Donald Schneider Science Watch® Newsletter Interview
For centuries astronomers have used survey
techniques to compile catalogs and inventories of
celestial objects. The greatest observers in antiquity,
Hipparchus and Ptolemy, produced the first catalogs. In
the eighteenth century Messier compiled a list of 110
nebulae, which William Herschel expanded to several
thousand, discovering Uranus serendipitously in the
process. In 1887 astronomers met in Paris to agree on a
global collaboration, the Carte du Ciel, which would
map millions of stars and galaxies. The project
continued for decades, producing weighty catalogs, but
little of astrophysical or cosmological significance;
the mere acquisition of data could not unlock the
secrets of the universe. In the 1950s the famous
Palomar Sky Survey demonstrated the superiority of
photographic deep surveys, following which the
remarkable growth of space astronomy and observational
cosmology was propelled by surveys across the entire
For centuries astronomers have used survey techniques to compile
catalogs and inventories of celestial objects. The greatest observers in
antiquity, Hipparchus and Ptolemy, produced the first catalogs. In the
eighteenth century Messier compiled a list of 110 nebulae, which William
Herschel expanded to several thousand, discovering Uranus serendipitously
in the process. In 1887 astronomers met in Paris to agree on a global
collaboration, the Carte du Ciel, which would map millions of
stars and galaxies. The project continued for decades, producing weighty
catalogs, but little of astrophysical or cosmological significance; the
mere acquisition of data could not unlock the secrets of the universe. In
the 1950s the famous Palomar Sky Survey demonstrated the superiority of
photographic deep surveys, following which the remarkable growth of space
astronomy and observational cosmology was propelled by surveys across the
entire electromagnetic spectrum.
"By some measures, the
scientific impact of the SDSS over the past
decade is comparable to or exceeds that of
the Hubble Space Telescope," says Donald
Schneider of Pennsylvania State
In the past decade, a dedicated 2.5-meter telescope at Apache Point
Observatory, New Mexico, has transformed our knowledge of the inventory of
the universe by producing the Sloan Digital Sky Survey (SDSS). The SDSS is
one of the most ambitious and influential surveys in the history of
astronomy. It has obtained deep, multicolor images covering more than a
quarter of the sky, and created three-dimensional maps containing over a
million galaxies and approximately 100,000 quasars. There is great interest
in what the SDSS data reveal about the state of the universe during the
first billion years after the Big Bang.
One member of the large SDSS team, Donald Schneider of Pennsylvania State
University, has recently distinguished himself in the Clarivate
database. Schneider currently ranks at #2 among the most-cited authors in
the Thomson Reuter's
Essential Science IndicatorsSM Space Science category,
based on papers published and cited over the last decade. He was also
featured earlier this year in the annual Science Watch roundup of
authors with multiple Hot Papers published over the last two years
Schneider received a bachelor's degree in physics and mathematics from the
University of Nebraska in 1976. He received a doctoral degree in astronomy
from the California Institute of Technology in 1982 and was a research
fellow there from 1982 to 1985. From 1985 to 1994, he was a member of the
Institute for Advanced Study in Princeton, New Jersey. He joined the
faculty at Penn State in 1994 as an associate professor of astronomy and
astrophysics, was promoted to professor in 1999, and named Distinguished
Professor in 2008.
From his Penn State
office in University Park, Schneider spoke to Science Watch
Physics correspondent Simon
What attracted you to research on high-redshift
In the late 1970s I was a graduate student at Caltech,
where my thesis adviser was James Gunn. He was one of the designers of
the first Wide-Field and Planetary Camera for the Hubble Space
Telescope, which involved the use of charge-coupled devices (CCDs),
which back then were novel. In my doctoral research I used prototype
CCDs to obtain images of clusters of galaxies, to determine if the most
luminous galaxies could be used as cosmological probes. CCDs were ideal
for that research. I finished that project in the early 1980s.
The potential of this new CCD technology fascinated Maarten Schmidt,
who in 1963 had first correctly interpreted the spectrum of the quasar
3C273 and determined its redshift (z=0.158). I was familiar with the
data-processing requirements of CCD surveys because at that time we had
to write our own software packages from first principles. I had the
good fortune to be hired by Schmidt as a postdoc, and over the next
fifteen years Jim, Maarten, and I performed a number of surveys for
high-redshift quasars—that is, with redshift larger than
four—in which Maarten was particularly interested.
How did you make those surveys?
We used a scanning technique in which the telescope is a transit
instrument, detecting objects as the rotation of the Earth sweeps them
across the field of view of the CCD. This approach allows the spectra
of a large number of objects to be obtained in a single night. I
suspect the success of this technique stimulated Jim Gunn to propose
the much bolder Sloan Digital Sky Survey. The SDSS camera, which Jim
designed, is operated in scan mode.
So, together you significantly increased the number
of high-redshift quasars?
Yes we did. Several times a year the three of us would travel to
the 5-meter telescope on Palomar, where we would scan strips of the
sky. We would then examine the data, trying to spot potential quasar
candidates, and then return to Palomar and use high-quality
spectroscopy to identify high-redshift quasars.
Would you describe the SDSS, and explain how it led
to great progress in extragalactic research?
SDSS was conceived by Jim Gunn, who has led the project
throughout its existence. Jim and the University of Chicago’s
Richard Kron and Donald York were the key people in the initial phase
of the SDSS; it has been a great privilege for me to work with them.
The scanning we’d done at Palomar suggested that much more could
be achieved with a more capable CCD camera.
We started scanning in 1984 with one CCD, then Jim developed an
instrument called the Four-Shooter for the Palomar 5-meter: it had four
highly sensitive CCDs, and our quasar survey, after several years of
effort, covered 1.5% of the sky. Jim soon envisaged a camera with 30
large CCDs that could image enormous areas of sky. SDSS was designed to
investigate extragalactic problems, so the survey area avoided the
galactic plane. In the end the SDSS covered one quarter of the sky.
The SDSS's great steps forward were obtaining digital, multicolor
images of a large area of the sky to brightness levels that were
significantly deeper than any previous survey with similar sky coverage
(for example, the Palomar Sky Survey), and high-quality spectra of more
than one million galaxies and quasars. The CCD camera was designed to
take images using five different filters from the ultraviolet to the
What was your role?
I was asked in 1990 to be the chairman of the quasar working
group; this was my primary contribution to the survey. At that time the
number of catalogued quasars was only a few thousand, so the SDSS
proposed far more than an order of magnitude increase in the quasar
count. Furthermore, the quality of the spectra we obtained was far
superior to most of the previously published quasar spectra. In 2002 I
also became the SDSS Scientific Publication Coordinator.
So, how many objects, and their spectra, can SDSS
grab on a clear night?
There are many amazing aspects of the Sloan survey. The camera is
a marvel. The telescope’s optics are extraordinary: the focal
plane has a diameter of 2.5 degrees, which is five times the apparent
diameter of the Moon.
Cited Papers by Donald P. Schneider
and Colleagues, Published Since
by total citations)
D.G. York, et al., "The
Sloan Digital Sky Survey: Technical
summary," Astronom. J.,
120(3): 1579-87, 2000.
C. Stoughton, et al.,
"Sloan Digital Sky Survey: Early
data release," Astronom.
J., 123(1): 485-548, 2002.
et al., "Cosmological
parameters from SDSS and WMAP,"
Phys. Rev. D, 69(10):
no. 103501, 2004.
D.J. Eisenstein, et al.,
"Detection of the baryon acoustic
peak in the large-scale correlation
function of SDSS luminous red
galaxies," Astrophys. J.,
633(2): 560-74, 2005.
M. Tegmark, et al., "The
three-dimensional power spectrum of
galaxies from the Sloan Digital Sky
Survey," Astrophys. J.,
606(2): 702-40, 2004.
The survey technique is firstly to do imaging, from which we identify
targets for spectroscopic investigation. Next we make an aluminum plate
about a meter in diameter that represents the 2.5-degree focal plane
view of the sky, and drill tiny holes exactly where the target objects
(quasars, galaxies, even stars) are going to appear on the focal plane.
Optical fibers are then attached to the holes; the spectrographs can
record 640 simultaneously. The fiber optics feed two double
spectrographs that obtain spectra covering the wavelength range between
380 nm and 920 nm.
Exposures are typically an hour in length, and SDSS has approximately
2,000 fields to view. On a really productive night the survey completes
several thousand spectra of extragalactic objects. The Survey
Coordinator, Steve Kent of the University of Chicago, did a masterful
job of orchestrating the interplay of the imaging and spectroscopic
observations to allow the data to be efficiently collected.
I have to relate that when I gave my first talk on performing this
quasar survey, at an astronomy meeting in 1991, initially the audience
just laughed—they did not believe we could do it!
A couple of the highly cited SDSS papers are on
quasar spectra. Why is the survey particularly valuable for
studying high-redshift objects?
My primary interest in the SDSS was its tremendous potential to
identify high-redshift quasars. When the SDSS was first proposed, there
were only about a dozen known objects with redshifts larger than four.
Luminous quasars are particularly interesting as they are thought to be
fuelled by supermassive black holes—those with a billion or more
solar masses. When we detect high-redshift quasars, we are viewing them
when the universe is less than a billion years old. What mechanism
allows such massive objects to be assembled so quickly?
High-redshift quasars are rare and faint, so, to find them, large areas
of sky needed to be scanned at high sensitivity. You also need to be
aware that when you attempt to detect high-redshift quasars, very
little of their radiation appears in the optical band; one must move to
near-infrared wavelengths to study these objects. The SDSS possessed
all three properties to find high-redshift quasars: coverage of large
area of sky, ability to detect faint objects, and imaging and
spectroscopic observations that reached to wavelengths longer than 900
nm. Our expectations in this area were exceeded: the SDSS had first
light in May 1998, and before the end of the year the SDSS team, led by
Xiaohui Fan, now at the University of Arizona, and Michael Strauss, of
Princeton University, had identified the most distant luminous quasar
(a redshift in excess of five).
The most highly cited paper from SDSS is the
technical summary, with Don York as the lead author. [Note: see
adjoining table, paper #1.] The paper describes the data products
and serves as an introduction to extensive technical online
documentation. That paper was followed by the #2 paper in the
table, on the early data release. What was the significance of
Paper #1 is the primer for the survey. It contains a description
of the survey goals, the instrumentation, and the observing
strategy—it is a must-read for anyone interested in the
possibility of using SDSS data. The early data-release paper, 108 pages
in length, describes the first public release of the SDSS data (in
2001) and contains a detailed description of the survey: e.g.,
definition of the survey coordinate system; construction of the object
catalogs; photometric and spectroscopic calibration; details of the
spectrograms; how to access the data, etc. The processing of the
imaging data was particularly challenging; this was overseen by Robert
Lupton of Princeton University. The exciting part—or the
thrilling or scary part, you might say—was this: how to create a
structure that has hundreds of gigabytes of data, and make it publicly
available in a manner that would be efficient from the point of view of
In 2001, Internet capacities were far from what they are today. We had
to anticipate the questions people would be asking of the data. Even
though the early release was only a tiny fraction of what we have now,
there was considerable concern in the team about the effectiveness of
the data-access mechanisms. The evidence suggests that we were, in
fact, very successful. If you examine the numbers of papers that are
based upon SDSS data, I believe you will find that authors who are
not members of the SDSS team are in the majority. Furthermore,
studies of scientific impact have shown that SDSS has had an enormous
influence: by some measures it is comparable to or exceeds that of the
Hubble Space Telescope over the past decade.
Four of SDSS’s top-ten most-cited papers,
including #2 in the table, describe the public release of the
data. Observational astronomers usually like to interpret their
data before releasing it publicly.
The SDSS team provides a public data release once per year. The
final SDSS-II release occurred in the fall of 2008. With each release
we publish documentation describing the data as well as any changes in
the processing software, calibration, etc., that are relevant for
interpreting the information. SDSS team members do have proprietary
access to the fully calibrated data for a few months before it is
released to the public, but the success of the survey would have been
considerably reduced if the data had not been released by the
The third-most highly cited paper, which is on the
cosmological parameters, has more than 800 citations. How is the
SDSS used as a cosmological probe?
Measurement of fundamental cosmological parameters was one of the
driving forces in the creation of the SDSS. To address many aspects of
this problem, one must assemble very large, extremely well-calibrated
databases—exactly what SDSS was designed to create. With the
positional (location on the sky) and redshift (distance from Earth)
measurements you know where all these objects are in three-dimensional
space; the SDSS allows us to view the locations of galaxies and quasars
on the largest scales. The structures we observe act as constraints on
For example, the cosmic microwave background, which in a sense is a
snapshot of the universe when it was only a few hundred thousand years
old, acts as a very important constraint on the geometry and the basic
parameters of the universe. The density perturbations in the microwave
background have characteristic length scale (often called baryonic
acoustic oscillations), which should be imprinted in the large-scale
distribution of matter. By deterFor Sloan Digital Sky Survey, Acclaim
Across the Universe this characteristic length at different redshifts,
one has a direct measurement of the expansion of the universe, which is
one of the fundamental constraints that any cosmological model must
Paper #5 in the table is on the three-dimensional
power spectrum of galaxies. How does SDSS data help us understand
the structure of the universe?
The fundamental SDSS observational quantity is a
three-dimensional map of galaxies and quasars, along with their
luminosities. We only have one universe, and since astronomy is a
passive science, with no possibility of repeat experiments, we must use
this snapshot to infer the properties that produced the observed
configuration. The time scales for the formation of stars and galaxies
are very long, so we do not observe changes in individual objects, but
because of the finite speed of light we are able to obtain views of the
universe at different times, which provides additional information to
constrain theoretical models. The intellectual challenge is to
determine the values the cosmological parameters must have to give rise
to the structures as we see them today.
Is the SDSS still in progress?
The original survey ran through to 2005; most of the originally
proposed imaging and spectroscopy had been completed at that time. For
three years starting in 2005 we had the second phase, SDSS-II, which
consisted of three projects: 1) the Legacy Survey, which completion of
the imaging and spectroscopy of the original survey; 2) a study of the
Milky Way galaxy by obtaining new imaging data, often near the Galactic
plane, and the spectra of tens of thousands of stars; and, 3) the SDSS
supernova survey, which used repeated sweeps of a 300-square degree
region of the SDSS supernovae at redshifts up to approximately 0.4.
The third phase of SDSS began in the summer of 2008. SDSS-III, guided
by Director Daniel Eisenstein of the University of Arizona, and Project
Scientist David Weinberg of Ohio State, consists of four distinct
projects, and for the first time uses the SDSS telescope at all lunar
phases: 1) an extension of the Milky Way structure survey, which will
end in July 2009; 2) a precision radial velocity survey, using an
instrument developed by Jian Ge of the University of Florida, to
monitor approximately 11,000 stars with the goal of detecting
exoplanets; 3) a survey, based upon the SDSS-I and SDSS-II imaging,
that will obtain spectra of. approximately 106 galaxies and
105 quasars with the expectation of measuring baryonic
acoustic oscillations at redshifts of 0.8 and 2.5; and 4) a Milky Way
survey using a high-resolution, infrared spectrograph to acquire
spectra of 100,000 stars to examine the chemical history of the galaxy.
SDSS continues to be incredibly productive.
Yes, and we have high hopes for increased capabilities. This
summer the spectrographs will be upgraded to improve their sensitivity
and they will also be able to obtain 1,000 spectra simultaneously in
preparation for starting the baryonic acoustic oscillation project in
late 2009. I believe the next five years will be exciting ones for the
Finally, what for you has been the most
satisfying aspect of SDSS?
Surveys provide the crucial, irreproducible archives that are
needed to understand the universe, and they often address questions
that had not even been conceived when the survey was completed. I truly
believe the SDSS observations will be used for centuries, which is
deeply satisfying. As I mentioned, surveys almost always produce
discoveries that were not anticipated. In the case of the SDSS, our
original proposal only mentioned stars briefly.
If you now examine the list of highly cited SDSS papers you'll find
several important works on stars in our galaxy (which inspired the
SDSS-II Milky Way project). The SDSS has greatly improved out
understanding of stellar streams, which are produced by tidal stripping
of dwarf galaxies that venture too close to the Milky Way. The SDSS has
also discovered a number of new dwarf companions to our galaxy. The
SDSS was not designed to address these questions about the Milky Way
and its neighborhood, so much credit must be given to the astronomers
who mined the data in an innovative way.
KEYWORDS: DONALD P. SCHNEIDER, DON SCHNEIDER, PENNSYLVANIA STATE
UNIVERSITY, PENN STATE, SLOAN DIGITAL SKY SURVEY, SDSS, ASTRONOMY,