Improved Ultraviolet Response, More Pixels, Expanded
Wavelength Coverage, Higher Charge Transfer Efficiency
Rates, Lower Readout Noise

SEATTLE, Washington — The world’s best planet-hunting
machine, the Keck High Resolution Echelle Spectrometer,
will be getting even better this year with an advanced
imaging array that will improve efforts to detect
extra-solar planets, examine distant quasars, measure
extragalactic stars and do other research requiring very
precise wavelength measurements of thousands of color
channels with one exposure.

HIRES, a visible-wavelength spectrometer responsible for
20 percent of the science on the Keck I telescope, will be
getting an advanced new detector that will increase photon
detection rates at ultraviolet wavelengths by a factor of
8 (at 3200 A), expand single-exposure wavelength coverage,
provide finer sampling of spectral features, and allow
faster readout rates to provide higher-quality science in
less time.

“From day-one, HIRES has been an outstanding instrument.
When it was first commissioned ten years ago there was no
other instrument like it,” said Frederic Chaffee, director
of the W.M. Keck Observatory. “This upgrade will bring
HIRES into the 21st century and keep this instrument
competitive with its peers in producing some of the
world’s best optical spectra for leading-edge science.”

The new CCD system will be a mosaic of three 2048 x 4096
CCD chips with 15-micron pixels arranged in a stacked
configuration for an image dimension of 6144 x 4096 pixels,
about 130 percent larger in area than the current CCD. The
detector will replace the current chip, which is a single
2048 x 2048 device that has larger 24-micron pixels. When
choosing a spectrograph for their science, astronomers
sometimes have to sacrifice wavelength coverage for
spectral resolution. By more than doubling the detector
size without reducing the resolution they will be able to
have their cake and eat it too.

“What we’re looking at when we’re searching for a planet
are wavelength shifts of one-thousandth of a single pixel,
less than a hundred atoms on the silicon crystal lattice
of the CCD,” said Dr. Steve Vogt, principal investigator
for HIRES and professor of astronomy and astrophysics at
UCO/Lick Observatory, University of California at Santa
Cruz. “Detecting these subtle changes — tiny fractions
of a single pixel — over time scales of many years, is
the art of planet finding.”

HIRES made history early in its commissioning when it was
able to examine objects 10 to 100 times fainter than any
other similar instrument in the world. Its spectral
capabilities enabled measurements of deuterium atoms that
provided direct evidence for a refined model of the Big
Bang theory, and it was a critical instrument in
establishing a new class of stars known as L & T dwarf
stars. Most well known is HIRES’ unique role in the
discovery and confirmation of nearly 50 extrasolar
planets, including the only known solar system other than
ours to contain a Jupiter-like planet in a Jupiter-like
orbit.

“The combination of Keck and HIRES have accounted for
nearly half of the published extrasolar planets,” said
Dr. Paul Butler, staff scientist at the Department of
Terrestrial Magnetism, Carnegie Institution of Washington.
“From glass and steel Steve Vogt has designed and built
an instrument capable of peering into the heart of
distant stars and detecting their unseen planets. With
the new CCD upgrade, the full genius of Steve Vogt’s
vision will finally be unlocked.”

Quantum efficiency rates for the ultraviolet region (the
ratio of number of photons detected to number of photons
present) are expected to rise from 8 percent to 65
percent (at 3200 A). This improvement in the ultraviolet
region will open a new window to astronomers interested
in very distant, very old extragalactic objects such as
quasars and studies relating to the structure and
characteristics of the early universe.

“This upgrade will be a dramatic improvement,” said Dr.
David Tytler, professor of physics at the Center for
Astrophysics and Space Sciences, University of California,
San Diego. “The much faster response rates will allow us
to do studies that otherwise would have been too difficult
by giving us much better data on objects that are at the
limit of what can be done today, and by allowing us to
examine many objects that are fainter than those we can
now observe. We are going to see major improvements in
the accuracy of our results, and some completely new
research programs.”

In addition to having smaller pixels (which allows finer
sampling of spectral features), the new HIRES CCDs will
have very low readout noise and very high charge transfer
efficiency (CTE). Readout-noise measures the certainty with
which a signal has been detected, and CTE is a measure of
how many electrons are “lost” as a pixel’s charge is
digitized and transferred across the CCD for readout. The
new assembly is designed for a readout noise of less than
3 electrons, compared to current noise of about five or
six electrons. HIRES will have a CTE level of at least
0.999997, which means that typically less than 1% of the
electrons will be “lost in the shuffle” across the 4096
pixels of the CCD during the readout process. New
electronics and parallel processing will provide exposure
readouts in less than 30 seconds, rather than the several
minutes it currently takes for each readout.

The HIRES upgrade will also eliminate a deficiency in the
current CCD caused by a slight curvature on the surface
of the old chip. This curvature required compromises to
be made in the optical light path. Much like the
improvements made to the Hubble Space Telescope, the new
CCD and optical prescription will bring HIRES back to
its original specifications of 12 micron FWHM resolution
images. Currently, HIRES images have a limiting
resolution of about 20 microns. “This detector upgrade
will finally enable HIRES to deliver the full power of
its original optical design,” said Vogt.

HIRES was designed and built by Steve Vogt and the
Instrument Development team at UCO/Lick Observatory,
University of California at Santa Cruz. Commissioned in
1993, the instrument is 18?feet square and weighs eight
tons, and is permanently mounted to the Nasmyth deck of
the Keck I telescope at the W.M. Keck Observatory in
Hawaii.

Funding for the W.M. Keck Observatory was provided by the
W.M. Keck Foundation. The Observatory is maintained and
operated by the California Association for Research in
Astronomy, a partnership of the California Institute of
Technology, the University of California, and the National
Aeronautics and Space Administration. For additional
information about HIRES and the W.M. Observatory, please
visit:

http://www.keck.hawaii.edu/realpublic/inst/hires/hires.html

or

http://www.keckobservatory.org/

IMAGE CAPTIONS:

[Image 1:
http://www.astro.caltech.edu/mirror/keck/news/HIRES2003/HIRESlightpathmid.jpg (69KB)]
HIRES Light path

[Image 2:
http://www.astro.caltech.edu/mirror/keck/news/HIRES2003/HIRESspectrummid.jpg (267KB)]
HIRES Spectrum