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.

BITMASKS

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 fundamental 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

MEMBER

The MEMBER tag is a convenience flag that can be used for preliminary identification of membership for some star clusters and for nearby dwarf galaxies. This membership is determined using spatial information (located within some fixed distance of cluster center), radial velocity (within some velocity of cluster velocity), Gaia proper motion, and Gaia distance. The list of clusters and galaxies in which stars have been identified is given in the MEMBER bitmask definition.

Dedicated webpages on the SAS provide a detailed set of visualizations for clusters and dwarf spheroidal galaxies. The positions and mean RVs were taken from Holtzman et al. (2018) for the canonical APOGEE Calibration clusters, Harris (2010) for all other globular clusters, and McConnachie (2012) for dwarf satellites. Mean proper motions (and distances) were then determined for RV members and used for another cleaning pass.

These classifications are provided for convenience and may not be the best choices for a study of the clusters or dwarfs. For those science applications using DR17, we refer users to the dedicated Open Clusters VAC and for dwarfs, to focused papers (Hasselquist et al. 2021 and Shetrone et al., in prep.).

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.

Main Red Star Sample BitMasks

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.

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.

Human Readable Tags

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.

BitMasks for External Calibration Targets
Mask & Bit	Meaning	Description
`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
Mask & Bit	Meaning	Description
`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 survey
`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)