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Special Target Selection


This page provides short summaries of the various target selection algorithms used for special plates.

During SDSS-I and SDSS-II a number of "special" plates were designed and observed, which were not part of the Legacy or SEGUE-1 surveys. These plates are designated with a special set of program names (programname in the plateX table in CAS or in the plates-dr8.fits file on SAS). In DR8 we have identified each target with a resulting spectrum using the SPECIAL_TARGET1 target bitmask (in the specObjAll table in CAS or in the specObj-dr8.fits file on SAS). Here we list the types of programs and special targets, with a short description of their goals and methods.

Special programs

The table below lists the various special programs, as well as the SPECIAL_TARGET1 flags set within each one. Note that some of the targets on each plate are "filler", sometimes using the standard Legacy target algorithms. In those cases, target bits in PRIMTARGET may be set; however there is no guarantee in those cases regarding the particular versions of target selection used or basis target set. When SPECIAL_FILLER is set, this means that the normal non-tiled targets from the legacy were included from SDSS imaging.

Program Name Description Bit names
apbias Galaxy aperture bias test plates APBIAS
commissioning Commissioning observations of stellar locus COMMISSIONING_STAR
disk Thick and thin disk stars DISKSTAR
fstars F star targets FSTAR
hyades Multi-pointing plate in Hyades HYADES_MSTAR
lowz Local galaxy clusters (Annis) LOWZ_ANNIS
lowz_loveday Photometric redshift test plate (Loveday) LOWZ_LOVEDAY
lowz_lrg Merged low-redshift galaxies and deep LRGs LOWZ_GALAXY, DEEP_GALAXY_RED, DEEP_GALAXY_RED_II, BCG
m31_fstars F-stars near M31 FSTAR, QSO_M31
msturnoff Main sequence turn-off MSTURNOFF
perseus Galaxies (and some stars) in Perseus-Pisces Cluster FSTAR, PERSEUS
photoz Photometric redshift test plate (Szalay) PHOTOZ_GALAXY, SPECIAL_FILLER
preboss Test plate in preparation for BOSS LRGs and QSOs PREBOSS_QSO, PREBOSS_LRG
premarvels_preselection Test pre-selection plate for MARVELS PREMARVELS
reddening Taurus and Orion reddening plates TAURUS_STAR, TAURUS_GALAXY
southern Merged southern Equatorial stripe targets
Double-lobed radio sources BENT_RADIO, STRAIGHT_RADIO
Faint quasars FAINT_QSO
High proper-motion stars HIPM
u-band survey U_PRIORITY, U_EXTRA, U_EXTRA2
taurus Low mass stars in Taurus TAURUS_STAR

Program Descriptions


The aperture bias plates targeted normal Legacy-type targets (marked APBIAS). The exact target algorithms used are stored in the PRIMTARGET and SECTARGET plates. However, these plates are not part of the normal Legacy survey and cannot be used for statistical analysis.

The plates were observed using multiple slightly-offset pointings in the different exposures, in an attempt to study the effects of fiber aperture bias. The resulting spectra were combined together in the reductions. For this reason, the resulting spectra are not calibratable and we have marked all these plates as "bad." However, redshifts are successfully recovered for a number of the objects on these plates.


The commissioning plates were early plates targeting the stellar locus. Point sources on the stellar locus were selected from the photometry (marked COMMISSIONING_STAR). In particular, grids of width 0.04 magnitude in the (u-g),(r-i) and (r-i),(i-z) color planes were set down, and where they existed, a single star was selected in each grid. The process was iterated until about 600 targets on each plate had been assigned. These plates are not part of any broader stellar survey, and cannot be used for statistical analysis.


The disk plates targeted stars in the thin and thick disk of our Galaxy, to study kinematics. We targeted a complete sample of bright, red stars in fifteen plates using three adjacent pointings. It provides an in situ sample of thin and thick disk stars with radial velocities, proper motions, and spectroscopic metallicity determinations, densely sampling a single line of sight out to 2 kpc above the Galactic plane.

The plates are near (l = 123°, b = -63°), target point sources, and the color and flux cuts are: i < 18.26, i-z > 0.2 (marked DISKSTAR)


F stars are numerous in the Galaxy, have sharp spectral features allowing accurate radial velocities to be measured, are approximate standard candles if the stars are on the main sequence, and are of high enough luminosity that they can be seen to great distances. This program aims to use F star radial velocities to understand the kinematics of the outer parts of the Milky Way. This program was in the Southern Galactic Cap Equatorial Stripe, but there are similar targets in other programs in the vicinity of M31, and the Perseus cluster.

This program selected point sources with -0.3 < (g-r) < 0.3 and 19.0 < g < 20.5 (marked FSTAR).


This plate targeted M-stars in the Hyades cluster (marked HYADES_MSTAR). Most fibers on the plate are extremely low signal-to-noise, but there are ten high signal-to-noise spectra of M-star targets. Note that their spectra are somewhat contaminated by emission lines from their cluster environment.


This program is part of a group of programs that selected samples of low-redshift galaxies deeper than the Legacy Main sample, based on photometric redshifts.

Extended sources were selected to satisfy the following cuts iPetro ≤ 20, iPetro + (r-i)model ≥ 17.75, and photometric redshift zp ≤ 0.17-0.19, where the last condition was chosen plate-by-plate to give enough targets to match the available number of fibers. These targets are marked LOWZ_ANNIS.


This program is part of a group of programs that selected samples of low-redshift galaxies deeper than the Legacy Main sample, based on photometric redshifts.

Extended sources were selected to satisfy the following cuts: an EDR photometric redshift from Csabai et al. (2003) > 0.003, rPetro < 20, and estimated Mr > -18. These targets are marked LOWZ_LOVEDAY.


This program carried out a survey of low-redshift galaxies to 2 magnitudes fainter than the SDSS main sample limit in order to add more low-luminosity galaxies to the sample. It also included a deeper sample of LRGs and Brightest Cluster Galaxies. The exposures here were taken much deeper than ordinary SDSS exposures.

For the low-redshift galaxies, we used photometric redshifts derived from second-order polynomial fits to observed Petrosian r magnitudes and model colors, with separate fits done in bins of model g-r color. For Chunks 45, 52, and 62, we used the SDSS EDR photometry and spectroscopy then available to derive photometric redshifts, while for Chunks 74 and 97, we were able to derive improved photometric redshifts using catalog-coadded Stripe 82 SDSS photometry, combined with all available SDSS redshift data on the Southern Equatorial Stripe as of 11 July 2003. This included much of the data taken for the express purpose of calibrating the photometric redshift relation in the SDSS photometric system.

Galaxies were then chosen for observations based on their photometric redshift zp and Petrosian magnitude rPetro. In particular, the aim was to target as complete a sample as possible for 17.77 ≤ r < 19.0 and true redshift below 0.15, and sparse samples to higher redshifts, as well as at fainter magnitudes 19.0 ≤ r < 19.5. The specific target categories, in order of highest to lowest priority for fiber assignment, are listed in the table below. As there are many more such targets than available fibers, the available targets were sampled sparsely. The sparse sampling fraction values were chosen to get reasonable distributions of objects over the target categories and to keep approximately similar target distributions from chunk to chunk, which resulted in somewhat different sampling fractions for each of the five chunks in which this algorithm was used.

Sparse sampling fractions
magnitudes redshift chunk45 chunk52 chunk62 chunk74 chunk97
17.77 ≤ r < 19.0 0.00 ≤ zp < 0.15 1.0 0.85 0.7 1.0 1.0
17.77 ≤ r < 19.0 0.15 ≤ zp < 0.20 0.15 0.1275 0.105 0.15 0.3
17.77 ≤ r < 19.0 0.20 ≤ zp < 0.25 0.15 0.1275 0.105 0.15 0.3
19.0 ≤ r < 19.5 0.00 ≤ zp < 0.15 0.25 0.2 0.150.25 0.3
19.0 ≤ r < 19.5 0.15 ≤ zp < 0.20 0.25 0.16 0.1 0.25 0.3
19.0 ≤ r < 19.5 0.20 ≤ zp < 0.25 0.25 0.18 0.150.25 0.3
17.77 ≤ r < 19.0 0.25 ≤ zp 0.015 0.015 0.0150.17 0.3
17.77 ≤ r < 19.0 zp < 0.0 0.65 0.65 0.65 0 0
19.0 ≤ r < 19.5 0.25 ≤ zp 0.005 0.005 0.0050.15 0.3
19.0 ≤ r < 19.5 zp < 0.00 1.0 1.0 1.0 0 0

A caveat to note is that chunk45 used the star/galaxy separation criteria of the SDSS photometric pipeline to select galaxies, and this resulted in noticeable contamination of stars in several of the chunk45 plates located at lower galactic latitudes. The other chunks used the star/galaxy cut employed by the SDSS main sample target selection algorithm (rPSF - rmodel ≥ 0.3 or 0.24, depending on the version of the photometric pipeline), which is more conservative for selecting galaxies.

In these same plates, we targeted LRGs with z > 0.25 which satisfied the Cut I criteria. These spectra serve two purposes: first, to obtain higher signal-to-noise ratio velocity dispersion measurements; and second, to better constrain the spectra of galaxies around the discontinuity in the targeting algorithm at z ≅ 0.4 (the distinction between Cut I and Cut II; see Eisenstein et al. 2001). In detail, in these plates some Cut II galaxies were also included (except in chunk97). The Cut I galaxies targeted for deep observations are marked DEEP_GALAXY_RED; the Cut II galaxies targeted for deep observations are marked DEEP_GALAXY_RED and DEEP_GALAXY_RED_II.

Finally, in chunk74 and chunk97, these plates also targeted brightest galaxies in clusters. While LRGs are often the brightest galaxies in their clusters, they need not be. The MaxBCG method described by Bahcall et al. (2003) searches for galaxies with the apparent magnitudes and colors of LRGs, together with a red sequence of fainter ellipticals in the vicinity (cf., Gladders & Yee 2000). The BCG program targeted BCG candidates found with this method, with estimated redshifts in the range 0.4 < z < 0.7. The MaxBCG algorithm was run on photometry derived from co-adding the detections (at the catalog level) of multiple scans of the Southern Equatorial Stripe. Such targets are marked BCG.


Near the direction of M31 we targeted F-stars and low-redshift quasars. Low-redshift quasars were targeted on the M31 imaging data using the standard quasar selection algorithm (Richards et al. 2002), but excluding the high-redshift candidates selected from the griz cube (marked QSO_M31). The confirmed quasars can be used to probe gas in the halo of M31. The plates used for this program also included F-star targets similar to those in the fstars program (marked FSTAR).

The M31 (e.g., Zucker et al. 2004a and Zucker et al. 2004b) imaging data are available in DR8.


The main-sequence turnoff program was designed to study the kinematics and metallicities of high-latitude thick disk and halo stars. One of the plates overlaps with the area of the disk program described below and uses the extra color cut i-z>0.2 to avoid overlap with that program.

One set of spectra is near (l,b) = (64°,-45°), and imposes the conditions that r < 19.15 and g-r < 0.8. The other set is near (l,b) = (114°,-62°) and imposes the additional condition that i-z > 0.2. In both cases, they targets are marked MSTURNOFF.


This program targeted low mass stars in Orion, selected by Peregrine McGehee. There were three classes of low-mass starts selected for, M1 to M3 (marked ORION_EARLY), greater than M3 (marked ORION_LATE) and candidate brown dwarfs (marked ORION_BD).

Early M stars needed to satisfy the cuts i < 19.5, r-i > 1, and i-z > 0.6.

Late M stars needed to satisfy the cuts i < 19.5, r-i > 1.2, i-z > 1.2 (and anything classified as late M-star was not classified as an early M star).

Candidate brown dwarfs needed to satisfy the cuts i < 19.0, r-i > 1.8, i-z > 1.8 (and anything classified as a candidate brown dwarf was not classified as an M star).

Additionally, standard targets were chosen here (marked SPECIAL_FILLER, with detailed target selection flags in PRIMTARGET).


This program targeted galaxies and F-stars around the Perseus-Pisces cluster. The F-star selection was the same as that in the fstars program.

Imaging scans were taken centered roughly on the Perseus cluster at z ≅ 0.018, and were used to target galaxies with a slightly deeper version of the Legacy Main Sample target selection. The double exposure plates 1665, 1666 targeted, and obtained redshifts for, approximately 400 galaxies as faint as rfiber = 18.8 in a region centered on the cluster. The majority of the galaxies are associated with the cluster, although there are 50 objects in a background overdensity at z ~ 0.05 (Brunzendorf & Meusinger 1999).

The Perseus-Pisces imaging scans covering this area are available in DR8.


This program included galaxies likely to be at higher redshifts than the Main sample for the purpose of calibrating photometric redshift techniques. The SDSS five-band photometry goes substantially fainter than does the spectroscopy, allowing photometric redshifts for vastly more objects than have spectroscopy (see, for example, Csabai et al. 2003). Calibrating the photometric redshift relation requires a training sample exploring the same range of apparent magnitudes and colors as the objects for which photometric redshifts will eventually be derived. The SDSS LRG sample (Eisenstein et al. 2001) obtains spectra for red faint (r < 19.5) galaxies; photometric redshifts of this relatively uniform population (e.g., Eisenstein et al. 2003) are fairly robust (e.g., Padmanabhan et al. 2005). The photoz program aimed to create a corresponding sample of faint blue galaxies.

The basic set of cuts applied were:

For objects that satisfied the above cuts, the quantity exp[c((g-r) - (0.40 +0.6(u-g)))] was calculated; if it was larger than a random number chosen between zero and one, the object was targeted for spectroscopy (marked PHOTOZ_GALAXY). The coefficient c=0.1411 was chosen to obtain an appropriate density of targets. Note that plates 672 and 809 have the same center, and some of the same objects were inadvertently observed twice.

Additionally, standard targets were chosen here (marked SPECIAL_FILLER, with detailed target selection flags in PRIMTARGET).


These plates were pre-BOSS plates meant to be used in testing throughput and target selection issues. Roughly speaking, the luminous red galaxy targets satisfied an early version of the LRG target selection, using preliminary calibrations of the photometry (marked PREBOSS_LRG). The quasar targets were blue point sources roughly in the color range of quasars at redshifts 2 to 3 (marked PREBOSS_QSO).


These plates were pre-MARVELS plates used for stellar pre-selection. The stars included were ones selected as suitable candidates for potential MARVELS followup (marked PREMARVELS). A number of these exposures were very short, due to the fact that we targeted rather bright stars.


The reddening program observed stars (marked TAURUS_STAR) in the Taurus and Orion regions for the purposes of testing the dust reddening law. In addition, some galaxies in these fields were targeted as well (marked TAURUS_GALAXY).


The southern program was executed on the Equatorial stripe in the Southern Galactic Cap, or Stripe 82 in SDSS parlance. It consisted of a number of different targeting classes that in fact evolved over time, as described here. In particular, there were three different chunk (or "tile run") defined over time within this program: chunk22 in 2001, chunk48 in 2002, and chunk73 in 2003. The targeting classes and details of the targeting criteria often vary among these chunks.


The taurus program targeted stars in Taurus program, mainly selected to be M-dwarfs or potential brown dwarfs using similar cuts to those in the orion program (marked TAURUS_STAR).