The Extreme ultraviolet Imaging Telescope (EIT) is an
instrument on the
SOHO
SoHo, short for "South of Houston Street, Houston Street", is a neighborhood in Lower Manhattan, New York City. Since the 1970s, the neighborhood has been the location of many artists' lofts and art galleries, art installations such as The Wall ...
spacecraft
A spacecraft is a vehicle that is designed spaceflight, to fly and operate in outer space. Spacecraft are used for a variety of purposes, including Telecommunications, communications, Earth observation satellite, Earth observation, Weather s ...
used to obtain high-resolution
image
An image or picture is a visual representation. An image can be Two-dimensional space, two-dimensional, such as a drawing, painting, or photograph, or Three-dimensional space, three-dimensional, such as a carving or sculpture. Images may be di ...
s of the
solar corona
In astronomy, a corona (: coronas or coronae) is the outermost layer of a star's Stellar atmosphere, atmosphere. It is a hot but relatively luminosity, dim region of Plasma (physics), plasma populated by intermittent coronal structures such as so ...
in the
ultraviolet
Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...
range. The EIT instrument is sensitive to
light
Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
of four different
wavelength
In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats.
In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
s: 17.1, 19.5, 28.4, and 30.4
nm, corresponding to light produced by highly
ionized
iron
Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...
(XI)/(X), (XII), (XV), and
helium
Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...
(II), respectively. EIT is built as a single telescope with a quadrant structure to the entrance mirrors: each quadrant reflects a different colour of EUV light, and the wavelength to be observed is selected by a shutter that blocks light from all but the desired quadrant of the main telescope.
The EIT wavelengths are of great interest to solar physicists because they are emitted by the very hot
solar corona
In astronomy, a corona (: coronas or coronae) is the outermost layer of a star's Stellar atmosphere, atmosphere. It is a hot but relatively luminosity, dim region of Plasma (physics), plasma populated by intermittent coronal structures such as so ...
but not by the relatively cooler
photosphere
The photosphere is a star's outer shell from which light is radiated. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately , or equivalently, a depth from which 50% of light will esc ...
of the Sun; this reveals structures in the corona that would otherwise be obscured by the brightness of the Sun itself. EIT was originally conceived as a
viewfinder
In photography, a viewfinder is a device on a camera that a photographer uses to determine exactly where the camera is pointed, and approximately how much of that view will be photographed. A viewfinder can be mechanical (indicating only direct ...
instrument to help select observing targets for the other instruments on board SOHO, but EIT is credited with a good fraction of the original science to come from SOHO, including the first observations of traveling wave phenomena in the corona, characterization of
coronal mass ejection
A coronal mass ejection (CME) is a significant ejection of plasma mass from the Sun's corona into the heliosphere. CMEs are often associated with solar flares and other forms of solar activity, but a broadly accepted theoretical understandin ...
onset, and determination of the structure of
coronal hole
Coronal holes are regions of the Sun's corona that emit low levels of ultraviolet and X-ray radiation compared to their surroundings. They are composed of relatively cool and tenuous plasma (physics), plasma permeated by magnetic fields that are o ...
s. Before mid-2010, EIT obtained an Fe XII (19.5 nm wavelength) image of the Sun about four times an hour, around the clock; these were immediately uplinked as time-lapse movies to th
SOHO web sitefor immediate viewing by anyone who is interested. (Since the summer of 2010, when Thorpe commissioning of the
Solar Dynamics Observatory was completed, its
Atmospheric Imaging Assembly has been able to take much higher resolution solar images much more frequently. The white-light coronagraphs on
SOHO
SoHo, short for "South of Houston Street, Houston Street", is a neighborhood in Lower Manhattan, New York City. Since the 1970s, the neighborhood has been the location of many artists' lofts and art galleries, art installations such as The Wall ...
are thus able to take images more frequently: they share a CPU and telemetry bandwidth with EIT. The images are used for long-duration studies of the Sun, for detailed structural analyses of solar features, and for real-time
space weather
Space weather is a branch of space physics and aeronomy, or heliophysics, concerned with the varying conditions within the Solar System and its heliosphere. This includes the effects of the solar wind, especially on the Earth's magnetosphere, ion ...
prediction by the
NOAA
The National Oceanic and Atmospheric Administration (NOAA ) is an American scientific and regulatory agency charged with forecasting weather, monitoring oceanic and atmospheric conditions, charting the seas, conducting deep-sea exploratio ...
Space Weather Prediction Center.
Technology
EIT is the first long-duration instrument to use
normal incidence multilayer
coated optics to image the
Sun
The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
in
extreme ultraviolet
Extreme ultraviolet radiation (EUV or XUV) or high-energy ultraviolet radiation is electromagnetic radiation in the part of the electromagnetic spectrum spanning wavelengths shorter than the hydrogen Lyman-alpha line from 121 nm down to ...
. This portion of the spectrum is extremely difficult to reflect, as most
matter
In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic pa ...
absorbs the light very strongly. Conventionally these wavelengths have been reflected either using
grazing incidence (as in a
Wolter telescope for imaging
X-rays
An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...
) or a
diffraction grating
In optics, a diffraction grating is an optical grating with a periodic structure that diffraction, diffracts light, or another type of electromagnetic radiation, into several beams traveling in different directions (i.e., different diffractio ...
(as in the jocularly-termed
overlappograph flown on
Skylab
Skylab was the United States' first space station, launched by NASA, occupied for about 24 weeks between May 1973 and February 1974. It was operated by three trios of astronaut crews: Skylab 2, Skylab 3, and Skylab 4. Skylab was constructe ...
in the mid-1970s). Modern
vacuum deposition technology allows mirrors to be coated with extremely thin layers of nearly any material. The multilayer mirrors in an EUV telescope are coated with alternate layers of a light "spacer" element (such as
silicon
Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...
) that absorbs EUV light only weakly, and a heavy "scatterer" element (such as
molybdenum
Molybdenum is a chemical element; it has Symbol (chemistry), symbol Mo (from Neo-Latin ''molybdaenum'') and atomic number 42. The name derived from Ancient Greek ', meaning lead, since its ores were confused with lead ores. Molybdenum minerals hav ...
) that absorbs EUV light very strongly. Perhaps 100 layers of each type might be placed on the mirror, with a thickness of around 10
nm each. The layer thickness is tightly controlled, so that at the desired wavelength, reflected photons from each layer interfere constructively. In this way, reflectivities of up to ~50% can be attained.
The multilayer technology allows conventional telescope forms (such as the
Cassegrain or
Ritchey–Chrétien designs) to be used in a novel part of the spectrum. Solar imaging with multilayer EUV optics was pioneered in the 1990s by the
MSSTA
The Multi-Spectral Solar Telescope Array (MSSTA) was a sounding rocket payload built by Professor Arthur B. C. Walker Jr. at Stanford University in the 1990s to test ultraviolet, EUV/XUV imaging of the Sun using normal incidence EUV-reflective m ...
and
NIXT
The NIXT, or Normal Incidence X-ray Telescope, was a sounding rocket payload flown in the 1990s by Professor Leon Golub (astrophysicist), Leon Golub of the Smithsonian Astrophysical Observatory, to prototype normal-incidence (conventional) optical ...
sounding rocket
A sounding rocket or rocketsonde, sometimes called a research rocket or a suborbital rocket, is an instrument-carrying rocket designed to take measurements and perform scientific experiments during its sub-orbital flight. The rockets are often ...
s, each of which flew on several five-minute missions into space. Multilayer EUV optics are also used in terrestrial
nanolithography
Nanolithography (NL) is a growing field of techniques within nanotechnology dealing with the engineering (patterning e.g. etching, depositing, writing, printing etc) of Nanometre, nanometer-scale structures on various materials.
The modern term r ...
rigs for fabrication of
microchip
An integrated circuit (IC), also known as a microchip or simply chip, is a set of electronic circuits, consisting of various electronic components (such as transistors, resistors, and capacitors) and their interconnections. These components a ...
s.
The EIT detector is a conventional
CCDs that are back-illuminated and specially thinned to admit the EUV photons. Because the detector is about equally sensitive to EUV and visible photons, and the Sun is about one
billion
Billion is a word for a large number, and it has two distinct definitions:
* 1,000,000,000, i.e. one thousand million, or (ten to the ninth power), as defined on the short scale. This is now the most common sense of the word in all varieties of ...
(10
9) times brighter in visible light than in EUV, special thin foil filters are used to block the visible light while admitting the EUV. The filters are made of extremely thin
aluminum foil, about 200 nm (0.2 micrometre) thick, and transmit about half of the incident EUV light while absorbing essentially all of the incident visible light.
History
EIT was a difficult sell to the scientific funding agencies, as it was not clear in the early 1990s that simple imaging of the corona would be scientifically useful (most of the other instruments on board SOHO are
spectrographs of various kinds). The EIT
PI,
Jean-Pierre Delaboudiniere, was forced to scrounge funding and resources from several locations to construct and launch the instrument. For example, EIT alone of the SOHO instruments does not have its own
flight computer
A flight computer is a form of slide rule used in aviation and one of a very few analog computers in widespread use in the 21st century. Sometimes it is called by the make or model name like E6B, CR, CRP-5 or in German, as the ''Dreieckrechner' ...
; it is connected to the
LASCO instrument flight computer, and is treated operationally as an additional LASCO camera. No funding was available for a pointing adjustment mechanisms, so EIT is bolted directly to the spacecraft and hence forms the SOHO pointing reference: the other instruments all align themselves to the EIT images. Focus adjustment is achieved by
thermal expansion
Thermal expansion is the tendency of matter to increase in length, area, or volume, changing its size and density, in response to an increase in temperature (usually excluding phase transitions).
Substances usually contract with decreasing temp ...
: the internal survival heaters (found in most spaceborne instruments) are used to achieve microscopic changes in the size of the telescope structure and hence the mirror spacing. EIT was originally allocated only about 1 kbit/s of data—about the same speed as a 110
baud
In telecommunications and electronics, baud (; symbol: Bd) is a common unit of measurement of symbol rate, which is one of the components that determine the speed of communication over a data channel.
It is the unit for symbol rate or modulat ...
teletype
A teleprinter (teletypewriter, teletype or TTY) is an electromechanical device that can be used to send and receive typed messages through various communications channels, in both point-to-point and point-to-multipoint configurations.
Init ...
—but after its utility became clear much more
telemetry
Telemetry is the in situ collection of measurements or other data at remote points and their automatic transmission to receiving equipment (telecommunication) for monitoring. The word is derived from the Greek roots ''tele'', 'far off', an ...
bandwidth
Bandwidth commonly refers to:
* Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range
* Bandwidth (computing), the rate of data transfer, bit rate or thr ...
was allocated to it.
Related instruments
The technology in EIT is based on prototype instruments that were flown on the
sounding rocket
A sounding rocket or rocketsonde, sometimes called a research rocket or a suborbital rocket, is an instrument-carrying rocket designed to take measurements and perform scientific experiments during its sub-orbital flight. The rockets are often ...
payloads
MSSTA
The Multi-Spectral Solar Telescope Array (MSSTA) was a sounding rocket payload built by Professor Arthur B. C. Walker Jr. at Stanford University in the 1990s to test ultraviolet, EUV/XUV imaging of the Sun using normal incidence EUV-reflective m ...
and
NIXT
The NIXT, or Normal Incidence X-ray Telescope, was a sounding rocket payload flown in the 1990s by Professor Leon Golub (astrophysicist), Leon Golub of the Smithsonian Astrophysical Observatory, to prototype normal-incidence (conventional) optical ...
. The first multilayer telescope to image the full disk of the Sun in EUV was flown by
A.B.C. Walker and team in 1987. The
TRACE,
STEREO
Stereophonic sound, commonly shortened to stereo, is a method of sound reproduction that recreates a multi-directional, 3-dimensional audible perspective. This is usually achieved by using two independent audio channels through a configurat ...
, and
Proba-2 spacecraft (launched in 1998, 2006, and 2009, respectively) carry similar multilayer imagers, as does the
Solar Dynamics Observatory mission.
External links
Latest EIT full-field images
References
[{{cite journal, last1=Delaboudinière, first1=J.-P., last2=Artzner, first2=G.E., last3=Gabriel, first3=A.H., last4=Hochedez, first4=J.F., last5=Millier, first5=F., last6=Song, first6=X.Y., last7=and 18 others, journal=Solar Physics, date=1995, volume=162, issue=1–2, pages=291–312, doi=10.1007/BF00733432,
title=EIT: Extreme-Ultraviolet Imaging Telescope for the SOHO Mission, bibcode=1995SoPh..162..291D, hdl=2268/16267 , s2cid=121667959, url=https://apps.dtic.mil/sti/pdfs/ADA530511.pdf, archive-url=https://web.archive.org/web/20170923075405/http://www.dtic.mil/get-tr-doc/pdf?AD=ADA530511, url-status=live, archive-date=September 23, 2017]
Solar and Heliospheric Observatory
Space telescopes
Extreme ultraviolet telescopes
Telescope instruments