The Cosmic Origins Spectrograph (COS) is a science instrument that was installed on the

Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...

during Servicing Mission 4 (

STS-125 STS-125, or HST-SM4 (Hubble Space Telescope Servicing Mission 4), was the fifth and final Space Shuttle mission to the Hubble Space Telescope (HST) and the last solo flight of the Space Shuttle Atlantis, Space Shuttle ''Atlantis''. The launch of ...

) in May 2009. It is designed for

ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...

(90–320 nm)

spectroscopy Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...

of faint point sources with a resolving power of ≈1,550–24,000. Science goals include the study of the origins of large scale structure in the universe, the formation and evolution of galaxies, and the origin of stellar and planetary systems and the cold interstellar medium. COS was developed and built by the Center for Astrophysics and Space Astronomy (CASA-ARL) at the

University of Colorado at Boulder The University of Colorado Boulder (CU Boulder, CU, or Colorado) is a public research university in Boulder, Colorado. Founded in 1876, five months before Colorado became a state, it is the flagship university of the University of Colorado sys ...

and the Ball Aerospace and Technologies Corporation in

Boulder, Colorado Boulder is a home rule city that is the county seat and most populous municipality of Boulder County, Colorado, United States. The city population was 108,250 at the 2020 United States census, making it the 12th most populous city in Color ...

. COS is installed into the axial instrument bay previously occupied by the Corrective Optics Space Telescope Axial Replacement (

COSTAR CoStar Group, Inc. is a Washington, DC-based provider of information, analytics and marketing services to the commercial property industry in the United States, Canada, the United Kingdom, France, Germany, and Spain. Founded in 1987 by Andrew C. ...

) instrument, and is intended to complement the Space Telescope Imaging Spectrograph (

STIS The Space Telescope Imaging Spectrograph (STIS) is a spectrograph, also with a camera mode, installed on the Hubble Space Telescope. Aerospace engineer Bruce Woodgate of the Goddard Space Flight Center was the principal investigator and crea ...

) that was repaired during the same mission. While STIS operates across a wider wavelength range, COS is many times more sensitive in the UV.COS Instrument Handbook
/ref>

Instrument overview

The Cosmic Origins Spectrograph is an ultraviolet

spectrograph An optical spectrometer (spectrophotometer, spectrograph or spectroscope) is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify mate ...

that is optimized for high sensitivity and moderate

spectral resolution The spectral resolution of a spectrograph, or, more generally, of a frequency spectrum, is a measure of its ability to resolve features in the electromagnetic spectrum. It is usually denoted by \Delta\lambda, and is closely related to the resolvi ...

of compact (point like) objects (stars, quasars, etc.). COS has two principal channels, one for

Far Ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...

(FUV) spectroscopy covering 90–205 nm and one for

Near Ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...

(NUV) spectroscopy spanning 170–320 nm. The FUV channel can work with one of three

diffraction grating In optics, a diffraction grating is an optical component with a periodic structure that diffracts light into several beams travelling in different directions (i.e., different diffraction angles). The emerging coloration is a form of structura ...

s, the NUV with one of four, providing both low and medium resolution spectra (table 1). In addition, COS has a narrow field of view NUV imaging mode intended for target acquisition. One key technique for achieving high sensitivity in the FUV is minimizing the number of optics. This is done because FUV reflection and transmission efficiencies are typically quite low compared to what is common at visible wavelengths. In accomplishing this, the COS FUV channel uses a single (selectable) optic to diffract the light from HST, correct for the Hubble spherical aberration, focus the diffracted light onto the FUV detector and correct for astigmatism typical of this sort of instrument. Since aberration correction is performed after the light passes into the instrument, the entrance to the spectrograph must be an extended aperture, rather than the traditional narrow entrance slit, in order to allow the entire aberrated HST image from a point source to enter the instrument. The 2.5 arc second diameter entrance aperture allows ≈ 95% of the light from compact sources to enter COS, yielding high sensitivity at the design resolution for compact sources. Post launch performance closely matched expectations. Instrument sensitivity is close to pre-launch calibration values, and detector background is exceptionally low (0.16 counts per resolution element per 1000 seconds for the FUV detector, and 1.7 counts per resolution element per 100 seconds for the NUV detector). FUV resolution is slightly lower than pre-launch predictions due to mid-frequency polishing errors on the HST primary mirror, while NUV resolution exceeds pre-launch values in all modes. Thanks to the minimal number of reflections, the G140L mode, and G130M central wavelength settings added after 2010, can observe light at wavelengths down to ~90 nm, and shorter, despite the very low reflectivity of the MgF₂ coated optics at these wavelengths.

Science Goals

The Cosmic Origins Spectrograph is designed to enable the observation of faint, point-like UV targets at moderate spectral resolution, allowing COS to observe hot stars ( OB stars,

white dwarfs A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes fro ...

cataclysmic variables In astronomy, cataclysmic variable stars (CVs) are stars which irregularly increase in brightness by a large factor, then drop back down to a quiescent state. They were initially called novae (), since ones with an outburst brightness visible t ...

and

binary star A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...

s) in the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...

and to observe the absorption features in the spectra of

active galactic nuclei An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much-higher-than-normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not prod ...

. Observations are also planned of extended objects. Spectroscopy provides a wealth of information about distant astronomical objects that is unobtainable through imaging:

Spectroscopy lies at the heart of astrophysical inference. Our understanding of the origin and evolution of the cosmos critically depends on our ability to make quantitative measurements of physical parameters such as the total mass, distribution, motions, temperatures, and composition of matter in the Universe. Detailed information on all of these properties can be gleaned from high-quality spectroscopic data. For distant objects, some of these properties (e.g., motions and composition) can only be measured through spectroscopy.

Ultraviolet (UV) spectroscopy provides some of the most fundamental diagnostic data necessary for discerning the physical characteristics of planets, stars, galaxies, and interstellar and intergalactic matter. The UV offers access to spectral features that provide key diagnostic information that cannot be obtained at other wavelengths.University of Colorado COS Web Page
/ref>

Obtaining

absorption spectra Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating ...

of interstellar and intergalactic gas forms the basis of many of the COS science programs. These spectra will address questions such as how was the

Cosmic Web The observable universe is a ball-shaped region of the universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because the electromagnetic radiation from these obj ...

formed, how much mass can be found in interstellar and intergalactic gas, and what is the composition, distribution and temperature of this gas. In general, COS will address questions such as:The Cosmic Origins Spectrograph and the Future of Ultraviolet Astronomy
/ref> * What is the large-scale structure of matter in the Universe? * How did

galaxies A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

form out of the intergalactic medium? * What types of galactic halos and outflowing winds do star-forming galaxies produce? * How were the chemical elements for life created in massive stars and

supernovae A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when a ...

? * How do stars and planetary systems form from dust grains in

molecular cloud A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen ...

s? * What is the composition of

planetary atmospheres An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A ...

and

comets A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ar ...

in our

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...

(and beyond)? Some specific programs include the following: Large-Scale Structure of

Baryonic Matter In particle physics, a baryon is a type of composite subatomic particle which contains an odd number of valence quarks (at least 3). Baryons belong to the hadron family of particles; hadrons are composed of quarks. Baryons are also classified ...

: With its high FUV spectroscopic sensitivity, COS uniquely suited for exploring the

Lyman-alpha forest The Lyman-alpha line, typically denoted by Ly-α, is a spectral line of hydrogen (or, more generally, of any one-electron atom) in the Lyman series. It is emitted when the atomic electron transitions from an ''n'' = 2 orbital to the gr ...

. This is the ‘forest’ of

seen in the spectra of distant

and

quasars A quasar is an extremely luminous active galactic nucleus (AGN). It is pronounced , and sometimes known as a quasi-stellar object, abbreviated QSO. This emission from a galaxy nucleus is powered by a supermassive black hole with a mass ranging ...

caused by intergalactic gas clouds, which may contain the majority of baryonic matter in the universe. Because the most useful absorption lines for these observations are in the far ultraviolet and the sources are faint, a high sensitivity FUV spectrograph with wide wavelength coverage is needed to perform these observations. By determining the

redshift In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase in f ...

and line width of the intervening absorbers, COS will be able to map out the temperature, density and composition of dark

baryonic matter In particle physics, a baryon is a type of composite subatomic particle which contains an odd number of valence quarks (at least 3). Baryons belong to the hadron family of particles; hadrons are composed of quarks. Baryons are also classified ...

in the

. Warm–hot

intergalactic medium Intergalactic may refer to: * "Intergalactic" (song), a song by the Beastie Boys * ''Intergalactic'' (TV series), a 2021 UK science fiction TV series * Intergalactic space * Intergalactic travel, travel between galaxies in science fiction and ...

: Absorption line studies of highly

ionized Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule ...

(hot) gas ( O IV, N V, etc.) and broad

Lyman-alpha The Lyman-alpha line, typically denoted by Ly-α, is a spectral line of hydrogen (or, more generally, of any one-electron atom) in the Lyman series. It is emitted when the atomic electron transitions from an ''n'' = 2 orbital to the gr ...

will explore the ionization state and distribution of hot intergalactic gas.

Great Wall The Great Wall of China (, literally "ten thousand Li (unit), ''li'' wall") is a series of fortifications that were built across the historical northern borders of ancient Chinese states and Imperial China as protection against Eurasian noma ...

Structure: Background

will be used to study intergalactic absorbers to estimate their transverse size and physical density and determine how the distribution of material correlates with nearby galaxy distributions in the CFA2 Great Wall. He II Reionization: Highly redshifted

helium will be used study the

reionization In the fields of Big Bang theory and cosmology, reionization is the process that caused matter in the universe to reionize after the lapse of the " dark ages". Reionization is the second of two major phase transitions of gas in the universe (t ...

process at a

(z) of ≈ 3.

Additional instrument design details

COS has two channels, the

(FUV) covering 90–205 nm and the

(NUV) covering 170–320 nm. All COS optics are reflective (except for the bright object aperture filter and NUV order sorters) to maximize efficiency and to avoid

chromatic aberration In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the wave ...

. Principal COS observing modes are summarized in table 1. Light from the Hubble Space Telescope enters the instrument through either the Primary Science Aperture (PSA) or the Bright Object Aperture (BOA). The BOA introduces a

neutral density filter In photography and optics, a neutral-density filter, or ND filter, is a filter that reduces or modifies the intensity of all wavelengths, or colors, of light equally, giving no changes in hue of color rendition. It can be a colorless (clear) ...

to the optical path that attenuates the light by approximately a factor of one hundred (five astronomical magnitudes). Both apertures are oversized (2.5 arc second clear aperture) permitting more than 95% of the light from a point source to enter the

. After passing through the PSA or BOA the light travels to one of the optics on the first of two optic select wheels, either one of the three FUV diffraction gratings or the first of the NUV collimation mirrors (table 1), depending on whether an FUV, NUV, or target acquisition channel is selected. All optics on the first wheel have an

aspheric An aspheric lens or asphere (often labeled ''ASPH'' on eye pieces) is a lens whose surface profiles are not portions of a sphere or cylinder. In photography, a lens assembly that includes an aspheric element is often called an aspherical lens. ...

profile to correct for the Hubble spherical aberration. The FUV channel has two medium and one low resolution spectroscopy modes. The FUV channels are modified

Rowland Circle X-ray fluorescence (XRF) is the emission of characteristic "secondary" (or fluorescent) X-rays from a material that has been excited by being bombarded with high-energy X-rays or gamma rays. The phenomenon is widely used for elemental analysis ...

spectrographs in which the single holographically ruled

concave Concave or concavity may refer to: Science and technology * Concave lens * Concave mirror Mathematics * Concave function, the negative of a convex function * Concave polygon, a polygon which is not convex * Concave set * The concavity In ca ...

simultaneously focuses and diffracts the incoming light and corrects both for the HST spherical aberration and for aberrations introduced by the extreme off-Rowland layout. The diffracted light is focused onto a 170x10 mm cross delay line microchannel plate detector. The FUV detector active area is curved to match the spectrograph's focal surface and is divided into two physically distinct segments separated by a small gap. The NUV channel has three medium and one low resolution spectroscopy modes as well as an imaging mode with an approximately 1.0 arc second unvignetted field of view. The NUV channels utilize a modified Czerny-Turner design in which collimated light is fed to the selected grating, followed by three camera mirrors that direct the diffracted light onto three separate stripes on a 25×25 mm Multi Anode Microchannel Array (MAMA) detector. The imaging mode is primarily intended for target acquisition.

References

External links

* * Th
Cosmic Origins Spectrograph Web Site
at the

University of Colorado The University of Colorado (CU) is a system of public universities in Colorado. It consists of four institutions: University of Colorado Boulder, University of Colorado Colorado Springs, University of Colorado Denver, and the University of Co ...

* th
COS Web site
at th
Space Telescope Science Institute
(STScI) {{Hubble Space Telescope Ultraviolet telescopes Hubble Space Telescope instruments Spectrographs

Instrument overview

Science Goals

Additional instrument design details

See also

References

External links