By Alex Filippenko, UC Berkeley Professor of Astronomy, and
Linda Copman
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Photo: Keck Observatory summit technician
servicing LRIS, the Low-Resolution Imaging Spectrometer,
on the Keck I telescope. Photo by Rick Peterson.
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The physical nature of dark energy is often said to be the
number one observationally motivated, unsolved problem in all
of physics, requiring a unification of the laws of physics
to understand. Astronomers at Keck Observatory are working
to set constraints on the properties of dark energy by conducting
ever more detailed measurements of supernovae using the Keck
I Telescope and its most productive instrument, LRIS, the Low-Resolution
Imaging Spectrometer. Technical enhancements are now underway
for LRIS, with first light scheduled for January, 2009, a time
eagerly anticipated by the Keck science community.
What researchers desperately need for their observations is
an improved “red channel” CCD, or charge-coupled device, in
LRIS. A CCD is an electronic imaging detector, like the ones
found in every digital camera. All CCDs produce an image that
is somewhat corrupted by random variations in the amount of
feeble light they collect from an object, by imperfections
in the detector electronics, and by sources of light in Earth’s
atmosphere. The difference between the true image and the measured
image is called “noise,” which astronomers always want to minimize.
Detectors are at the “heart” of the LRIS instrument, limiting
the sensitivity of the instrument and the quality of the scientific
data it produces.
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| Image: Illustration showing the improved
coverage area for the new LRIS red channel detector. |
Technical enhancements planned for LRIS will provide a larger
field of view and greater clarity. Even more importantly, LRIS
will have far greater sensitivity at near-infrared wavelengths
(the “red channel”), a capability which is crucial for getting
spectra of the most distant galaxies and supernovae because
their light has been “redshifted” by the expansion of the universe.
The upgrade will also allow much more reliable subtraction
of light from the Earth’s atmosphere, thus permitting astronomers
to obtain spectra of exceedingly faint objects. This is not
currently possible with the existing LRIS.
The spectrum, or spread-out component colors of a star (think
of a rainbow produced from sunlight), indicates the surface
temperature of that star, the elements which make up the star,
and the speed at which the star is moving toward or away from
Earth. Each star has a unique spectral "fingerprint" which
helps scientists to understand a great deal about the composition
and properties of that star. Spectra of galaxies, or gravitationally
bound collections of billions of stars, provide similarly vital
information about both the galaxies themselves and the expansion
of intervening space. Spectra are so important to astronomers
that taking spectra occupies about 75 percent of the time at
most of the world's major observatories -- Keck Observatory
included.
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| Image: A spectrum of distant supernova “SN
1999ff” compared with a spectrum of the nearby supernova “SN
1989B.” The spectra are very similar, showing that the
detailed properties of the 1999 supernova are essentially
identical to those of the 1989 supernova (and to similar
supernovae which have been discovered more recently).
Though “SN 1999ff” was quite faint, this excellent spectrum
was obtained with an exposure of just one hour, using
the LRIS instrument on the Keck I telescope. Astronomers
would like to obtain such high-quality LRIS spectra of
even fainter objects, as will be possible with an upgraded
red-channel CCD. Image by Alex Filippenko and Alison
Coil. |
LRIS is the single most requested and most productive instrument
on the Keck I Telescope, as measured by the number of science
articles published annually based on observations taken with
it. LRIS is an extremely versatile instrument, because it is
equipped with a short wavelength or blue channel, as well as
a long wavelength or red channel. Stars with spectra which
peak in the blue or ultraviolet wavelengths are hotter than
the sun, and stars with spectra which peak in the red wavelengths
are cooler than the sun. LRIS has the capability to acquire
spectra or images of more than 100 astronomical targets simultaneously.
LRIS has facilitated many of the most significant astronomical
discoveries made in the past decade, including the fact that
the expansion rate of the universe is speeding up with time.
Researchers have also used LRIS to explore the sources of gamma
ray bursts and the properties of these phenomena. LRIS
has been crucial to studies of both the evolution
of galaxies and the amount of mysterious “dark matter” contained
in clusters of galaxies.
In 2002 a new blue-channel detector was installed on LRIS,
an upgrade that profoundly increased the instrument’s capabilities
in the blue and near-ultraviolet wavelengths. This permitted,
for example, much more comprehensive observations of intergalactic
gas clouds and the star-formation history of distant galaxies.
Since that time, making a comparable upgrade to the instrument’s
red-channel detector has been a high priority for the Observatory.
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| Image: Graph showing the increased photon
detection efficiency, or quantum efficiency (QE), for
the new LRIS red-channel CCD detector versus the current
detector. |
Thanks to tremendous recent advances in electronic detector
technology, advanced red-channel CCDs are now available for
LRIS — just as the quality of personal digital cameras has
grown by leaps and bounds in the past decade. These new detectors
have been developed by a visionary partnership between UC Santa
Cruz and the Lawrence Berkeley National Laboratory. The next
step is to install the new red-channel detector in LRIS, an
upgrade which requires some modifications to the existing configuration.
The new red-channel detector on LRIS will enable astronomers
to observe distant galaxies at the observable fringes of the
expanding universe. It will also allow the study of cool stars
that emit most of their radiation in the red and near-infrared
wavelengths. Finally, it will permit the study of “brown dwarfs,” very
cool objects (sometimes called “failed stars”) that are larger
than planets but smaller than normal stars. These are some
of the exciting and dynamic realms that astronomers at Keck
Observatory are waiting to explore.
“I eagerly look forward to using the upgraded
LRIS on Keck to determine the nature of dark energy and
provide clues to a unified theory of forces. It'll be a
blast!” — Alex Filippenko
Over $200,000 in funding has been received to support the LRIS
upgrade via recent grants from the Change Happens Foundation
and from private individuals. Additional philanthropic support
is needed. Contact the Office
of Advancement at Keck Observatory to learn more. 
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