SEGUE Target Selection

Using the photometry from SDSS, the SEGUE survey designed a series of color, magnitude, and proper motion criteria to identify stellar targets that were of particular interest. Each photometric target is labeled with distinctive hexadecimal codes (bitmasks) which indicate which stellar types they are associated with. For each individual stellar category, a certain number of photometric targets that fulfill the selection criteria are observed spectroscopically. For example, around 25 stars that meet the SEGUE White Dwarf criteria are observed along each line-of-sight.

When using SEGUE data for large-scale studies of Galactic structure, be sure to read Accounting for observational biases in SEGUE.

Below, we list the different selection criteria for the different programs in the SEGUE survey. The SEGUE sample consists of three different surveys, SEGUE-1, SEGUE-2, and SDSS stellar targeting originating from the SDSS Legacy Program. Each of these also consists of different observing programs, with different targeting goals and a different target-selection algorithm. For example, low latitude plates focus on K giants, with different criteria than K giants observed as part of the segue and seguefaint programs.

In addition to variation from program-to-program, the target selection algorithm for SEGUE-1 plates evolved over time. More detailed information on the evolution of target criteria is available on the Past Versions page. Here we list the most current version of the target selection algorithm. The target identification for each star listed in sppParams, SpecObjAll, and segueTargetAll is based on the latest version of the selection criteria.

Finally, the photometry and proper motions for SDSS stars has evolved over time. SEGUE-1 selected targets for spectroscopic based on a series of SEGUE imaging stripes in conjunction with proper motions from various astrometric catalogs (see Getting Started with SEGUE: Target Selection Differences and Yanny et al. 2009 for more detail). SEGUE-2 selected targets based on photometry and astrometry from SDSS DR7. These data sets have changed for DR9-DR12. Targets that may have fulfilled the criteria for a SEGUE target type in DR7 may no longer, thanks to photometric errors, among other things. Thus, there is variation in the number of targets for each category between DR7 and DR9-DR12 (due to the astrometry errors in SDSS DR8, this SEGUE data set should not be used).

Below, we detail the different targets examined in SEGUE-1, SEGUE-2, and SDSS stellar targeting originating from the SDSS Legacy Program. Each program has a table with summary information for each target type and a description of the different targeting criteria. Finally, we also include a brief summary of the calibration fiber identifications.

SEGUE-1 Target Selection

Example color-color diagram of SEGUE 1 target selection categories.
Example color-color diagram of SEGUE 1 target selection categories.

Each star observed in photometry (g < 21 or z < 21) is assigned a bitmask, segue1_target1, which indicates the different selection criteria it fulfills, regardless of whether or not it receives a SEGUE fiber. These are in segueTargetAll. There is also a segue1_target1 parameter in sppParams and SpecObjAll; however, rather than listing all of the different target categories a star belongs to, this parameter indicates why each of the observed stars received a spectroscopic fiber. For example, if a star fulfills the selection criteria of both the G-dwarf and low-metallicity categories but is assigned a fiber as a G dwarf, segueTargetAll.segue1_target1 will mark the stars as both types, whereas sppParams.segue1_target1 and specObjAll.segue1_target1 will indicate the G-dwarf category alone.

The SEGUE-1 survey consisted of a number of different programs, which utilize different target-selection algorithms. Each of the programs and their associated plates are listed here. Plates in the segue or seguefaint programs make up the majority of the observations. The segcluster, segpointed, and segpointedf plates focused on open and globular clusters and Milky Way substructure. Any additional spectroscopic fibers not used for these were assigned to stars according to the target selection categories of segue and seguefaint. For a uniform SEGUE-1 sample over a large portion of the sky, only use the plates targeted under the segue and seguefaint programs.

These target selection criteria have evolved over time. We have the latest version (v4.6) here. However, for information about how these have evolved over time, please see Past Versions.

SEGUE-1 v4.6 Target Selection: segue and seguefaint programs

SEGUE 1 Target Selection Categories: segue and seguefaint
Category Hex Integer Binary Criteria Per
LoS		 Total*
F/G 0x80000200 -2147483136 29 g < 20.2
0.2 < (g-r) < 0.48		 50 55189
Main Sequence/White dwarf pairs** 0x80001000 -2147479552 212 15.0 < g < 20
(u-g) < 2.25
-0.2 < (g-r) < 1.2
0.5 < (r-i) < 2.0
(g-r) > -19.78(r-i)+11.13
(g-r) < 0.95(r-i)+0.5
(i-z) > 0.5 for (r-i) > 1
(i-z) > 0.68(r-i)-0.18 for (r-i) ≥ 1		 5 338
A/Blue Horizontal Branch 0x80002000 -2147475456 213 g < 20.5
0.8 < (u-g) < 1.5
-0.5 < (g-r) < 0.2
u detection
Color-gravity index condition		 150 15626
K giants (l and red) 0x80004000 -2147467264 214 r < 20.2
0.35 < (g-r) < 0.8
0.15 < (r-i) < 0.6
l-color > 0.07
PMtotal < 11 mas/yr
good PM		 70 17924
K dwarf 0x80008000 -2147450880 215 14.5 < r < 19.0
0.55 < (g-r) < 0.75		 95 28580
Low Metallicity 0x80010000 -2147418112 216 14 < r < 19.0
-0.5 < (g-r) < 0.75
0.6 < (u-g) < 3.0
l-color > 0.135
u detection		 150 21741
Cool White Dwarf 0x80020000 -2147352576 217 14.5 < r < 20.5
(g-i) > -2
(g-i) < 1.7 if no neighbor with g > 22 within 7''
Otherwise, (g-r) < 0.12
good PM
Hg > 16.05+2.9(g-i)
Hg > 17.5		 10 789
G dwarf 0x80040000 -2147221504 218 14.0 < r < 20.2
0.48 < (g-r) < 0.55		 375 43339
White Dwarf 0x80080000 -2146959360 219 g < 20.3
-1 < (g-r) < -0.2
-1 < (u-g) < 0.7
(u-g) + 2(g-r) < -0.1		 25 2997
Metal-poor main sequence turnoff, subdwarfs 0x80100000 -2146435072 220 g < 20.3
-0.7 < P1(s) < -0.25
0.4 < (u-g) < 1.4
0.2 < (g-r) < 0.7		 200 35617
L, T brown dwarfs 0x80200000 -2145386496 221 z < 19.5
(i-z) > 1.7
u > 21
g > 22
r > 21		 5 59
M subdwarfs 0x80400000 -2143289344 222 14.5 < r < 19.0
(g-r) > 1.6
0.95 < (r-i) < 1.3		 5 238
AGB 0x80800000 -2139095040 223 14.0 < r < 19.0
2.5 < (u-g) < 3.5
0.9 < (g-r) <1.3
s-color < -0.06
u detection		 10 198
Extreme M subdwarfs* 0x81000000 -2130706432 224 (r-i) < 0.787(g-r)-0.356
(r-i) < 0.9
Hr >17
1.8 < (g-i) < 2.4		 40 596

*This is the number of unique SEGUE-1 observations of this type on segue or seguefaint plates. We use the bitmasks from the DR9 segueTargetAll table. Individual stars can belong to multiple categories.
**All magnitudes for MS/WD pairs are the reddened values
Notes:

  • All ugriz magnitudes listed below are dereddened PSF mags, unless otherwise specified. This means that a check of objc_flag[2] indicates the target has 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.
  • Proper motions have been included for objects which have matches to the USNO-B catalog and are used in various selection categories. Many criteria state "good PM." This uses parameters from the Proper Motions catalog and is defined below:
    • Match = 1
    • dist22 > 7.0 arcsec (i.e., the nearest neighbor brighter than 22nd magnitude is more than 7'' away)
    • pmSigmaRa < 525 mas/yr
    • pmSigmaDec < 525 mas/yr
    • nFit = 6 (nFit is the number of measurements used in the proper motion fit)
  • Low Latitude Algorithms implemented:
    • AGB type objects at low latitude
    • K III type objects at low latitude
    • Bluetip objects at low latitude
    • AMNH high proper motion objects at low latitude
  • Principal component color definitions:
    • l-color = -0.436u + 1.129g - 0.119r - 0.574i + 0.1984 and is a photometry metallicity indicator for stars in the color range 0.5 < (g-r) < 0.8.
      Reference: Lenz et al. (1998)
    • s-color = -0.249u + 0.794g - 0.555r + 0.234
    • P1(s) = P1 = 0.91(u-g) + 0.415(g-r) -1.280
      Reference: Helmi et al. (2003)
    • Hg = g + 5 + 5log10(PM (arcsec/year))
    • Hr = r + 5 + 5log10(PM (arcsec/year))

SEGUE-1 v4.6 Target Selection: seglowlat and seglowlatf programs

SEGUE 1 Target Selection Categories: seglowlat
Category Hex Integer Binary Criteria Per
LoS		 Total*
Low latitude K giant 0x80000400 -2147482624 210 0.55 < (g-r) < 0.9
g < 19
PMtotal < 11 mas/yr		 300 2699
Low latitude Blue Stellar Locus 0x80000800 -2147481600 211 (g-r) < 0.25 800 1940
Low latitude AGB/M giants 0x88000000 -2013265920 227 s-color > -0.13
3.5 > (u-g) > 2.6
0.8 < (g-r) <1.3		 50 115

*This is the number of unique SEGUE-1 observations of this type on seglowlat plates. We use the bitmasks from the DR9 segueTargetAll table. Individual stars can belong to multiple categories.

SEGUE-1 v4.6 Target Selection: segcluster, segclusterf

A number of SEGUE plates (mostly in SEGUE-1, but also in SEGUE-2) specifically targeted known globular or open clusters. Since these clusters are generally quite bright, the exposure times of these plates were often shorter than the normal 45 min to 2 hours exposure time, in some cases as short as two minutes. Such a short exposure time can result in lack of sky signal, and the wavelength calibration for these plates as well as the spectrophotometric flux calibration must be carefully be done by hand. While we have taken many precautions in getting these calibrations correct, users are cautioned to regard these with care. The spectra and measured SSPP output parameters from these cluster stars, combined with their ugriz or other photometry is the basis for putting the log(g), [Fe/H] and Teff systems on uniform system. The list of plates targeted in this fashion with their target cluster is here.

SEGUE-1 v4.6 Target Selection: segpointed and seguepointedf

The segpointed and segpointedf programs focused on regions of the sky with probably Milky Way substructure. Any spectroscopic fibers not devoted to the substructure were assigned to targets using the selection criteria of segue and seguefaint. Just like the segcluster plates, we caution that these plates should not be used when composing a uniform SEGUE sample over the sky. The list of plates targeted in this fashion is here.

SEGUE-1 v4.6 Target Selection: segtest,segtestf

These plates were done to test the S/N and other constraints for the SEGUE target selection algorithm. While the spectra from these plates are fine, these targets should not be included in larger SEGUE samples. The list of plates targeted in this fashion is here.

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SEGUE-2 Target Selection

The SEGUE-2 survey consisted only of the segue2 program. This focused on different target categories than the various programs in SEGUE-1. In contrast to SEGUE-1 stars, the parameter segue2_target1 in sppParams and SpecObjAll identifies all of the different stellar categories each star fulfills at the time of target selection (e.g., using DR7 photometry). It does not exclusively reflect why a star received a fiber. segueTargetAll.segue2_target1 lists all of the different SEGUE-2 categories each photometric target (g < 21 or z < 21) fulfills in DR9 data, regardless of whether it was observed spectroscopically or as part of SEGUE-1. SEGUE-2 plates are listed here.

SEGUE-2 Target Selection

Hex and Decimal values for segue2_target1 bitmaps
Category Hex Integer Binary Criteria Per
LoS		 Total*
MS turnoff 0x80000001 -2147483647 20 18 < g < 19.5
0.1 < (g-r) < 0.48
psfmagerr(g,r,i) < 0.05		 100 35466
Red K giants 0x80000002 -2147483646 21 15.5 < g < 18.5
r > 15
psfmagerr(g,r,i) < 0.05
good PM
0.8 < (g-r) < 1.3
(u-g) > 0.84(g-r)+1.758
(u-g) < 2.4(g-r)+0.73
PMtotal < 11 mas/yr		 < 50 805
l-color K giants 0x80000004 -2147483644 22 15.5 < g < 18.5
r > 15
psfmagerr(g,r,i) < 0.05
good PM
0.7 < (u-g) < 3
0.5 < (g-r) < 0.8
l-color > 0.09
0.1 < (r-i) < 0.6
PMtotal < 11 mas/yr		 100 16016
Proper motion only K giants 0x80000008 -2147483640 23 15.5 < g < 18.5
r > 15
psfmagerr(g,r,i) < 0.05
good PM
0.8 < (u-g) < 1.2
PMtotal < 7 mas/yr
(u-g) > 2.375(g-r)-0.45
(u-g) < 0.84(g-r)+1.758		 150 14729
Low Metallicity 0x80000010 -2147483632 24 0.3 < (g-r) < 0.8
0.5 < (u-g) < 2.0
15.5 < g < 18
r > 15
psfmagerr(g,r,i) < 0.05
psfmagerr(u) < 0.2
l-color > 0.115
good PM		 100 17020
Hypervelocity 0x80000020 -2147483616 25 17 < g < 20
PMtotal > 8 mas/yr
good PM
Box 1 criteria or Box 2 criteria		 <5 226
Cool Extreme and Ultra Subdwarfs** 0x80000040 -2147483584 26 g > 15.5
(g-r) > 1.4
(r-i) > 0.4
15 < r < 20
(r-i) < 3.0(g-r)-3.5
PMtotal < 10 mas/yr
Hr > 10.0+2.5(g-r)
psfmagerr(r,i) < 0.05
good PM		 50 7396
M giants 0x80000080 -2147483520 27 (g-r) > 1.3
(u-g) > 1.8+0.9(g-r)
i > 14.5
15.5 < g < 19.25
PMtotal < 11 mas/yr
psfmagerr(g,r) < 0.05
good PM		 <50 344
Halo high velocity 0x80000100 -2147483392 28 17 < g < 20
0.1 < (g-r) < 0.48
Vtan > 300 km/s
Vtan/ σ(Vtan) > 3.0		 <5 3432
Blue Horizontal Branch 0x80002000 -2147475456 213 15.5 < g < 20.3
-0.5 < (g-r) < 0.1
0.8 < (u-g) < 1.5		 < 100 8602
Ultra-Cool CIA White Dwarfs 0x80020000 -2147352576 217 15.5 < r < 20.5
g > 15.5
-2.0 < (g-i) > 1.7
good PM
Hg > 17.5
Hg > 16.05+2.9(g-i)		 < 10 107

*This is the number of unique SEGUE-2 observations of this type on segue2 plates. We use the bitmasks from the DR9 segueTargetAll table. Individual stars can belong to multiple categories.

**The magnitudes used for cool extreme and ultra subdwarfs are uncorrected for extinction (i.e., reddened).

Notes:

  • All ugriz magnitudes listed below are dereddened PSF mags, unless otherwise specified. This means that a check of objc_flag[2] indicates the target has 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.
  • Proper motions have been included for objects which have matches to the USNO-B catalog and are used in various selection categories. Many criteria state "good PM." This uses parameters from the Proper Motions catalog and is defined below:
    • Match = 1
    • dist22 > 7.0 arcsec (i.e., the nearest neighbor brighter than 22nd magnitude is more than 7'' away)
    • pmSigmaRa < 525 mas/yr
    • pmSigmaDec < 525 mas/yr
    • nFit = 6 (nFit is the number of measurements used in the proper motion fit)
  • Principal component color definitions:
    • l-color = -0.436u + 1.129g - 0.119r - 0.574i + 0.1984
      and is a photometry metalicity indicator for stars in the color range 0.5 < (g-r) < 0.8.
      Reference: Lenz et al. (1998)
    • s-color = -0.249u + 0.794g - 0.555r + 0.234
    • P1(s) = P1 = 0.91(u-g) + 0.415(g-r) -1.280
      Reference: Helmi et al. (2003)
    • Hg = g + 5 + 5log10(PM (arcsec/year)) is the reduced proper motion with respect to g.
    • Hr = r + 5 + 5log10(PM (arcsec/year)) is the reduced proper motion with respect to r.
    • Vtan is the tangential velocity, computed from the measured proper motions and an estimated distance from photometric parallax.
    • σ(Vtan) is the error on the tangential velocity calculation, based on the errors of the proper motions and distances.

SDSS Stellar Targeting - Legacy

Approximately 200,000 stars were spectroscopically observed as part of the SDSS and SDSS-II Legacy program. These targets are briefly described in Yanny et al. 2009,and the target categories are explained in Stoughton et al. 2002 There is a brief description of the overall Legacy target selection algorithm here.

The table below lists the possible legacy1_target1 (located in the sppParams and SpecObjAll tables) bit settings available on each spectrum if this object was a Legacy target. Unlike SEGUE-1 and SEGUE-2, every target in the Legacy component is identified by the target type criteria it fulfills, rather than why it was assigned a fiber.

SDSS Stellar Targeting Selection Categories
Category Hex Integer Binary Color, Mag Cuts Total Fibers
ROSAT C 0x00000800 2048 211 r < 16.5
ROSAT X-ray source within 60''
(u-g) < 1.1		 8000
BHB Star 0x00002000 8192 213 -0.4 < (g-r) < 0
0.8 < (u-g) < 1.8		 19887
Carbon Star 0x00004000 16384 214 (g-r) > 0.85
(r-i) > 0.05
(i-z) > 0
(r-i) < -0.4+0.65(g-r)
(g-r) > 1.75		 4453
Brown Dwarf 0x00008000 32768 215 z < 19
psfmagerr(i) < 0.3
(r-i) > 1.8
(i-z) > 1.8		 667
Subdwarf 0x00010000 65536 216 (g-r) > 1.6
0.95 < (r-i) < 1.6
psfmagerr(g) < 0.1		 1482
Cataclysmic Variable* 0x00020000 131072 217 g < 19.5
(u-g) < 0.45
(g-r) < 0.7
(r-i) > 0.3
(i-z) > 0.4
(u-g)-1.314(g-r) < 0.61
(r-i) > 0.91
(i-z) > 0.49		 8959
Red Dwarf 0x00040000 262144 218 i < 19.5
psfmagerr(i) < 0.2
(r-i) > 1.0
(r-i) > 1.8		 14649
White Dwarf 0x00080000 524288 219 (g-r) < -0.15
(u-g)+2(g-r) < 0
(r-i) < 0
Hg > 19
Hg > 16.136 + 2.727(g-i)
(g-i) > 0		 6059
Serendipity Blue 0x00100000 1048576 220 (u-g) < 0.8
(g-r) < -0.05		 81937
Serendipity FIRST 0x00200000 2097152 221 FIRST radio source within 1.5'' 14689
Serendipity Red* 0x00400000 4194304 222 (r-i) > 2.7
(i-z) > 1.6		 4179
Serendipity Distant* 0x00800000 8388608 223 (g-r) > 2
(r-i) < 0		 11820
ROSAT E 0x08000000 134217728 227 ROSAT source within 60'' 8000
PN Star 0x10000000 268435456 228 (g-r) > 0.4
(r-i) < -0.6
(i-z) > -0.2
16 < r < 20.5		 20

*The magnitudes for these targets are not dereddened.

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Calibration Fibers in SEGUE

A number of stellar targets are assigned fibers for calibration purposes. This is indicated in the parameter segue1_target2 in the SpecObjAll and sppParams tables for SEGUE-1 targets and in segue2_target2 for SEGUE-2.

The bitmask assignments for segue1_target2 are listed below:

Hex and Integer values for segue1_target2 bitmasks
Category Hex Integer Binary
Science Target 0x40000000 1073741824 230
Reddening Standard 0x2 2 21
QA Duplicate Observation* 0x8 8 23
Sky 0x10 16 24
Spectrophotometric Standard 0x20 32 25

The bitmask assignments for segue2_target2 are listed below:

Hex and Integer values for segue2_target2 bitmasks
Category Hex Integer Binary
Light Trap 0x80000001 -2147483647 20
Reddening Standard 0x80000002 -2147483646 21
Test 0x80000004 -2147483644 22
QA Duplicate Observation* 0x80000008 -2147483640 23
Sky 0x80000010 -2147483632 24
Spectrophotometric Standard 0x80000020 -2147483616 25
Guide Star 0x80000040 -2147483584 26
Bundle Hole 0x80000080 -2147483520 27
Quality Hole 0x80000100 -2147483392 28
Hot Standard 0x80000200 -2147483136 29
Stellar Cluster Target 0x80000400 -2147482624 210
Stetson Standard 0x80000800 -2147481600 211

*QA duplicate observations are no longer marked in segue1_target2 bitmasks. These are only marked in sourcetype in SpecObjAll.

The bitmask assignments for legacy_target2 are listed below:

Hex and Integer values for legacy_target2 bitmasks
Category Hex Integer Binary
Light Trap 0x80000001 -2147483647 20
Reddening Standard 0x80000002 -2147483646 21
Test 0x80000004 -2147483644 22
QA Duplicate Observation* 0x80000008 -2147483640 23
Sky 0x80000010 -2147483632 24
Spectrophotometric Standard 0x80000020 -2147483616 25
Guide Star 0x80000040 -2147483584 26
Bundle Hole 0x80000080 -2147483520 27
Quality Hole 0x80000100 -2147483392 28
Hot Standard 0x80000200 -2147483136 29
Southern Survey Target 0x80000000 -2147483648 231

*QA duplicate observations are no longer marked in legacy_target2 bitmasks.

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