Sky & Telescope - January 2023 - 24

Astronomy's Workhorse
Las Campanas Observatory in Chile, which hosts a telescope
that to astronomers is almost identical to the Sloan Foundation
Telescope. Now, we enjoy a full-sky view of stars in the
Milky Way. SDSS V continues this work with the Milky Way
Mapper, which is measuring spectra using either the infrared
spectrograph from APOGEE or the optical one from BOSS -
or both - for more than 4 million stars.
The Changing Universe
Over the years, SDSS has sometimes surveyed the same
regions of sky again and again. In these data, astronomers
can look for sources that change with time, either because
they move or because their brightness changes. Repeated
measurements thus turned out to be crucial to studying the
changing universe.
Even individual SDSS images are carried out over a period
of time: During imaging, the telescope is fixed, purposefully
letting the sky drift by while its set of five filters consecutively
transmit a specific part of the spectrum, from the ultraviolet
to the infrared. These monochrome images are stitched
together to make color images, but they're actually taken at
slightly different times. Anything that's moving across the
frame ends up looking like a little traffic light in the composite
image, with blue, green, and red light slightly spread out.
Astronomers have used this signature to discover more than
100,000 asteroids as they move across the sky.
SDSS began making more deliberate movies of the sky by
repeatedly imaging narrow stripes of sky in an effort to discover
and measure the light from Type Ia supernovae, which
astronomers can use to measure large distances. SDSS has
also run programs that take multiple spectra of objects, such
as a reverberation mapping survey of distant quasars. Astronomers
gauge how quickly these spectra vary - and, thus, how
long it takes light to travel across the quasar's domain - to
reveal the mass of the supermassive black hole and the size
of the gaseous disk around it. This technique powers another
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Right Ascension
FIELD OF STREAMS In this all-sky image, which contains SDSS data through 2011, the brightness indicates the density of certain types of stars.
Color indicates distance, with blue marking stars out to 50,000 light-years away, green for stars out to 60,000 light-years, and red for stars out to
88,000 light-years. The several stellar streams visible were once dwarf galaxies or globular clusters but were torn apart by the Milky Way's gravity.
24 JANUARY 2023 * SKY & TELESCOPE
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Are you a teacher interested in incorporating SDSS
into your astronomy curriculum? Check out sdss.org/
education. In addition to providing various activity
ideas, SDSS distributes its old plug plates for free
(while supplies last) for use in education.
core component of SDSS V, Black Hole Mapper, which will
make this kind of measurement for 300,000 quasars (a big
leap over the not-quite-1,000 done to date).
The Robots Are Taking Over
I've been working within the SDSS collaboration for more
than a decade now, and one of my pet peeves has been
incorrect descriptions of SDSS as a " robotic " or " automated "
telescope. I presume this idea comes from thinking that SDSS
data are so vast and perfect that only an automaton could
have produced them. It's flattering in that way. But I also feel
it diminishes the vast scale of the human effort that went
into SDSS.
Two human observers have run every single night of spectroscopic
data-taking: one " warm " in the control room and
one " cold, " venturing outside periodically to change over the
plug plates. Those plates are flat metal circles about the size of
a coffee table, each of which is custom-drilled with holes to
hold up to about 1,000 fiber-optic cables. Celestial light travels
through each cable to the spectroscopes. To prepare for a
night's observing, multiple people throughout a dayshift load
these plates into cartridges and then plug in the hundreds of
fiber optics by hand. The all-time record was nine plates in a
single night, representing 10,000 fiber optics plugged during
the day - ouch. I once tried to plug just one plate by myself,
and it wasn't an easy job!
The funny thing about my pet peeve is that the future
of SDSS actually is robots, and the future is now. The last
observation with a plug plate at Apache Point Observatory
Declination
Palomar 5
Orphan stream
Sagittarius stream

http://www.sdss.org/education http://www.sdss.org/education

Sky & Telescope - January 2023

Table of Contents for the Digital Edition of Sky & Telescope - January 2023