Targets/Samples

This page discusses how to use targeting-related tags to recover targeting decisions.

If you are curious as to why your objects of interest were observed, you are in the right place! If you are attempting to do a detailed study mapping our stars to an underlying population, some of the information here will be useful. Still, you should also review the page on Selection Bias. If why your target or sample was observed is irrelevant to your science, then carry on! The philosophy of targeting is presented on Targeting Information.

How Targeting Choices are Communicated

Many of the science goals for APOGEE require careful understanding of selection functions (see Selection Bias). In contrast, APOGEE achieves other goals via careful targeting over the entire footprint, and yet other goals require no knowledge of why a target was sampled. Thus, we maintain detailed records of how and why stars were targeted for observation. This information is encoded by several means that we summarize before discussing their use for generic science applications.

Alogrithms

Summaries of how targets are selected are given in Targeting Information with more detailed discussions and motivations given in Technical References.

Targeting Files

As detailed on Data Access, the data files from which targets are drawn, their priorities, and other relevant information are provided on the SAS for each field. The Targeting section of Data Access describes the files, their locations, and has links to the data models.

Target-by-Target information is provided in the targeting bitmasks. The target bitmasks used in APOGEE-2 are called APOGEE2_TARGET1, APOGEE2_TARGET2, and APOGEE2_TARGET3. The target bitmasks used for APOGEE-1 targets are APOGEE_TARGET1 and APOGEE_TARGET2 (and an unused one called APOGEE_TARGET3). The right-hand navigation bar has links to bitmask descriptions. The bits within the flags are not exactly aligned between the two generations of APOGEE.

Targets from APOGEE-1 will only have APOGEE1 flags and vice versa.

The tag TARGFLAGS in the summary files is a string with a list of the target flags for a given object.

Summary File Tags

Information is also encoded into the PROGRAMNAME, FIELD, TELESCOPE, and SURVEY tags available for each target in the summary files. These convey when or where the specific target was observed and its primary science program (see Targeting Information).

EXTRATARG

The EXTRATARG tag is filled during the pipeline processing to flag specific items. This is a tag created for convenience and not a funamental targeting flag. Generally, EXTRATARG helps to simplify idenficiation of key samples as a uniform flagging between APOGEE1 and APOGEE2.

If no bits in EXTRATARG are set, then this star meets the criteria for the Main Red Star Sample. It may, however, also be a special target (e.g., selected using other non-main red star sample criteria). Note that having no bits set is distinct from having bit 0 set!

Descripton of EXTRATARG
Bit Tag Bit Description
NOT_MAIN 0 Not part of the main red star sample
COMMISSIONING 1 Data obtained during instrument commissioning.
TELLURIC 2 Target selected for determining the telluric correction.
APO1M 3 NMSU-1m observation
DUPLICATE 4 Duplicate observation

Dissecting the Main Red Star Sample

The main red star sample follows a uniform set of target selection criteria, primarily relying upon photometry. As described in Targeting Information for the Main Red Star Sample, the specific criteria vary by the science goal in a particular field. Moreover, whenever there are unallocated science fibers on a given plate, these algorithms are employed to “fill” those fibers. Whether or not these details are relevant depends on your specific science goal. Here we summarize the targeting bits applied to the main sample.

Stars in the main red star sample should all have APOGEE2_NORMAL_SAMPLE set, which is apogee2_target1 bit 14.

BitMasks for the Main Red Sample
Selection Decision Bits Description
Magnitude Bin

apogee_target1
or
apogee2_target1

 11 APOGEE2_SHORT 12 APOGEE2_MEDIUM 13 APOGEE2_LONG 29 APOGEE2_FAINT_EXTRA
These bits indicate the $H$ mag limit used to select these stars within their respective structural component as described in.
Magnitude Bin

apogee2_target2

 3 APOGEE2_MOD_BRIGHT_LIMIT 16 APOGEE2_MANGA_LED
Unique to APOGEE-2. A modified bright limit indicates a star in a cohort with a bright limits of $H \ge 10$ mag rather than the standard $H \ge 7$ mag. MaNGA led fields have a brighter faint limit due to differences in dithering strategy.
$(J-K_{s})_{0}$ Bin

apogee_target1
or
apogee2_target1

 0 APOGEE2_ONEBIN_GT_0_5 1 APOGEE2_TWOBIN_0_5_TO_0_8 2 APOGEE2_TWOBIN_GT_0_8 16 APOGEE2_ONEBIN_GT_0_3
These bits indicate the (J-K$_{s}$)$_{0}$ dereddened color-selection used to select this star.
Dereddening Method
apogee_target1
or
apogee2_target1
 3 APOGEE2_IRAC_DERED 4 APOGEE2_WISE_DERED 5 APOGEE2_SFD_DERED 6 APOGEE2_NO_DERED
These bits indicate how or if $E(J-K)_{0}$ was determined. Either IRAC and WISE photometry are used with the RJCE method. If no MIR data are available, then SFD is used or no dereddening is applied. See dereddened color-selection.
The bits in apogee_target1 and apogee2_target1 are aligned.

Halo Selection

Targeting in the halo is complicated by having to look through the disk, which can dominate the star counts over the halo. Thus, in halo-focused fields, additional selection criteria are employed. These are given below.

BitMasks for Unique to the Halo Sample
Selection Decision Bits Description
Wash+D Class
apogee_target1
or
apogee2_target1
 7 APOGEE2_WASH_GIANT 8 APOGEE2_WASH_DWARF
These bits indicate the classification of the star using Wash+D photometry.
Wash+D Class
apogee2_target1
 17 APOGEE2_WASH_NOCLASS
Unique to APOGEE-2. This bit indicates that the star did not have a Wash+D classification.

Processing that creates the summary catalogs also converts many of the bitmasks into human readable tags. Similar to EXTRATARG this serves to unify key items across APOGEE1 and APOGEE2 as well as to merge information from targeting input files into the summary catalog.

Tag in Summary File Options and Description
AK_TARG_METHOD
Note: this refers to the targeting catalog. The summary catalogs may not be filled from the same input targeting catalog. Original targeting values are available in the targeting input files. Various WISE releases are documented here.
 none No dereddening SFD Using $E(B-V)$ from SFD, value given in SFD_EBV RJCE_IRAC RJCE selection using IRAC photometry RJCE_WISE_OPS RJCE selection using a pre-release WISE catalog RJCE_WISE_OPS2 RJCE selection using a pre-release WISE catalog RJCE_WISE_PARTSKY RJCE selection used a pre-release WISE catalog RJCE_WISE RJCE selection using WISE photometry RJCE_WISE_ALLSKY RJCE selection using the WISE All Sky Release
COMMISS Set to 1 for data taken during spectrograph commissioning.
 TARGFLAGS  Verbose/text form of target flags
 SURVEY
 apogee APOGEE-1 observation apo1m Observation with the NMSU 1-meter apogee-marvels APOGEE-1 observation co-observing with MARVELS apogee2 APOGEE-2 observation apogee2-manga APOGEE-2 observation co-observing with MaNGA, APOGEE-2 led. manga-apogee2 APOGEE-2 observation co-observing with MaNGA, MaNGA led.
Target Selection Mag-Color Ranges
 MIN_H Bright H limit for target selection for this object MAX_H Faint H limit for target selection for this object MIN_JK Blue (J-K) limit for target selection for this object MAX_JK Red (J-K) limit for target selection for this object
 WASH_DDO51_GIANT_FLAG  Flagged as a giant for targeting purposes based on Washington/DDO51 photometry
 WASH_DDO51_STAR_FLAG  Flagged as a star for targeting purposes based on Washington/DDO51 photometry

Potentially Useful Samples

Here we provide brief descriptions of intentional samples indicated with targeting bits that may be of use for scientific applications.

External Calibration Stars

A number of stars with published stellar parameters or chemical abundances derived from (usually optical) spectroscopy are observed in order to calibrate the ASPCAP pipeline. These targets are selected via a number of routes.

APOGEE2_TARGET2 bit 2 APOGEE2_STANDARD_STAR star with previous stellar parameters or chemical abundance measurements
APOGEE_TARGET2
or
APOGEE2_TARGET2 bit 3
APOGEE_RV_STANDARD
or
APOGEE2_RV_STANDARD
star with previous radial velocity measurements
APOGEE2_TARGET2 bit 13 APOGEE2_LITERATURE_CALIB overlap with high-resolution literature studies
APOGEE2_TARGET2 bit 5 APOGEE2_EXTERNAL_CALIB generic flag for external survey calibration, see Overlap with other Surveys for more details
APOGEE2_TARGET2 bit 22 APOGEE2_1M_TARGET Stars with $H \leq$ 7 mag observed with APOGEE-N instrument's fiber-link to NMSU's 1-meter telescope. Those stars in the calibration program have FEILD set to "calib." See Special Programs.

Internal Calibration Stars

A number of stars were intentionally observed in both APOGEE-1 and APOGEE-2 or both with APOGEE-N and APOGEE-S for the purpose of internal calibration, either to monitor instrument stability over time or provide a comparison between instruments. Stars deliberately targeted for these purposes are flagged as APOGEE2_INTERNAL_CALIB in APOGEE2_TARGET2 bit 6.

A number of stars were unintentionally observed multiple times and processed independently. These sources can be identified using EXTRATARG.

Overlap with other Surveys

APOGEE-2's footprint overlaps with those of several other stellar surveys and we intentionally target stars in common with these surveys. Stars chosen for this purpose are flagged with APOGEE2_TARGET2 bit 5, 13, 14, 15, 16, 17, and/or 18, as described further below:

BitMasks for Overlap with Other Surveys
APOGEE_TARGET1 bit 30 APOGEE_SEGUE_OVERLAP Star selected for overlap with SEGUE observations.
APOGEE2_TARGET2 bit 5 APOGEE2_EXTERNAL_CALIB General flag for external overlap.
APOGEE2_TARGET2 bit 13 APOGEE2_LITERATURE_CALIB
APOGEE2_TARGET2 bit 14 APOGEE2_GES_OVERLAP Targets intentionally common with the Gaia-ESO
APOGEE2_TARGET2 bit 15 APOGEE2_ARGOS_OVERLAP Targets intentionally common with the Abundances and Radial velocity Galactic Origins Survey (ARGOS); see Freeman et al. (2013)
APOGEE2_TARGET2 bit 16 APOGEE2_GAIA_OVERLAP
APOGEE2_TARGET2 bit 17 APOGEE2_GALAH_OVERLAP Targets intentionally common with the GALactic Archaeology with HERMES (GALAH); see Martell et al. (2016)
APOGEE2_TARGET2 bit 18 APOGEE2_RAVE_OVERLAP Targets intentionally common with the RAdial Velocity Experiment (RAVE); see Steinmetz et al. (2006)