(imaging survey of 500,000,000 objects in 35% of sky)
The Sloan Digital Sky Survey (SDSS)
can be considered a continuing series of surveys
that began with photometry and continued with spectrography
as well.
The series is named for the Alfred P. Sloan Foundation.
It images all types of astronomical objects, but a major initial goal was
redshifts of galaxies, i.e., to produce a vast 3D map.
Surveying began in 2000 and has grown to 35% sky coverage,
with photometry of over 500 million objects
and spectra of over a million.
The median redshift of the galaxies
is redshift z = 0.1, reaching as far as z = 0.7,
with quasars to redshift z = 6 and beyond.
The original survey (now called SDSS-I) produced images using
the ugriz photometric system of five passbands:
Spectrography was carried out on selected galaxies and quasars.
The survey was a milestone, providing an improved set of freely-available
all-sky research-quality astronomical data, produced using 2000s
technology, carried out with a dedicated telescope
(the Sloan 2.5m Telescope) at Apache Point Observatory (APO),
New Mexico. Subsequent efforts use additional telescopes,
including the du Pont Telescope at Las Campanas Observatory,
Chile, adding coverage of more of the southern hemisphere.
SDSS efforts:
MaNGA (Mapping Nearby Galaxies at APO) - IFU observations of 10,000 nearby galaxies, yielding a map of information for each galaxy, e.g., showing whether the SF is in the center versus in the arms.
To carry out spectrography on so many objects, for years SDSS
used a partially manual process with their multi-object spectrograph:
fibers were positioned by hand-plugging them into aluminum plates
(plug plates) with holes that were pre-drilled at positions
corresponding to locations of objects within the field of view.
The holes were drilled and their positions verified using
machine-controlled equipment.
SDSS has now replaced this procedure with robotic fiber positioners.
Spectra have been collected on millions of objects.