Cosmic Origins Spectrograph
Encyclopedia
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
during Servicing Mission 4 (STS-125
STS-125
STS-125, or HST-SM4 , was the fifth and final space shuttle servicing mission to the Hubble Space Telescope .Launch occurred on 11 May 2009 at 2:01 pm EDT...
) in May 2009. It is designed for ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
(115–320 nm) spectroscopy
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
of faint point sources with a resolving power
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...
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
University of Colorado at Boulder
The University of Colorado Boulder is a public research university located in Boulder, Colorado...
and the Ball Aerospace and Technologies Corporation
Ball Aerospace
Ball Aerospace & Technologies Corp. is a manufacturer of spacecraft, components, and instruments for national defense, civil space and commercial space applications. The company is a wholly owned subsidiary of Ball Corp...
in Boulder, Colorado
Boulder, Colorado
Boulder is the county seat and most populous city of Boulder County and the 11th most populous city in the U.S. state of Colorado. Boulder is located at the base of the foothills of the Rocky Mountains at an elevation of...
.
COS is installed into the axial instrument bay previously occupied by the Corrective Optics Space Telescope Axial Replacement (COSTAR) instrument, and is intended to complement the Space Telescope Imaging Spectrograph (STIS) that was repaired during the same mission. While STIS operates across a wider wavelength range, COS is many times more sensitive in the UV.
Instrument overview
Spectrometer
A spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization...
that is optimized for high sensitivity and moderate spectral resolution
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...
of compact (point like) objects (stars, quasars, etc.). COS has two principal channels, one for Far Ultraviolet (FUV) spectroscopy covering 115–205 nm and one for Near Ultraviolet (NUV) spectroscopy spanning 170–320 nm. The FUV channel can work with one of three diffraction grating
Diffraction grating
In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...
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.
| Grating (Channel) | Approximate Useful Wavelength Wavelength In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a... Range |
Resolving power 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... (λ/Δλ) |
|---|---|---|
| G130M (FUV) | 115–145 nm | 16,000–21,000 |
| G160M (FUV) | 141–178 nm | 16,000–21,000 |
| G140L (FUV) | <90–205 nm | 1,500–4,000 |
| G185M (NUV) | 170–210 nm | 22,000–28,000 |
| G225M (NUV) | 210–250 nm | 28,000–38,000 |
| G285M (NUV) | 250–320 nm | 30,000–41,000 |
| G230L (NUV) | 170–320 nm | 2,100–3,900 |
| TA1 (target acquisition imager) | 170–320 nm | ~0.05 arc sec. angular resolution |
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. In addition, thanks to the minimal number of reflections, the G140L mode can observe light at wavelengths shorter than 90nm 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, cataclysmic variables and binary stars) in the Milky WayMilky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
and to observe the absorption features in the spectra of active galactic nuclei. 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.
Obtaining absorption spectra 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 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:
- What is the large-scale structure of matter in the Universe?
- How did galaxies 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?
- How do stars and planetary systems form from dust grains in molecular cloudsMolecular cloudA molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen ....
? - What is the composition of planetary atmospheresAtmosphereAn atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
and comets in our Solar SystemSolar SystemThe Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
(and beyond)?
Some specific programs include the following:
Large-Scale Structure of Baryonic Matter
Baryon
A baryon is a composite particle made up of three quarks . Baryons and mesons belong to the hadron family, which are the quark-based particles...
: With its high FUV spectroscopic sensitivity, COS uniquely suited for exploring the Lyman-alpha forest
Lyman-alpha forest
In astronomical spectroscopy, the Lyman-alpha forest is the sum of absorption lines arising from the Lyman-alpha transition of the neutral hydrogen in the spectra of distant galaxies and quasars....
. This is the ‘forest’ of absorption spectra seen in the spectra of distant galaxies and quasars 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
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
and line width of the intervening absorbers, COS will be able to map out the temperature, density and composition of dark baryonic matter
Baryon
A baryon is a composite particle made up of three quarks . Baryons and mesons belong to the hadron family, which are the quark-based particles...
in the Cosmic Web.
Warm–hot intergalactic medium: Absorption line studies of highly ionized (hot) gas (O
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
IV, N
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
V, etc.) and broad Lyman-alpha
Lyman series
In physics and chemistry, the Lyman series is the series of transitions and resulting ultraviolet emission lines of the hydrogen atom as an electron goes from n ≥ 2 to n = 1...
will explore the ionization state and distribution of hot intergalactic gas.
Great Wall
Great Wall (astronomy)
The Great Wall , sometimes specifically referred to as the CfA2 Great Wall, is one of the largest known super-structures in the Universe...
Structure: Background active galactic nuclei 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 ionized helium will be used study the reionization
Reionization
In Big Bang cosmology, reionization is the process that reionized the matter in the universe after the "dark ages," and is the second of two major phase changes of gas in the universe. As the majority of baryonic matter is in the form of hydrogen, reionization usually refers to the reionization of...
process at a redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
(z) of ≈ 3.
Additional instrument design details
COS has two channels, the Far Ultraviolet (FUV) covering 115–205 nm and the Near Ultraviolet (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 aberrationChromatic aberration
In optics, chromatic aberration is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have a different refractive index for different wavelengths of light...
. 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
Neutral density filter
In photography and optics, a neutral density filter or ND filter can be a colorless or grey filter. An ideal neutral density filter reduces and/or modifies intensity of all wavelengths or colors of light equally, giving no changes in hue of color rendition.The purpose of standard photographic...
to the optical path that attenuates the light by approximately a factor of one hundred (five astronomical magnitudes
Absolute magnitude
Absolute magnitude is the measure of a celestial object's intrinsic brightness. it is also the apparent magnitude a star would have if it were 32.6 light years away from Earth...
). Both apertures are oversized (2.5 arc second clear aperture) permitting more than 95% of the light from a point source to enter the spectrograph
Spectrograph
A spectrograph is an instrument that separates an incoming wave into a frequency spectrum. There are several kinds of machines referred to as spectrographs, depending on the precise nature of the waves...
.
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 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 spectrographs in which the single holographically ruled
Holographic grating
A holographic grating is a type of diffraction grating formed by an interference-fringe field of two laser beams whose standing-wave pattern is exposed to a polished substrate coated with photoresist. Processing of the exposed medium results in a pattern of straight lines with a sinusoidal cross...
aspheric concave diffraction grating
Diffraction grating
In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...
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
Microchannel plate detector
A micro-channel plate is a planar component used for detection of particles and impinging radiation . It is closely related to an electron multiplier, as both intensify single particles or photons by the multiplication of electrons via secondary emission...
. 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.
See also
- Wide Field and Planetary CameraWide Field and Planetary CameraThe Wide Field/Planetary Camera was a camera installed on the Hubble Space Telescope until December 1993. It was one of the instruments on Hubble at launch, but its functionality was severely impaired by the defects of the main mirror optics which afflicted the telescope...
- Wide Field and Planetary Camera 2Wide Field and Planetary Camera 2The Wide Field and Planetary Camera 2 is a baby grand piano sized camera built by the Jet Propulsion Laboratory and formerly installed on the Hubble Space Telescope. It was installed by servicing mission 1 in 1993, replacing the telescope's original Wide Field and Planetary Camera...
- Wide Field Camera 3Wide Field Camera 3The Wide Field Camera 3 is the Hubble Space Telescope's last and most technologically advanced instrument to take images in the visible spectrum...
- Advanced Camera for SurveysAdvanced Camera for SurveysThe Advanced Camera for Surveys is a third generation axial instrument aboard the Hubble Space Telescope . The initial design and scientific capabilities of ACS were defined by a team based at Johns Hopkins University. ACS was assembled and tested extensively at Ball Aerospace & Technologies Corp...
- Near Infrared Camera and Multi-Object SpectrometerNear Infrared Camera and Multi-Object SpectrometerThe Near Infrared Camera and Multi-Object Spectrometer is a scientific instrument for infrared astronomy, installed on the Hubble Space Telescope , operating from 1997 to 1999, and from 2002 to 2008...
- Faint Object CameraFaint Object CameraThe Faint Object Camera was a camera installed on the Hubble Space Telescope from launch in 1990 until 2002. It was replaced by the Advanced Camera for Surveys.The camera was built by Dornier GmbH and was funded by the European Space Agency...
External links
- The Cosmic Origins Spectrograph Web Site at the University of Colorado
- the COS Web site at the Space Telescope Science Institute (STScI)