The plate list table
denotes which version of the target selection was used for each plate.
Notes at the beginning of each version of the target selection
highlight changes from previous version. In addition, there are
occasional special target selection cuts utilized for specific plates/targets.
SEGUE has as one of its goals a kinematic and stellar population study
of the high-latitude thick disk and halo of the Milky Way. The halo
is sampled sparsely with a series of seven deg2 tiles both in the
SEGUE imaging stripes and the main Legacy survey area, with centers
separated by roughly 10 deg. Each such tile is sampled with two
pointings, split at r=17.8, approximately the median target
magnitude.
Half of the ~1180 targets on each line of sight are
selected using straightforward color-color and color-magnitude cuts designed
to
sample across the main sequence from g-r = 0.75 (K-dwarfs at T
eff ~ 5000K) through the main sequence turnoff. To this sample we
add metal-poor main sequence turnoff stars selected by their blue
ugr colors, essentially an ultraviolet excess cut that is extremely
efficient at separating the halo from the thick disk near the turnoff.
At r = 19.5, the average star that makes this selection is at a
heliocentric distance of 10 kpc for [Fe/H] = -1.54. To reach to
greater distances, we use the strength of the Balmer jump to select
field blue horizontal branch (BHB) stars in the ugr color-color
diagram.
The halo BHB sample extends to distances of 40
kpc, and we select all available BHB candidates in our high-latitude
fields.
The most distant spectroscopic target category in SEGUE is halo red
giants. We select halo giant candidates by the photometric offset in
the ugr color-color diagram caused by their ultraviolet excess and
weak MgH relative to foreground disk dwarfs, augmented by a 3σ
proper motion cut using a
recalibrated version of the USNO-B catalog.
Spectroscopic identification of true giants using the methodology in
has shown that the giant selection is roughly
50% efficient at g < 17, the current limit to which we can reliably
identify giants in the spectra. The halo giant sample identified in
this way reaches distances of 40 kpc from the sun.
The spectroscopic selection also includes smaller categories of rare
but interesting objects. These include cool white dwarfs selected
with the recalibrated USNO-B reduced proper motion diagram, which can
be used to date the age of the Galactic disk, and high proper motion
targets from the SUPERBLINK
catalog, which have uncovered some of the
most extreme M subdwarfs known and aided in
the calibration of their metallicity scale using common proper motion
pairs.
All ugriz magnitudes listed below are dereddened PSF mags
objc_flag[2] check:
No bright, no edge, no bad interp, no bad deblends
For selections which involve u band, we require that
the u flags indicate a u band detection.
The 'regular SDSS bright end cut' is: No spectra of
objects with g,r or i fiber mag < 14.5 allowed.
For SEGUE v2_0 we cut on r only at rfiber < 14.5.
This resulted in fiber warnings for
red stars where r-i > 0 → i < 14.5.
The spectra were ok however and scattered light was not
harmful to adjacent fibers on the CCDs.
For SEGUE v3_0 we cut on ifiber < 14.2.
This allows bright blue things to get spectra and still avoids
putting very red bright stars on the plate to scatter.
Also, proper motions have been included for objects
which have matches to the USNO-B catalog and are used
in the K giant and Cool White dwarf selection categories.
Weighting: weight on the 'v' parameter so object with v color
of 0 is 4 times as likely to get fiber as v of 0.4.
Target count Goal: 150 per plate pair (get ~100 per pair)
Weighting: magslope 1.8 and faint plate targets sorted by
magnitude to 'pack' them, so that brightest ones targeted before
fainter ones.
Target count Goal: 150 per plate pair
For selections which involve u band, we require that
the u flags indicate a u band detection.
The 'regular SDSS bright end cut' is: No spectra of
objects with g, r or i fiber mag < 14.5 allowed.
For SEGUE v2.0 we cut on r only at rfiber < 14.5,
this resulted in "whopping" fiber warnings for
red stars where r-i > 0 → i < 14.5.
The spectra were ok however and scattered light was not
harmful to adjacent fibers on the CCDs.
For SEGUE v3_0 we are cutting on ifiber < 14.2.
This allows bright blue things to get spectra and still avoids
putting very red bright stars on the plate to scatter.
Also, proper motions have been included for objects
which have matches to the USNO-B catalog and are used
in the K giant and Cool White dwarf selection categories.
Significant changes to Cool White Dwarf, M dwarf
Addition of Brown dwarf category
Specific category changes (in bold):
White Dwarf:
g < 20.3 , -1 < g-r < -0.2, -1 < u-g < 0.7
u-g + 2(g-r) < -0.1
Weighting: magslope 1.2
Target goal: 25 per plate pair
Cool White Dwarf:
Color cuts:
14.5 < r < 20.5
-2 < g-i
g-i < 1.7 (if no neighbor with g > 22 within 7 arcsec)
otherwise g-i < 0.12
Require reliable proper motion (μ) measurement:
one-to-one match to USNO_B
good proper motion fit (small rms in each coordinate)
be detected in all 5 epochs in USNO-B
no neighbor brighter than g = 22 within 7 arcsec
(to avoid blending problems)
OR
blue enough (g-i < 0.12) that target is likely white dwarf
