurring quartz crystals of high purity at approximately 2000 °C, using either an electrically heated furnace (electrically fused) or a gas/oxygen-fuelled furnace (flame fused). Fused quartz is normally transparent. The optical and thermal properties of fused quartz are superior to those of other types of glass due to its purity. For these reasons, it finds use in situations such as semiconductor fabrication and laboratory equipment. It has better ultraviolet transmission than most other glasses, and so is used to make lenses and other optics for the ultraviolet spectrum.

## Silicas

Fused silica is produced using high-purity silica sand as the feedstock, and is normally melted using an electric furnace, resulting in a material that is translucent or opaque. (This opacity is caused by very small air bubbles trapped within the material.)

Synthetic fused silica is made from a silicon-rich chemical precursor usually using a continuous flame hydrolysis process which involves chemical gasification of silicon, oxidation of this gas to silicon dioxide, and thermal fusion of the resulting dust (although there are alternative processes). This results in a transparent glass with an ultra-high purity and improved optical transmission in the deep ultraviolet.

"UV grade" synthetic fused silica (sold under various tradenames including "HPFS", "Spectrosil" and "Suprasil") has a very low metallic impurity content making it transparent deeper into the ultraviolet. An optic with a thickness of 1 cm will have a transmittance of about 50% at a wavelength of 170 nm, which drops to only a few percent at 160 nm. However, its infrared transmission is limited by strong water absorptions at 2.2 μm and 2.7 μm. "Infrared grade" fused quartz (tradenames "Infrasil", "Vitreosil IR" and others) which is electrically fused, has a greater presence of metallic impurities, limiting its UV transmittance wavelength to around 250 nm, but a much lower water content, leading to excellent infrared transmission up to 3.6 μm wavelength. All grades of transparent fused quartz/fused silica have nearly identical physical properties. The water content (and therefore infrared transmission of fused quartz and fused silica) is determined by the manufacturing process. Flame fused material always has a higher water content due to the combination of the hydrocarbons and oxygen fueling the furnace forming hydroxyl (OH) groups within the material. An IR grade material typically has an [OH] content of <10 parts per million.

The optical dispersion of fused silica can be approximated by the following Sellmeier equation:[4]

${\displaystyle \varepsilon =n^{2}=1+{\frac {0.69616630\lambda ^{2}}{\lambda ^{2}-0.0684043^{2}}}+{\frac {0.4079426\lambda ^{2}}{\lambda ^{2}-0.11624140^{2}}}+{\frac {0.8974794\lambda ^{2}}{\lambda ^{2}-9.896161^{2}}},}$

where the wavelength ${\displaystyle \lambda }$  is measured in micrometers.

This equation is valid between 0.21 and 3.71 micrometers and at 20 °C.[4] Its validity was confirmed for wavelengths up to 6.7 ${\displaystyle \mu }$ m.[5] Experimental data for the real (refractive index) and imaginary (absorption index) parts of the complex refractive index of fused quartz is available over the spectral range from 30 nm to 1000 ${\displaystyle \mu }$ m.[5]online.

## Lenses

Diagram of the focal ratio of a simple optical system where ${\displaystyle f}$  is the focal length and ${\displaystyle D}$  is the diameter of the objective lens. Credit: Vargklo.

This is a double-convex, thick lens diagram. Credit: Tamasflex.

A 35 mm lens is set to ${\displaystyle f/11}$ , as indicated by the white dot above the f-stop scale on the aperture ring. Credit: MarkSweep.

For the case of a [double-convex] lens of thickness d in air, and surfaces with radii of curvature R1 and R2, the effective focal length f is given by:

${\displaystyle {\frac {1}{f}}=(n-1)\left[{\frac {1}{R_{1}}}-{\frac {1}{R_{2}}}+{\frac {(n-1)d}{nR_{1}R_{2}}}\right],}$

where n is the refractive index of the lens medium. The quantity 1/f is also known as the optical power of the lens.

In most photography and all telescopy, where the subject is essentially infinitely far away, longer focal length (lower optical power) leads to higher magnification and a narrower angle of view; conversely, shorter focal length or higher optical power is associated with a wider angle of view.

The 35 mm lens in the image at right has an aperture range of ${\displaystyle f/2.0}$  to ${\displaystyle f/22.}$

The lens at right uses a standard f-stop scale, which is an approximately geometric sequence of numbers that corresponds to the sequence of the powers of the square root of 2:   ${\displaystyle f/1,}$  ${\displaystyle f/1.4}$  ${\displaystyle f/2,}$  ${\displaystyle f/2.8,}$  ${\displaystyle f/4,}$  ${\displaystyle f/5.6,}$  ${\displaystyle f/8,}$  ${\displaystyle f/11,}$  ${\displaystyle f/16,}$  [and] ${\displaystyle f/22.}$

The sequence above is obtained by approximating the following exact geometric sequence:

${\displaystyle f/1={\frac {f/1}{({\sqrt {2}})^{0}}},}$
${\displaystyle f/1.4={\frac {f/1}{({\sqrt {2}})^{1}}},}$
${\displaystyle f/2={\frac {f/1}{({\sqrt {2}})^{2}}},}$
${\displaystyle f/2.8={\frac {f/1}{({\sqrt {2}})^{3}}},}$

## Mirrors

Typical requirements for grinding and polishing a curved mirror ... require the surface to be within a fraction of a wavelength of light of a particular conic shape.

Telescopes use front silvered or first surface mirrors, where the reflecting surface is placed on the front (or first) surface of the glass (this eliminates reflection from glass surface ordinary back mirrors have). Some of them use silver, but most are aluminium, which is more reflective at short wavelengths than silver. All of these coatings are easily damaged and require special handling. They reflect 90% to 95% of the incident light when new. The coatings are typically applied by vacuum deposition. A protective overcoat is usually applied before the mirror is removed from the vacuum, because the coating otherwise begins to corrode as soon as it is exposed to oxygen and humidity in the air. Front silvered mirrors have to be resurfaced occasionally to keep their quality.

Cold mirrors are dielectric mirrors that reflect the entire visible light spectrum, while efficiently transmitting infrared wavelengths. These are the converse of hot mirrors.

Hot mirrors reflect infrared light while allowing visible light to pass. These can be used to separate useful light from unneeded infrared to reduce heating of components in an optical device. They can also be used as dichroic beamsplitters. (Hot mirrors are the converse of cold mirrors.)

## Actuators

Actuators are part of the active optics of the Gran Telescopio Canarias. Credit: Vesta.

Active optics is a technology used with reflecting telescopes developed in the 1980s[6], which actively shapes a telescope's mirrors to prevent deformation due to external influences such as wind, temperature, mechanical stress. Without active optics, the construction of 8 metre class telescopes is not possible, nor would telescopes with segmented mirrors be feasible.

This slow motion simulation is of typical adaptive optics operation at a telescope. Credit: Rnt20.

Def. an optical system in telescopes that reduces atmospheric distortion by dynamically measuring and correcting wavefront aberrations in real time, often by using a deformable mirror is called adaptive optics.

"Already it has allowed ground-based telescopes to produce images with sharpness rivalling those from the Hubble Space Telescope. The technique is expected to revolutionize the future of ground-based optical astronomy."[7]

The slow-motion simulation at right is typical for adaptive optics operation of a telescope. "The left hand side shows what a point source (e.g. small star) would look like through a ground-based telescope without adaptive optics correction. The right hand side shows what is seen after adaptive optics correction has been applied. Note that the adaptive optics corrected image is normally very compact, but occasionally it "breaks up". If long exposure images are taken, the adaptive optics correction produces a sharp point at the centre of the image, while the uncorrected image is just a large fuzzy blob. Note that the pattern changes much more quickly when adaptive optics correction is applied. This can make it difficult to use adaptive optics corrected telescopes in an astronomical interferometer or for speckle imaging."[8]

## Colors

Color Frequency Wavelength
violet 668–789 THz 380–450 nm
blue 631–668 THz 450–475 nm
cyan 606–630 THz 476–495 nm
green 526–606 THz 495–570 nm
yellow 508–526 THz 570–590 nm
orange 484–508 THz 590–620 nm
red 400–484 THz 620–750 nm

The visible spectrum is the portion of the electromagnetic spectrum that is visible to (can be detected by) the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm.[9] In terms of frequency, this corresponds to a band in the vicinity of 400–790 THz. A light-adapted eye generally has its maximum sensitivity at around 555 nm (540 THz), in the green region of the optical spectrum (see: luminosity function).

There are several cases of astronomers who claimed that following a cataract operation, they could see shorter wavelengths than other people, slightly into the ultraviolet.

## Entities

This image is a portrait of Johannes Kepler. Credit: Dr. Manuel.

Johannes Kepler's Rudolphine Tables (1627) are regarded as the most accurate and comprehensive star catalogue and planetary tables published up until that time. It contained the positions of over 1000 stars and directions for locating the planets within our solar system. Kepler finished the work in 1623 and dedicated it to his patron, the Emperor Rudolf II, but actually published it in 1627. The table's findings support Kepler's laws and the theory of a heliocentric astronomy.

## Sources

"Electrons moving along a Birkeland current may be accelerated by a plasma double layer. If the resulting electrons approach relativistic velocities (i.e. if they approach the speed of light) they may subsequently produce a Bennett pinch, which in a magnetic field causes the electrons to spiral and emit synchrotron radiation that may include radio, optical (i.e. visible light), x-rays, and gamma rays.

## Objects

This is a photograph taken in 1910 during the passage of Halley's comet. Credit: The Yerkes Observatory.

A photo of the planet Mars is taken in Straßwalchen (Austria) on September 19, 2003, shortly after its closest approach. Credit: Rochus Hess, http://members.aon.at/astrofotografie.

The Earth and Moon is imaged by the Mars Global Surveyor on May 8, 2003, at 12:59:58 UTC.

The telescope photograph of the Great Andromeda Nebula is taken around 1899. Credit: Isaac Roberts.

The 1910 approach, which came into naked-eye view around 10 April[10] and came to perihelion on 20 April,[10] was notable for several reasons: it was the first approach of which photographs exist, and the first for which spectroscopic data were obtained.[11] Furthermore, the comet made a relatively close approach of 0.15AU,[10] making it a spectacular sight. Indeed, on 19 May, the Earth actually passed through the tail of the comet.[12][13] One of the substances discovered in the tail by spectroscopic analysis was the toxic gas cyanogen,[14] which led astronomer Camille Flammarion to claim that, when Earth passed through the tail, the gas "would impregnate the atmosphere and possibly snuff out all life on the planet."[15] His pronouncement led to panicked buying of gas masks and quack "anti-comet pills" and "anti-comet umbrellas" by the public.[16] In reality, as other astronomers were quick to point out, the gas is so diffuse that the world suffered no ill effects from the passage through the tail.[15]

"It is quite possible that [faint streamers preceding the main tail and lying nearly in the prolonged radius vector] may have touched the Earth, probably between May 19.0 and May 19.5, [1910,] but the Earth must have passed considerably to the south of the main portion of the tail [of Halley's comet]."[17]

Of the other planets of the solar system, Mercury, Mars, Jupiter, Saturn, Uranus, and Neptune, none has apparently produced as much drama and excitement recently on Earth among some of the intelligent life forms as Halley's comet.

Mars made its closest approach to Earth and maximum apparent brightness in nearly 60,000 years, 55,758,006 km (0.372719 AU), magnitude −2.88, on 27 August 2003 at 9:51:13 UT.

Still much further away from the Earth than the Sun or Neptune are the many stars and nebulae that make up the Milky Way. Beyond the confines of our galaxy is the Andromeda Galaxy imaged on the left.

Of the Local Group, “[i]ts two dominant galaxies, the Milky Way and Andromeda (M31), are separated by a distance of ~700 kpc and are moving toward each other with a radial velocity of about -117 km s-1 (Binney & Tremaine 1987, p. 605).”[18] "making [Andromeda] one of the few blueshifted galaxies. The Andromeda Galaxy and the Milky Way are thus expected to collide in about 4.5 billion years, although the details are uncertain since Andromeda's tangential velocity with respect to the Milky Way is only known to within about a factor of two.[19] A likely outcome of the collision is that the galaxies will merge to form a giant elliptical galaxy.[20] Such events are frequent among the galaxies in galaxy groups. The fate of the Earth and the Solar System in the event of a collision are currently unknown. If the galaxies do not merge, there is a small chance that the Solar System could be ejected from the Milky Way or join Andromeda.[21]

## Strong forces

"The molecular cloud Cepheus B is subject to strong forces both trying to compress and to disrupt it simultaneously."[22]

## Weak forces

"The observation of a neutrino burst within 3 h of the associated optical burst from supernova 1987A in the Large Magellanic Cloud provides a new test of the weak equivalence principle, by demonstrating that neutrinos and photons follow the same trajectories in the gravitational field of the galaxy."[23]

## Continua

An active galactic nuclear optical continuum emission is visible whenever there is a direct view of the accretion disc. Jets can also contribute to this component of the AGN emission. The optical emission has a roughly power-law dependence on wavelength.

## Absorptions

The figure shows the absorption length as a function of depth. Credit: P. Buford Price and the Amanda Collaboration.

The figure shows the average scattering coefficent (1/scattering length) as a function of depth. Credit: P. Buford Price and the Amanda Collaboration.

"The right figure shows the absorption length as a function of depth. The bulk of the scientifically-useful optical sensors in AMANDA are embedded between 1500 and 1900 m beneath the surface."[24]

"The optical properties of in situ ice beneath the south pole are measured by a combination of in situ N2 lasers, DC lamps, and YAG laser pulses from the surface. The properties vary with depth due to climatological variation such as ice ages. [...] The two properties that most strongly affect the reconstruction capabilities of AMANDA-II are absorption and scattering."[24]

"The absorption strongly depends on wavelength. Notice [in the figure at right] that the absorption also depends on depth at wavelengths where the absorption coefficient is relatively small. For short wavelengths, the absorption coefficient is small and dust contributes significantly, which is responsible for the depth dependence. At 532nm, the absorption coefficient is large, and the value is largely determined by intrinsic properties of ice (i.e, the roll of dust is less obvious)."[24]

"The [second figure at right] shows the average scattering coefficent (1/scattering_length) as a function of depth. Note that the effective scattering length, L_eff, is (approximately) the average length to isotropize the direction of all but 1/e of the photons. This important parameter for diffusion calculations is related to the geometric scattering length by L_eff=L_geo/(1-<cos(angle)>). The solid curve shows the coefficient of the scattering length for 400 nm light. Other colors behave differently due to the slight dependence of the scattering length on wavelength."[24]

"At depths below 1400m, dust is responsible for light scattering in ice. The rapid rise in scattering at shallow depths (relative to the surface) is due to onset of air bubbles trapped in the ice. The dashed blue line shows the intrinsic scattering from dust in the region dominated by air bubbles."[24]

The "maximum absorption length is slightly more than 100m at AMANDA-II depths, but the scattering length is only 20m for wavelengths that correspond to the longest absorption lengths."[24]

"For quenched galaxies, the Hα absorption trough is deep and can be traced through the nucleus and along the major axis. It extends to a radius at or beyond 2 Rd [where Rd is the galaxy disk scale length] in all but three cases. This makes it possible to determine a velocity width from the optical spectrum as is done for emission line flux, with appropriate corrections between stellar and gas velocities (see discussion in Paper I, also Neistein, Maoz, Rix, & Tonry, 1999). In the few cases where a velocity width can also be measured from the H I data, it is found to be in good agreement with that taken from the Hα absorption line flux."[25]

## Bands

Spectral bands are part of optical spectra of polyatomic systems, including condensed materials, large molecules, etc. Each line corresponds to one level in the atom splits in the molecules. When the number of atoms is large, one gets a continuum of energy levels, the so called "spectral bands". They are often labeled in the same way as the monatomic lines.

## Meteors

Micrometeoroids are extremely common in space. These tiny particles are a major contributor to space weathering processes. When they hit the surface of the Moon, or any airless body (Mercury, the asteroids, etc.), the resulting melting and vaporization causes darkening and other optical changes in the regolith.

## Electrons

The diagram shows a delta electron knocked out by a 180 GeV muon at the SPS at CERN Credit: Wilcokoppert.

"Each [optical module] OM contains a 10 inch [photo-multiplier tube] PMT that detects individual photons of Cerenkov light generated in the optically clear ice by muons and electrons moving with velocities near the speed of light."[26]

## Gamma rays

Gamma-ray bursts (GRBs) are flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies. They are the most luminous electromagnetic events known to occur in the universe. Bursts can last from ten milliseconds to several minutes, although a typical burst lasts 20–40 seconds. The initial burst is usually followed by a longer-lived "afterglow" emitted at longer wavelengths (X-ray, ultraviolet, optical, infrared, microwave and radio).[27]

## Indices

Sample calibration colors[28]
Class B–V U–B V–R R–I Teff (K)
O5V –0.33 –1.19 –0.15 –0.32 42,000
B0V –0.30 –1.08 –0.13 –0.29 30,000
A0V –0.02 –0.02 0.02 –0.02 9,790
F0V 0.30 0.03 0.30 0.17 7,300
G0V 0.58 0.06 0.50 0.31 5,940
K0V 0.81 0.45 0.64 0.42 5,150
M0V 1.40 1.22 1.28 0.91 3,840

The color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature. To measure the index, one observes the magnitude of an object successively through two different filters, such as U and B, or B and V, where U is sensitive to ultraviolet rays, B is sensitive to blue light, and V is sensitive to visible (green-yellow) light (see also: UBV system). The set of passbands or filters is called a photometric system. The difference in magnitudes found with these filters is called the U-B or B–V color index, respectively. The smaller the color index, the more blue (or hotter) the object is. Conversely, the larger the color index, the more red (or cooler) the object is. This is a consequence of the logarithmic magnitude scale, in which brighter objects have smaller (more negative) magnitudes than dimmer ones. For comparison, the yellowish Sun has a B–V index of 0.656 ± 0.005,[29] while the bluish Rigel has B–V –0.03 (its B magnitude is 0.09 and its V magnitude is 0.12, B–V = –0.03).[30] The passbands most optical astronomers use are the UBVRI filters, where the U, B, and V filters are as mentioned above, the R filter passes red light, and the I filter passes infrared light. These filters were specified as particular combinations of glass filters and photomultiplier tubes.

## Filters

A Photometric system is a set of well-defined passbands (or filters), with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used. For each photometric system a set of primary standard stars is provided.

Filter Letter Effective Wavelength Midpoint λeff For Standard Filter[31] Full Width Half Maximum[31] Variant(s) Description
Ultraviolet
U 365nm 66nm u, u', u* "U" stands for ultraviolet.
Visible
B 445nm 94nm b "B" stands for blue.
V 551nm 88nm v, v' "V" stands for visual.
G g, g' "G" stands for green (visual).
R 658nm 138nm r, r', R', Rc, Re, Rj "R" stands for red.
Near-Infrared
I 806nm 149nm i, i', Ic, Ie, Ij "I" stands for infrared.

## Polars

Light given off by a star is un-polarized, i.e. the direction of oscillation of the light wave is random. However, when the light is reflected off the atmosphere of a planet, the light waves interact with the molecules in the atmosphere and they are polarized.[32]

## Ultraviolets

This is an image of the star-forming region R136 in NGC 2070 with the Hubble Space Telescope in ultraviolet and visible light. Credit: NASA, ESA, F. Paresce (INAF-IASF, Bologna, Italy), R. O'Connell (University of Virginia, Charlottesville), and the Wide Field Camera 3 Science Oversight Committee.

Ultraviolet astronomy is radiation astronomy applied to the ultraviolet phenomena of the sky, especially at night. It is also conducted above the Earth's atmosphere and at locations away from the Earth as a part of explorational (or exploratory) ultraviolet astronomy.

In ultraviolet-optical astronomy, images may yield important information. The image at right "is the most detailed view of the largest stellar nursery in our local galactic neighborhood. The massive, young stellar grouping, called R136, is only a few million years old and resides in the 30 Doradus Nebula, a turbulent star-birth region in the Large Magellanic Cloud (LMC), a satellite galaxy of our Milky Way. There is no known star-forming region in our galaxy as large or as prolific as 30 Doradus. Many of the diamond-like icy blue stars are among the most massive stars known. Several of them are over 100 times more massive than our Sun. These hefty stars are destined to pop off, like a string of firecrackers, as supernovas in a few million years. This image, taken in ultraviolet, visible, and red light by Hubble's Wide Field Camera 3, spans about 100 light-years. The nebula is close enough to Earth that Hubble can resolve individual stars, giving astronomers important information about the birth and evolution of stars in the universe. The Hubble observations were taken Oct. 20-27, 2009. The blue color is light from the hottest, most massive stars; the green from the glow of oxygen; and the red from fluorescing hydrogen."[33]

## Visuals

This image shows the 26-inch Warner & Swasey refracting telescope at the United States Naval Observatory. Credit: Waldon Fawcett.

What is “the “old-fashioned” spirit of real-time visual astronomy”?[34] “I think everyone can conjure up a mental image of astronomers at every level and place in history, gazing through the eyepieces of their telescopes at sights far away - true visual astronomy.”[34]

## Violets

"In 1997 we observed comet Hale-Bopp with the 2.6 m Nordic Optical Telescope on La Palma, Canary Islands, with a view to estimating the 12C/13C abundance ratio. About twenty high-resolution (λ /Δ λ ~ 70000) spectra of the strong CN Violet (0,0) band were secured with the SOFIN spectrograph from 7 to 13 April. The heliocentric and geocentric distances of the comet were then close to 0.9 AU and 1.4 AU, respectively. While the data do show the expected lines of the 13C14N isotopic molecule, we have been surprised to find in addition a number of very weak features, which are real and turn out to be positioned very near to the theoretical wavelengths of lines pertaining to the R branch of 12C15N."[35]

## Infrareds

This Hubble image of the Egg Nebula shows one of the best views to date of the brief but dramatic preplanetary, or protoplanetary nebula phase in a star’s life. Credit: ESA/Hubble & NASA.

Infrared astronomy deals with the detection and analysis of infrared radiation (wavelengths longer than red light). Except at wavelengths close to visible light, infrared radiation is heavily absorbed by the atmosphere, and the atmosphere produces significant infrared emission. Consequently, infrared observatories have to be located in high, dry places or in space. The infrared spectrum is useful for studying objects that are too cold to radiate visible light, such as planets and circumstellar disks. Longer infrared wavelengths can also penetrate clouds of dust that block visible light, allowing observation of young stars in molecular clouds and the cores of galaxies.[36] Some molecules radiate strongly in the infrared. This can be used to study chemistry in space; more specifically it can detect water in comets.[37]

"The NASA/ESA Hubble Space Telescope has been at the cutting edge of research into what happens to stars like our Sun at the ends of their lives. One stage that stars pass through as they run out of nuclear fuel is the preplanetary, or protoplanetary nebula. This Hubble image at right of the Egg Nebula shows one of the best views to date of this brief but dramatic phase in a star’s life.

The preplanetary nebula phase is a short period in the cycle of stellar evolution — over a few thousand years, the hot remains of the star in the centre of the nebula heat it up, excite the gas, and make it glow as a planetary nebula. The short lifespan of preplanetary nebulae means there are relatively few of them in existence at any one time. Moreover, they are very dim, requiring powerful telescopes to be seen. This combination of rarity and faintness means they were only discovered comparatively recently. The Egg Nebula, the first to be discovered, was first spotted less than 40 years ago, and many aspects of this class of object remain shrouded in mystery.

At the centre of this image, and hidden in a thick cloud of dust, is the nebula’s central star. While we can’t see the star directly, four searchlight beams of light coming from it shine out through the nebula. It is thought that ring-shaped holes in the thick cocoon of dust, carved by jets coming from the star, let the beams of light emerge through the otherwise opaque cloud. The precise mechanism by which stellar jets produce these holes is not known for certain, but one possible explanation is that a binary star system, rather than a single star, exists at the centre of the nebula.

The onion-like layered structure of the more diffuse cloud surrounding the central cocoon is caused by periodic bursts of material being ejected from the dying star. The bursts typically occur every few hundred years.

The distance to the Egg Nebula is only known very approximately, the best guess placing it at around 3000 light-years from Earth. This in turn means that astronomers do not have any accurate figures for the size of the nebula (it may be larger and further away, or smaller but nearer). This image is produced from exposures in visible and infrared light from Hubble’s Wide Field Camera 3.

Infrared and optical astronomy are often practiced using the same telescopes, as the same mirrors or lenses are usually effective over a wavelength range that includes both visible and infrared light.

## Submillimeters

Stars "believed to have circumstellar disks similar to the primitive solar nebula [are] based on the criteria [...]:

1. high far-infrared optical depths around visible stars,
2. shallow spectral energy densities longward of 5 µm, and
3. large millimeter-wave flux densities indicative of ≳ 0.01 M of H2."[38]

## Superluminals

"The arrival times of the Cerenkov photons in 6 optical sensors determine the direction of the muon track."[26]

"The optical requirements on the detector medium are severe. A large absorption length is needed because it determines the required spacing of the optical sensors and, to a significant extent, the cost of the detector. A long scattering length is needed to preserve the geometry of the Cerenkov pattern. Nature has been kind and offered ice and water as natural Cerenkov media. Their optical properties are, in fact, complementary. Water and ice have similar attenuation length, with the roles of scattering and absorption reversed. Optics seems, at present, to drive the evolution of ice and water detectors in predictable directions: towards very large telescope area in ice exploiting the long absorption length, and towards lower threshold and good muon track reconstruction in water exploiting the long scattering length."[26]

"The Baikal experiment represents a proof of concept for future deep ocean projects that have the advantage of larger depth and optically superior water."[26]

"With the attenuation length peaking at 55m near 470 nm, the site is optically similar to that of the best deep water sites investigated for neutrino astronomy."[26]

"Astronomy, whether in the optical or in any other wave-band, thrives on a diversity of complementary instruments, not on “a single best instrument”."[26]

## Rocky objects

The SkyMapper telescope at Siding Spring Observatory is in the foreground, with the 2.3 m telescope in the background. Credit: Iridia.

"The formation of Earth-like planets through collisional accumulation of rocky objects within a disk has mainly been explored in theoretical and computational work in which post-collision ejecta evolution is typically ignored1,2,3, although recent work has considered the fate of such material4. [...] An important ingredient in understanding the vanishing mid-infrared excess emission toward TYC 8241 2652 1 is the initial state of its disk system. Given an age of ~10 Myr, the star could have been host to either an accreting protoplanetary disk rich in gas and dust or a second-generation debris disk formed from the collisions of rocky objects orbiting the star13. [...] the dusty material orbiting TYC 8241 2652 1 is the result of the collisions of rocky objects."[39]

"To determine the age of TYC 8241 2652 1 we obtained high-resolution optical spectra over four epochs from February 2008 to January 2009 with an echelle spectrograph mounted on the Siding Spring Observatory 2.3-m telescope. [...] The spectral type and effective temperature is determined from line ratios28 in the Siding Spring Observatory echelle spectra."[39]

## Shelters

The Canada-France-Hawaii Telescope is located at the Mauna Kea Observatory in Hawai'i. Credit: Fabian_RRRR.

"The Canada-France-Hawaii Telescope (CFHT) is a 3.6 m optical-infrared telescope located on the summit of Mauna Kea on the island of Hawaii."[40] The CFHT is at an altitude of 4,204 meters. Mauna Kea last erupted 4,000 to 6,000 years ago [~7,000 b2k]. The Mauna Kea Observatories are used for scientific research across the electromagnetic spectrum from visible light to radio, and comprise the largest such facility in the world.

## Spectroscopy

Astronomical spectroscopy is the technique of spectroscopy used in astronomy. The object of study is the spectrum of electromagnetic radiation, including visible light, which radiates from stars and other celestial objects. Spectroscopy can be used to derive many properties of distant stars and galaxies, such as their chemical composition, but also their motion by Doppler shift measurements.

## Mercury

This is a full-color image of Mercury from the first MESSENGER flyby. Credit: NASA/ Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

"Optical reflectance studies of Mercury provide evidence for Mg silicates."[41]

## Earth

"For outside wave fields of the Moon and the Earth accept seismic and acoustic waves which characteristic frequencies first of all coincide with frequencies of orbital and own rotation of cosmogony objects (planets and their satellites, multiple star systems, pulsars)."[42]

"Besides wave (acoustic) processes in the top Earth atmosphere and also those from that its are accompanied also by optical effects (polar lights) and strongly connected with gas dust streams acting [4]."[42]

## Asteroids

When narrowband spectrophotometric observations are made of outer-belt asteroids separated by "their average distance from the Sun (3.21, 3.64, 3.78 and 4.29 au respectively) ... the slope in their 0.5-1 μm spectra (scaled to 1 at 0.7 μm) increases with solar distance. ... this compositional gradation [may extend] through the orbits of Saturn, Uranus and Neptune. ... Because the ion solar flux varies as the inverse square of the distance from the Sun, one expects, as in fact observed, the surfaces of D and P type asteroids become blacker as the distance from the Sun decreases."[43]

## Iapetus

The optical reflectance spectra from "the leading hemisphere of Iapetus" over a range of ~0.35 to ~0.82 μm is rather closely matched by optical reflectance spectra "from frozen CH4 (T ~ 10 K) bombarded by ~ 1016 protons (1.5 MeV)cm-2 [organic 1]".[43]

"The spectrum of the Iapetus (dark) leading hemisphere is redder and resembles that of D-type asteroids (Cruikshank et al., 1983). The spectrum of organic 1 fits that at the lower λ but is more reflecting at λ > 0.7 μm. As a matter of fact the spectrum of the Iapetus leading hemisphere is the sum of two contributions: that from the dark units and from the ice-rich bright polar caps."[43]

## Phoebe

The optical reflectance spectra from Phoebe over a range of ~0.35 to ~0.82 μm is very closely matched by optical reflectance spectra "from frozen CH4 (T ~ 10 K) bombarded by ... ~ 1017 protons [1.5 MeV] cm-2 [organic 2]".[43]

"Phoebe's spectrum ... is very similar to that of C-type asteroids ... and is well fitted by the laboratory carbonized material (organic 2)."[43]

## Neptune

Observations by NASA's Hubble Space Telescope reveal an increase in Neptune's brightness in the southern hemisphere. Credit: NASA, L. Sromovsky, and P. Fry (University of Wisconsin-Madison).

At right is a set of images from different years for Neptune. These images "show that Neptune's brightness has increased significantly since 1996. The rise is due to an increase in the amount of clouds observed in the planet's southern hemisphere. These increases may be due to seasonal changes caused by a variation in solar heating. Because Neptune's rotation axis is inclined 29 degrees to its orbital plane, it is subject to seasonal solar heating during its 164.8-year orbit of the Sun. This seasonal variation is 900 times smaller than experienced by Earth because Neptune is much farther from the Sun. The rate of seasonal change also is much slower because Neptune takes 165 years to orbit the Sun. So, springtime in the southern hemisphere will last for several decades! Remarkably, this is evidence that Neptune is responding to the weak radiation from the Sun. These images were taken in visible and near-infrared light by Hubble's Wide Field and Planetary Camera 2."[44]

## Mira B

This is a NASA Hubble Space Telescope image of the cool red giant star Mira A (right), officially called Omicron Ceti in the constellation Cetus, and its nearby hot companion (left) taken on December 11, 1995 in visible light using the European Space Agency's Faint Object Camera (FOC). Credit: NASA.

Some data suggest that Mira B is a normal main sequence star of spectral type K and roughly 0.7 solar masses, rather than a white dwarf as first envisioned.[45]

Analysis in 2010 of rapid optical brightness variations has indicated that Mira B is in fact a white dwarf.[46]

## Camelopardalis

This is an optical image of U Camelopardalis from the Hubble Space Telescope. Credit: ESA/Hubble, NASA and H. Olofsson (Onsala Space Observatory).

The official IAU sky chart for the constellation Camelopardalis is at right with notable sources marked and located.

"A bright star [in the image at left] is surrounded by a tenuous shell of gas in this unusual image from the NASA/ESA Hubble Space Telescope. U Camelopardalis, or U Cam for short, is a star nearing the end of its life. As it begins to run low on fuel, it is becoming unstable. Every few thousand years, it coughs out a nearly spherical shell of gas as a layer of helium around its core begins to fuse. The gas ejected in the star’s latest eruption is clearly visible in this picture as a faint bubble of gas surrounding the star."[47]

"U Cam is an example of a carbon star. This is a rare type of star whose atmosphere contains more carbon than oxygen. Due to its low surface gravity, typically as much as half of the total mass of a carbon star may be lost by way of powerful stellar winds."[47]

"Located in the constellation of Camelopardalis (The Giraffe), near the North Celestial Pole, U Cam itself is actually much smaller than it appears in Hubble’s picture. In fact, the star would easily fit within a single pixel at the centre of the image. Its brightness, however, is enough to overwhelm the capability of Hubble’s Advanced Camera for Surveys making the star look much bigger than it really is. The shell of gas, which is both much larger and much fainter than its parent star, is visible in intricate detail in Hubble’s portrait. While phenomena that occur at the ends of stars’ lives are often quite irregular and unstable (see for example Hubble’s images of Eta Carinae, potw1208a), the shell of gas expelled from U Cam is almost perfectly spherical."[47]

"The image was produced with the High Resolution Channel of the Advanced Camera for Surveys [using the 606 nm and 814 nm filters]."[47]

## Carina

This two micron image is of NGC 3199 in Carina. Credit: 2MASS.

At left is the IAU sky chart for the constellation Carina with notable sources located.

"NGC 3199, in the constellation Carina, [...] is the wind-blown partial "ring" around the Wolf-Rayet (W-R) star WR 18 (aka HD 89358), the easternmost (leftmost) of the three bright blue stars near the center of the 2MASS image. NGC 3199 and WR 18 are at a distance of about 3.6 kpc (11,736 light years) from us. W-R stars represent the final evolutionary stages of very massive stars (with ~30 solar masses or greater). The nebula shows an asymmetric appearance, i.e., only one side (the western one) of the shell is bright, both in the optical and the near-infrared. The fainter, eastern side is there, but is much fainter. Some W-R ring nebulae can be seen in 2MASS images, such as the more complete ring around M1-67. But, NGC 3199 is particularly bright in the 2MASS data. Dyson & Ghanbari (1989, A&A, 226, 270) provided an explanation for the ring's appearance through a model where a moving WR 18 is blowing a strong stellar wind into a surrounding uniform interstellar medium. Vigorous mass loss of 10-5 to 10-4 solar masses per year is characteristic of W-R stars, as the star approaches the end of its short life, although not all are surrounded by ring nebulae."[48]

## Centaurus

This is the planetary nebula SuWt 2. Credit: NASA, NOAO, H. Bond and K. Exter (STScI/AURA).

"This image [at left] of the planetary nebula SuWt 2 reveals a bright ring-like structure encircling a bright central star. The central star is actually a close binary system where two stars completely circle each other every five days. The interaction of these stars and the more massive star that sheds material to create the nebula formed the ring structure. The burned out core of the massive companion has yet to be found inside the nebula. The nebula is located 6,500 light-years from Earth in the direction of the constellation Centaurus. This color image was taken on Jan. 31, 1995 with the National Optical Astronomy Observatory's 1.5-meter telescope at the Cerro Tololo Inter-American Observatory in Chile."[49]

## Gemini

This multiwavelength composite shows the supernova remnant IC 443, also known as the Jellyfish Nebula. Credit: NASA/DOE/Fermi LAT Collaboration, NOAO/AURA/NSF, JPL-Caltech/UCLA.

At left is a multiwavelength composite that shows the supernova remnant IC 443, also known as the Jellyfish Nebula. Fermi GeV gamma-ray emission is shown in magenta, optical wavelengths as yellow, and infrared data from NASA's Wide-field Infrared Survey Explorer (WISE) mission is shown as blue (3.4 microns), cyan (4.6 microns), green (12 microns) and red (22 microns). Cyan loops indicate where the remnant is interacting with a dense cloud of interstellar gas.

## Standard candles

Def. an astronomical object that has a known luminosity is called a standard candle.

Def. any astronomical object of known absolute magnitude is called a standard candle.

Standard candles are objects that belong to some class that [has] a known brightness. By comparing the known luminosity of the latter to its observed brightness, the distance to the object can be computed using the inverse square law. These objects of known brightness are termed standard candles. ... The apparent magnitude, or the magnitude as seen by the observer, can be used to determine the distance D to the object in kiloparsecs (where 1 kpc equals 1000 parsecs) as follows:

${\displaystyle {\begin{smallmatrix}5\cdot \log _{10}{\frac {D}{\mathrm {kpc} }}\ =\ m\ -\ M\ -\ 10,\end{smallmatrix}}}$

where m the apparent magnitude and M the absolute magnitude. ... [B]oth magnitudes must be in the same frequency band and there can be no relative motion in the radial direction. The difference between absolute and apparent magnitudes is called the distance modulus, and astronomical distances, especially intergalactic ones, are sometimes tabulated in this way.

## Standard stars

"An internally consistent and homogeneous list of standard stars on the Johnson-Kron-Cousins broadband UBVRI photometric system"[50] are published in a set of articles:

1. 223 stars in the magnitude range from 7 to 17,[50]
2. 526 stars in the magnitude range 11.5 < V < 16.0,[51] and
3. 202 stars completing the magnitude range 8.90 < V < 16.30.[52]

* Light green boxes: Technique applicable to star-forming galaxies. * Light blue boxes: Technique applicable to Population II galaxies. * Light Purple boxes: Geometric distance technique. * Light Red box: The planetary nebula luminosity function technique is applicable to all populations of the Virgo Supercluster. * Solid black lines: Well calibrated ladder step. * Dashed black lines: Uncertain calibration ladder step. Credit: .

The cosmic distance ladder (also known as the Extragalactic Distance Scale) is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an astronomical object is possible only for those objects that are "close enough" (within about a thousand parsecs) to Earth. The techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at close distances with methods that work at larger distances. Several methods rely on a standard candle, which is an astronomical object that has a known luminosity.

## Astrography

All four of the HESS telescope array in Namibia are in operation at night. Credit: H.E.S.S. collaboration.

Light pollution obscures the stars in the night sky for city dwellers [and] interferes with astronomical observatories. Light is particularly problematic for amateur astronomers, whose ability to observe the night sky from their property is likely to be inhibited by any stray light from nearby. Most major optical astronomical observatories are surrounded by zones of strictly enforced restrictions on light emissions.

In addition to skyglow, light trespass can impact observations when artificial light directly enters the tube of the telescope and is reflected from non-optical surfaces until it eventually reaches the eyepiece. This direct form of light pollution causes a glow across the field of view which reduces contrast. Light trespass also makes it hard for a visual observer to become sufficiently dark adapted. The usual measures to reduce this glare, if reducing the light directly is not an option, include flocking the telescope tube and accessories to reduce reflection, and putting a light shield (also usable as a dew shield) on the telescope to reduce light entering from angles other than those near the target. Under these conditions, some astronomers prefer to observe under a black cloth to ensure maximum dark adaptation.

H.E.S.S. is located on the Cranz family farm, Göllschau, in Namibia, near the Gamsberg, an area well known for its excellent optical quality. The first of the four telescopes of Phase I of the H.E.S.S. project went into operation in Summer 2002; all four were operational in December 2003.

## Astrohistory

The astronomer Clyde Tombaugh is the discoverer of Pluto. Credit: NASA.

Historical astronomy is the science of analysing historic astronomical data. The American Astronomical Society (AAS), established in 1899, states that its Historical Astronomy Division "...shall exist for the purpose of advancing interest in topics relating to the historical nature of astronomy. By historical astronomy we include the history of astronomy; what has come to be known as archaeoastronomy; and the application of historical records to modern astrophysical problems."[53] Historical and ancient observations are used to track theoretically long term trends, such as eclipse patterns and the velocity of nebular clouds.[54] Conversely, utilizing known and well documented phenomenological activity, historical astronomers apply computer models to verify the validity of ancient observations, as well as dating such observations and documents which would otherwise be unknown.

## Astromathematics

The diagram illustrates Kepler's three laws using two planetary orbits. Credit: Hankwang.
1. The orbit of every planet is an ellipse with the Sun at one of the two foci.
2. A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.[55]
3. The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.

The diagram at the right illustrates Kepler's three laws of planetary orbits: (1) The orbits are ellipses, with focal points ƒ1 and ƒ2 for the first planet and ƒ1 and ƒ3 for the second planet. The Sun is placed in focal point ƒ1. (2) The two shaded sectors A1 and A2 have the same surface area and the time for planet 1 to cover segment A1 is equal to the time to cover segment A2. (3) The total orbit times for planet 1 and planet 2 have a ratio a13/2 : a23/2.

## Astrophysics

This beautiful galaxy is tilted at an oblique angle on to our line of sight, giving a "birds-eye view" of the spiral structure. Credit: Hubble data: NASA, ESA, and A. Zezas (Harvard-Smithsonian Center for Astrophysics); GALEX data: NASA, JPL-Caltech, GALEX Team, J. Huchra et al. (Harvard-Smithsonian Center for Astrophysics); Spitzer data: NASA/JPL/Caltech/S. Willner (Harvard-Smithsonian Center for Astrophysics.

A soda straw in a glass of liquid shows the refraction of light in the liquid. Credit: Bcrowell.

Def. a straight line along which an observer has a clear view is called line of sight.

In the section on 'senses' above is a demonstration of the principle of 'line of sight'; i.e., "a line from an observer's eye to a distant point toward which [the observer] is looking"[56]. In the image on the left of rain beneath a dark cloud, there is a highway with a vehicle on it. The vehicle is further away from the observer than the right turn onto a side road. Is the blue sky behind the dark cloud? Is the line of trees in the background further away than the dark cloud? Many objects in this image and the others can be layered relative to the observer (some are closer by inspection than others). These layers or strata are strata along the line of sight. The principle of line of sight can be used to make deductions about the relative locations (or positions) of objects from the observer's perspective.

By observing many of the wandering lights in the night sky, an occasional occultation of the light of one astronomical object may occur by the intervention of another along a closer astronomical stratum. On April 25, 1838, an occultation of Mercury by the Moon occurred when Mercury was visible to the unaided eye after sunset.[57] An occultation of Venus by the Moon occurred "on the afternoon of October 14", 1874.[57] An earlier such occultation "occurred on May 23, 1587, and is thus recorded by [Tycho Brahe] in his Historia Celestis"[57]. "Thomas Street, in his Astronomia Carolina (A.D. 1661), mentions three occultations by Venus, being two occasions when the planet covered Regulus, and once when there was an occultation of Mars by Venus."[57] "[Thomas Street] describes [the occultation of Mars by Venus] as follows: "1590,. Oct. 2nd, 16h. 24s. Michael Mœstlin observed ♂ eclipsed by ♀.""[57]

Def. the turning or bending of any wave, such as a light or sound wave, when it passes from one medium into another of different optical density is called refraction.

## Sciences

This is an image of the title page of Johannes Kepler's Rudolphine Tables (1627). Credit: Johannes Kepler.

At right is a copy of the title page of Johannes Kepler's Rudolphine Tables (1627). It is regarded as the most accurate and comprehensive star catalogue and planetary tables published up until that time. It contained the positions of over 1000 stars and directions for locating the planets within our solar system.

## Technology

This is a schematic of a Keplerian refracting telescope which uses two different sizes of planoconvex lenses. Credit: .

Technology is the making, usage, and knowledge of tools, machines, techniques, crafts, systems or methods of organization in order to solve a problem or perform a specific function. It can also refer to the collection of such tools, machinery, and procedures.

## Telescopes

An Airy diffraction pattern as shown is generated by a plane wave falling on a circular aperture, such as the pupil of the eye. Credit: .

Def. a monocular optical instrument possessing magnification for observing distant objects is called a telescope.

An optical telescope gathers and focuses light mainly from the visible part of the electromagnetic spectrum (although some work in the infrared and ultraviolet).[58]

Most radiation telescopes, especially optical telescopes, combine a variety of lenses, mirrors, active and adaptive optics, filters, detectors, mounts, image processing, and observatories, in many locations.

A telescope's imaging system's resolution can be limited either by aberration or by diffraction causing blurring of the image. These two phenomena have different origins and are unrelated. Aberrations can be explained by geometrical optics and can in principle be solved by increasing the optical quality — and cost — of the system. On the other hand, diffraction comes from the wave nature of light and is determined by the finite aperture of the optical elements. The lens' circular aperture is analogous to a two-dimensional version of the single-slit experiment. Light passing through the lens interferes with itself creating a ring-shape diffraction pattern, known as the Airy pattern, if the wavefront of the transmitted light is taken to be spherical or plane over the exit aperture.

The interplay between diffraction and aberration can be characterised by the point spread function (PSF). The narrower the aperture of a lens the more likely the PSF is dominated by diffraction.

Two point sources are regarded as just resolved when the principal diffraction maximum of one image coincides with the first minimum of the other.[59]

## Angular resolution

This is a log-log plot of aperture diameter vs angular resolution at the diffraction limit for various light wavelengths compared with various astronomical instruments. Credit: Cmglee.

Def. a quantitative measure of the ability of an optical instrument to produce separable images" is called resolving power.

The angular resolution of an optical telescope is determined by the diameter of the primary mirror or lens gathering the light (also termed its "aperture").

The Rayleigh criterion for the resolution limit ${\displaystyle \alpha _{R}}$  (in radians) is given by

${\displaystyle \sin(\alpha _{R})=1.22{\frac {\lambda }{D}}}$

where ${\displaystyle \lambda }$  is the wavelength and ${\displaystyle D}$  is the aperture. For visible light (${\displaystyle \lambda }$  = 550 nm) in the small-angle approximation, this equation can be rewritten:

${\displaystyle \alpha _{R}={\frac {138}{D}}}$

Here, ${\displaystyle \alpha _{R}}$  denotes the resolution limit in arcseconds and ${\displaystyle D}$  is in millimeters. In the ideal case, the two components of a double star system can be discerned even if separated by slightly less than ${\displaystyle \alpha _{R}}$ . This is taken into account by the Dawes limit

${\displaystyle \alpha _{D}={\frac {116}{D}}}$

The equation shows that, all else being equal, the larger the aperture, the better the angular resolution. The resolution is not given by the maximum magnification (or "power") of a telescope. Telescopes marketed by giving high values of the maximum power often deliver poor images.

The log-log plot at right is of aperture diameter vs angular resolution at the diffraction limit for various light wavelengths compared with various astronomical instruments. For example, the blue star shows that the Hubble Space Telescope is almost diffraction-limited in the visible spectrum at 0.1 arcsecs, whereas the red circle shows that the human eye should have a resolving power of 20 arcsecs in theory, though normally only 60 arcsecs.

## Clocks

The FOCS 1 is a continuous cold caesium fountain atomic clock in Switzerland. Credit: .

An atomic clock is a clock device that uses an electronic transition frequency in the microwave, optical, or ultraviolet region[60] of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element. Atomic clocks are the most accurate time and frequency standards known, and are used as primary standards for international time distribution services, to control the wave frequency of television broadcasts, and in global navigation satellite systems such as GPS.

The FOCS 1 continuous cold cesium fountain atomic clock started operating in 2004 at an uncertainty of one second in 30 million years. The clock is in Switzerland.

## Motion calibrators

POA CALFOS is the improved Post Operational Archive version of the Faint Object Spectrograph (FOS) calibration pipeline ... The current version corrects for image motion problems that have led to significant wavelength scale uncertainties in the FOS data archive. The improvements in the calibration enhance the scientific value of the data in the FOS archive, making it a more homogeneous and reliable resource.

## Observatories

The Sphinx Observatory at the Jungfraujoch in the Swiss Alps is a high altitude observatory less affected by the atmosphere. Credit: Eric Hill from Boston, MA, USA.

The dome of the Grand Telescope is shown at sunset. Credit: Pachango.

This is an inside view of New Mexico Skies 24" Observatory, Mayhill, NM. Credit: Robert B. Denny.

Historically, observatories [are] as simple as [using or placing stably] an astronomical sextant (for measuring the distance between stars) or Stonehenge (which has some alignments on astronomical phenomena). ... Most optical telescopes are housed within a dome or similar structure, to protect the delicate instruments from the elements. Telescope domes have a slit or other opening in the roof that can be opened during observing, and closed when the telescope is not in use. In most cases, the entire upper portion of the telescope dome can be rotated to allow the instrument to observe different sections of the night sky. Radio telescopes usually do not have domes.

The "Great Observatories Origins Deep Survey (GOODS) [...] are primarily, but not exclusively, based on multi–band imaging data obtained with the Hubble Space Telescope (HST) and the Advanced Camera for Surveys (ACS). [...] Existing deep observations from the Chandra X-ray Observatory (CXO) and groundbased facilities are supplemented with new, deep imaging in the optical and near-infrared from the European Southern Observatory (ESO) and from the Kitt Peak National Observatory (KPNO) [includes deep] observations with the Space Infrared Telescope Facility (SIRTF) [and] at the Kitt Peak National Observatory, National Optical Astronomical Observatories".[61]

"Observations [of 64 neglected double stars] were made with the GRAS002 robotic [optical] telescope located at the Remote Astronomical Society Observatory [at] Mayhill, [New Mexico] NM, USA (http://www.remote-astronomical-society.org/)."[62]

The double stars [were] imaged using a Takahashi Mewlon 300 Dall-Kirkham cassegrainian reflector located at the Remote Astronomical Society Obseervatory in Mayhill, New Mexico."[62]

The GTC began its preliminary observations on 13 July 2007, using 12 segments of its primary mirror, made of Zerodur glass-ceramic by the German company Schott AG. Later the number of segments was increased to a total of 36 hexagonal segments fully controlled by an active optics control system, working together as a reflective unit.[63][64] Its Day One instrumentation [is the Optical System for Imaging and low Resolution Integrated Spectroscopy] OSIRIS. Scientific observations began properly in May 2009.[65]

## Deep space surveillance

The United States counterpart to the Okno system is GEODSS at Diego Garcia. Credit: U.S. Air Force photo/Senior Master Sgt. John Rohrer.

At left is an image of the US counterpart, the Ground-based Electro-Optical Deep Space Surveillance (GEODSS) base at Diego Garcia, British Indian Ocean Territory latitude 7.41173°S and longitude 72.45222°E.

## Printing

This photograph of a printing press reminds us that writing descriptions down preserves knowledge for the future. Credit: MatthiasKabel.

The invention of the printing press (an early example is shown at right) made it possible for scientists and politicians to communicate their ideas with ease, leading to the Age of Enlightenment; an example of technology as a cultural force.

The images show proof of concept using adaptive optics to enhance the resolution of the International Space Station. Credit: David Dayton, John Gonglewski, Sergio Restaino, Jeffrey Martin, James Phillips, Mary Hartman, Stephen Browne, Paul Kervin, Joshua Snodgrass, Nevin Heimann, Michael Shilko, Richard Pohle, Bill Carrion, Clint Smith, and Daniel Thiel.

"Adaptive optics (AO) systems have been in operation for roughly 30 years. As the field matures increased numbers of possible applications are being discovered however their practical implementation has been greatly limited by the high cost, high power consumption, and size of traditional adaptive optics systems."[66]

In "the development of micro-machined electro-mechanical systems (MEMS) based adaptive optics elements [... in] particular electrostatic membrane mirrors [1], and nematic liquid crystal based devices [2] [...] a proof-of-concept demonstration of these two devices [is] for imaging bright low earth orbit (LEO) solar illuminated satellites through atmospheric turbulence, performed on the Air Force Research Laboratory, Directed Energy Directorate’s 3.67 meter AEOS telescope on Haleakala Maui. The aperture was stopped done to 1.12 meters in order to match the number of actuators and dynamic range of the devices."[66]

## Hypotheses

1. Most of the early research was centered on improving optical astronomy.

## References

1. M. Villata, C. M. Raiteri, T. J. Balonek, M. F. Aller, S. G. Jorstad, O. M. Kurtanidze, F. Nicastro, K. Nilsson, H. D. Aller, A. Arai, A. Arkharov, U. Bach, E. Benítez, A. Berdyugin, C. S. Buemi, M. Böttcher, D. Carosati, R. Casas, A. Caulet, W. P. Chen, P.-S. Chiang, Y. Chou, S. Ciprini, J. M. Coloma, G. Di Rico, C. Díaz, N. V. Efimova, C. Forsyth, A. Frasca, L. Fuhrmann, B. Gadway, S. Gupta, V. A. Hagen-Thorn, J. Harvey, J. Heidt, H. Hernandez-Toledo, F. Hroch, C.-P. Hu, R. Hudec, M. A. Ibrahimov, A. Imada, M. Kamata, T. Kato, M. Katsuura, T. Konstantinova, E. Kopatskaya, D. Kotaka, Y. Y. Kovalev, Yu. A. Kovalev, T. P. Krichbaum, K. Kubota, M. Kurosaki, L. Lanteri, V. M. Larionov, L. Larionova, E. Laurikainen, C.-U. Lee, P. Leto, A. Lähteenmäki, O. López-Cruz, E. Marilli, A. P. Marscher, I. M. McHardy, S. Mondal, B. Mullan, N. Napoleone, M. G. Nikolashvili, J. M. Ohlert, S. Postnikov, T. Pursimo, M. Ragni, J. A. Ros, K. Sadakane, A. C. Sadun, T. Savolainen, E. A. Sergeeva, L. A. Sigua, A. Sillanpää, L. Sixtova, N. Sumitomo, L. O. Takalo, H. Teräsranta, M. Tornikoski, C. Trigilio, G. Umana, A. Volvach, B. Voss, and S. Wortel (July 3, 2006). "The unprecedented optical outburst of the quasar 3C 454.3 The WEBT campaign of 2004–2005". Astronomy & Astrophysics 453 (3): 817-22. doi:10.1051/0004-6361:20064817. Retrieved 2013-12-09.
2. McGraw-Hill Encyclopedia of Science and Technology (5th ed.). McGraw-Hill. 1993.
3. Moore, P. (1997). Philip's Atlas of the Universe. Great Britain: George Philis Limited. ISBN 0-540-07465-9.
4. I. H. Malitson (1965). "Interspecimen Comparison of the Refractive Index of Fused Silica". Journal of the Optical Society of America 55 (10): 1205. doi:10.1364/JOSA.55.001205.
5. R. Kitamura, L. Pilon, M. Jonasz (2007). "Optical Constants of Silica Glass From Extreme Ultraviolet to Far Infrared at Near Room Temperatures". Applied Optics 46 (33): 8118–8133. doi:10.1364/AO.46.008118.
6. John W. Hardy (June 1977). Active optics: A new technology for the control of light. Proceedings of the IEEE. pp. 110.
7. Roddier (1999). François Roddier (ed.). Adaptive Optics in Astronomy (PDF). Cambridge, United Kingdom: Cambridge University Press. p. 411. ISBN 0 521 55375 X. Retrieved 2012-02-15.
8. Rnt20 (June 10, 2011). File:Ao movie.gif. San Francisco, California: Wikimedia Foundation, Inc. Retrieved 2013-01-10.
9. Cecie Starr (2005). Biology: Concepts and Applications. Thomson Brooks/Cole. ISBN 053446226X.
10. D. K. Yeomans (1998). Great Comets in History. Jet Propulsion Laboratory. Retrieved 15 March 2007.
11. D. A. Mendis (1988). "A Postencounter view of comets". Annual Review of Astronomy and Astrophysics 26 (1): 11–49. doi:10.1146/annurev.aa.26.090188.000303.
12. Ian Ridpath (1985). Through the comet’s tail. Revised extracts from A Comet Called Halley by Ian Ridpath, published by Cambridge University Press in 1985. Retrieved 2011-06-19.
13. Brian Nunnally (May 16, 2011). This Week in Science History: Halley’s Comet. pfizer: ThinkScience Now. Retrieved 2011-06-19.
14. "Yerkes Observatory Finds Cyanogen in Spectrum of Halley's Comet". The New York Times. 8 February 1910. Retrieved 15 November 2009.
15. "Ten Notable Apocalypses That (Obviously) Didn't Happen". Smithsonian magazine. 2009. Retrieved 14 November 2009.
16. Interesting Facts About Comets. Universe Today. 2009. Retrieved 15 January 2009.
17. Heber D. Curtis (June 1910). "Photographs of Halley's Comet made at the Lick Observatory". Publications of the Astronomical Society of the Pacific 22 (132): 117-30.
18. Abraham Loeb, Mark J. Reid, Andreas Brunthaler, and Heino Falcke (November 2005). "Constraints on the Proper Motion of the Andromeda Galaxy Based on the Survival of Its Satellite M33". The Astrophysical Journal 633 (2): 894-8. doi:10.1086/491644. Retrieved 2011-11-14.
19. The Grand Collision, from the series: The Sky At Night, airdate: November 5, 2007
20. Cox, T. J.; Loeb, A. (2008). "The collision between the Milky Way and Andromeda". Monthly Notices of the Royal Astronomical Society 386 (1): 461–474. doi:10.1111/j.1365-2966.2008.13048.x.
21. Cain, F. (2007). When Our Galaxy Smashes Into Andromeda, What Happens to the Sun?. Retrieved 2007-05-16.
22. M.A. Moreno-Corral, C. Chavarria-K., E de Lara, and S. Wagner (June 1993). "Hα Interferometric Optical and Near IR Photometric Studies of Star Forming Regions I. The Cepheus-B/Sh2-155/Cepheus OB3 association complex". Astronomy and Astrophysics 273 (06): 619-32. Retrieved 2013-12-11.
23. Lawrence M. Krauss, Scott Tremaine (January 1988). "Test of the Weak Equivalence Principle for Neutrinos and Photons". Physical Review Letters 60 (3): 176–7. doi:10.1103/PhysRevLett.60.176.
24. P. Buford Price and the Amanda Collaboration (September 8, 2005). AMANDA-II Science Results. Irvine, California USA: University of California Irvine. Retrieved 2014-03-23.
25. Nicole P. Vogt and Martha P. Haynes, Riccardo Giovanelli, and Terry Herter (June 2004). "M/L, Hα Rotation Curves, and HI Gas Measurements for 329 Nearby Cluster and Field Spirals. III. Evolution in Fundamental Galaxy Parameters". The Astronomical Journal 127 (6): 3325-37. doi:10.1086/420703. Retrieved 2013-12-20.
26. Francis Halzen and Dan Hooper (June 12, 2002). "High-energy neutrino astronomy: the cosmic ray connection". Reports on Progress in Physics 65 (7): 1025-107. doi:10.1088/0034-4885/65/7/201. Retrieved 2014-02-08.
27. Vedrenne, G and Atteia, J.-L. (2009). Gamma-Ray Bursts: The brightest explosions in the Universe. Springer/Praxis Books. ISBN 978-3-540-39085-5.CS1 maint: multiple names: authors list (link)
28. Martin V. Zombeck (1990). "Calibration of MK spectral types". Handbook of Space Astronomy and Astrophysics (2nd ed.). Cambridge University Press. p. 105. ISBN 0-521-34787-4.
29. David F. Gray (1992), The Inferred Color Index of the Sun, Publications of the Astronomical Society of the Pacific, vol. 104, no. 681, pp. 1035-1038 (November 1992)
30. James Binney; Merrifield M. Galactic Astronomy, Princeton University Press, 1998, ch. 2.3.2, pp. 53
31. Schmid, H. M.; Beuzit, J.-L.; Feldt, M. et al. (2006). "Search and investigation of extra-solar planets with polarimetry". Direct Imaging of Exoplanets: Science & Techniques. Proceedings of the IAU Colloquium #200 1 (C200): 165–170. doi:10.1017/S1743921306009252.
32. F. Paresce (December 15, 2009). Hubble's Festive View of a Grand Star-Forming Region. HubbleSite. Retrieved 2013-01-10.
33. Antony Cooke (2005). Visual Astronomy Under Dark Skies: A New Approach to Observing Deep Space. London: Springer-Verlag. p. 180. ISBN 1852339012. Retrieved 2011-11-06.
34. C. Arpigny; R. Schulz; J. Manfroid; I. Ilyin; J. A. Stüwe; J.-M. Zucconi (October 2000). "The isotope ratios 12C/13C and 14N/15N in comet C/1995 O1 (Hale-Bopp)". Bulletin of the American Astronomical Society 32 (10): 1074. Retrieved 2013-12-20.
35. Staff (11 September 2003). Why infrared astronomy is a hot topic. ESA. Retrieved 11 August 2008.
36. Infrared Spectroscopy – An Overview. NASA/IPAC. Retrieved 11 August 2008.
37. Steven V. W. Beckwith and Anneila I. Sargent (November 1, 1991). "Particle Emissivity in Circumstellar Disks". The Astrophysical Journal 381 (11): 250-8. doi:10.1086/170646. Retrieved 2013-12-22.
38. Carl Melis, B. Zuckerman, Joseph H. Rhee, Inseok Song, Simon J. Murphy, Michael S. Bessell (July 5, 2012). "Rapid disappearance of a warm, dusty circumstellar disk". Nature 487 (7405): 74-6. doi:10.1038/nature11210. Retrieved 2013-12-13.
39. Paul Murdin (November 2000). Paul Murdin (ed.). Canada-France-Hawaii Telescope, In: Encyclopedia of Astronomy and Astrophysics. Bristol: Institute of Physics. Bibcode:2000eaa..bookE4166.. doi:10.1888/0333750888/4166. |access-date= requires |url= (help)
40. Theodore E. Madey, Robert E. Johnson, Thom M. Orlando (March 2002). "Far-out surface science: radiation-induced surface processes in the solar system". Surface Science 500 (1-3): 838-58. doi:10.1016/S0039-6028(01)01556-4. Retrieved 2012-02-09.
41. O. B. Khavroshkin and V. V. Tsyplakov (2009). "Rotation of cosmogony objects and Outside wave fields of the Moon and the Earth". EPSC Abstracts 4 (149): 1. Retrieved 2013-12-20.
42. G. Andronico, G. A. Baratta, F. Spinella, and G. Strazzulla (October 1987). "Optical evolution of laboratory-produced organics - applications to Phoebe, Iapetus, outer belt asteroids and cometary nuclei". Astronomy and Astrophysics 184 (1-2): 333-6. Retrieved 2013-09-25.
43. Phil Davis (October 9, 2009). Brighter Neptune. National Aeronautics and Space Administration. Retrieved 2012-07-20.
44. "First Planet-Forming Disk Found in the Environment of a Dying Star." Accessed 1/10/07. http://www.keckobservatory.org/article.php?id=99
45. Jennifer L. Sokoloski, Lars Bildsten (November 2010). "Evidence for the White Dwarf Nature of Mira B". The Astrophysical Journal 723 (2): 1188-94. doi:10.1088/0004-637X/723/2/1188.
46. H. Olofsson (July 2, 2012). Red giant blows a bubble. Maryland USA: SpaceTelescope Organization. Retrieved 2013-12-24.
47. S. Van Dyk (May 22, 2003). 2MASS Atlas Image Gallery: Miscellaneous Objects. California USA: IPAC, California Institute of Technology. Retrieved 2013-12-24.
48. H. Bond and K. Exter (June 3, 2008). White Dwarf Lost in Planetary Nebula. Maryland USA: Hubblesite Organization. Retrieved 2013-12-24.
49. Arlo U. Landolt (March 1983). "UBVRI photometric standard stars around the celestial equator". The Astronomical Journal 88 (3): 439-60. doi:10.1086/113329.
50. Arlo U. Landolt (July 1992). "UBVRI photometric standard stars in the magnitude range 11.5-16.0 around the celestial equator". The Astronomical Journal 104 (1): 340-71, 436-91. doi:10.1086/116242.
51. Arlo U. Landolt (May 2009). "UBVRI Photometric Standard Stars Around the Celestial Equator: Updates and Additions". The Astronomical Journal 137 (5): 4186-269. doi:10.1088/0004-6256/137/5/4186.
52. [1]
53. [2]
54. Bryant, Jeff; Pavlyk, Oleksandr. "Kepler's Second Law", Wolfram Demonstrations Project. Retrieved December 27, 2009.
55. Philip B. Gove, ed. (1963). Webster's Seventh New Collegiate Dictionary. Springfield, Massachusetts: G. & C. Merriam Company. p. 1221. |access-date= requires |url= (help)
56. Samuel J. Johnson (1874). "Occultations of and by Venus". Astronomical register 12: 268-70.
57. Barrie William Jones. The search for life continued: planets around other stars. p. 111.
58. Max Born and Emil Wolf (October 1999). Principles of Optics. Cambridge: Cambridge University Press. p. 461. ISBN 0-521-64222-1.
59. Dennis McCarthy, P. Kenneth Seidelmann (2009). TIME from Earth Rotation to Atomic Physics. Weinheim: Wiley-VCH.
60. M. Giavalisco, H. C. Ferguson, A. M. Koekemoer, M. Dickinson, D. M. Alexander, F. E. Bauer, J. Bergeron, C. Biagetti, W. N. Brandt, S. Casertano, C. Cesarsky, E. Chatzichristou, C. Conselice, S. Cristiani, L. Da Costa, T. Dahlen, D. De Mello, P. Eisenhardt, T. Erben, S. M. Fall, C. Fassnacht, R. Fosbury, A. Fruchter, Jonathan. P. Gardner, N. Grogin, R. N. Hook, A. E. Hornschemeier, R. Idzi, S. Jogee, C. Kretchmer, V. Laidler, K. S. Lee, M. Livio, R. Lucas, P. Madau, B. Mobasher, L. A. Moustakas, M. Nonino, P. Padovani, C. Papovich, Y. Park, S. Ravindranath, A. Renzini, M. Richardson, A. Riess, P. Rosati, M. Schirmer, E. Schreier, R. S. Somerville, H. Spinrad, D. Stern, M. Stiavelli, L. Strolger, C. M. Urry, B. Vandame, R. Williams, C. Wolf (January 2004). "The Great Observatories Origins Deep Survey: Initial Results From Optical and Near-Infrared Imaging". The Astrophysical Journal 600 (2): L93-8. doi:10.1086/379232. Retrieved 2013-12-10.
61. E. O. Wiley (Winter 2008). "Neglected Double Observations for 2006 No. 4: some 22nd Hour Doubles". Journal of Double Star Observations 4 (1): 14-9. Retrieved 2013-10-27.
62. Tests begin on Canaries telescope. BBC. 14 July 2007.
63. Giant telescope begins scouring space. July 14, 2007.
64. El Gran Telescopio CANARIAS comienza a producir sus primeros datos científicos. IAC Press release. June 16, 2009.
65. David Dayton, John Gonglewski, Sergio Restaino, Jeffrey Martin, James Phillips, Mary Hartman, Stephen Browne, Paul Kervin, Joshua Snodgrass, Nevin Heimann, Michael Shilko, Richard Pohle, Bill Carrion, Clint Smith, Daniel Thiel (December 16, 2002). "Demonstration of new technology MEMS and liquid crystal adaptive optics on bright astronomical objects and satellites". Optics 10 (25): 1508-19. Retrieved 2014-02-14.