APOGEE Instrument and Survey

APOGEE collects infrared spectra to carry out a systematic survey of all parts of the Milky Way using homogeneous selection criteria. With the APOGEE instrument, 300 spectra (including both science and calibration targets) are taken at a time. The APOGEE survey probed a large number of different locations in the sky; DR12 includes results for 437 fields. For most survey fields, multiple observations are made at different epochs to allow for the identification of binary stars through detection of their radial velocity variations. Combined spectra of these multiple observations are one of APOGEE’s primary data products. Stellar atmospheric parameters and chemical abundances are derived from these combined spectra using the APOGEE Stellar Parameters and Chemical Abundances Pipeline (ASPCAP).

The Spectrograph

Up until Data Release 10, all released SDSS spectroscopic data (Data Releases 1-9) — including the stellar data from the SEGUE and SEGUE-2 surveys — were taken with optical spectrographs. APOGEE spectra were first released as part of DR10, and are quite different from the optical spectra obtained with the SDSS/BOSS spectrographs. APOGEE spectra sample the infrared portion of the electromagnetic spectrum, covering wavelengths between 1.5 and 1.7 microns (in the infrared H-band). Moreover, the spectra are taken at relatively high spectral resolution — 10 times higher than the SDSS optical spectra. This higher resolution enables a more detailed look at the light emitted by stars, and allows the radial velocities, chemical compositions, and physical properties of the survey stars to be measured with greater precision and accuracy. For more information on the related instrumentation, see the detailed description of the APOGEE spectrograph.

The APOGEE Survey

The main APOGEE survey targets cool stars — particularly red giant stars — throughout the multiple components of the Milky Way: the thin disk, thick disk, bulge and halo. These targets have been selected using a simple color criterion. Other spectra collected for a set of ancillary science programs often utilize different targeting strategies. The way that the different samples have been selected is documented in a description of the target selection, which also describes how the targeting information is attached to the output data products through the use of target flags.

The APOGEE Spectra

APOGEE measures many spectra in a single observation — 300 at a time — with a unique, state-of-the-art, cryogenic, infrared spectrograph. This is done by means of a plugplate, an aluminum disk placed in the focal plane of the telescope. Each plate corresponds to a specific patch of sky and is pre-drilled with holes corresponding to the sky positions of objects in that area. Each area of the sky is covered by one or more unique plates. DR12 presents results for 750 different plugplates in 437 fields. For APOGEE, almost all plugplates are observed multiple times on separate nights to allow for the identification of binary star systems as objects that have a radial velocity that changes over time as a reflection of the binary star orbit. Each one of these observations is called a visit, and visits are identified with a plate number and the MJD (Modified Julian Date) on which that plate was observed.

Each hole on each plate corresponds to one object on the sky. Optical fibers plugged into each hole bring the light from the telescope focal plane to the (pseudo)slit of the spectrograph. Thus, each spectrum is also referenced by the number of the fiber (fiberID) at the pseudoslit with which it was collected. Plates used by the APOGEE survey have 300 fibers each. If a plate is observed on more than one MJD, in general the fibers will be replugged; thus, even for the same plugplate, a given fiberID on different MJDs may correspond to different objects on the sky and the same star will generally be observed with a different fiberID at each visit. The visit spectra for individual objects are available in apVisit FITS files.

Sample APOGEE spectra for stars of different spectral types (click for a larger image). Gaps in the spectra correspond to gaps in detector coverage.
Sample APOGEE spectra for stars of different spectral types (click for a larger image). Gaps in the spectra correspond to gaps in detector coverage.

In addition to the individual visit spectra, the sum of these spectra is available. The APOGEE software pipeline coadds the spectra for each particular star coming from different visits to the same field to produce a higher signal-to-noise spectrum of each object. These combined spectra are identified by the catalog name of each star. The APOGEE sample is selected almost exclusively from the 2MASS catalog, and thus the stellar identifications are by their APOGEE ID, which basically just encodes the position of the star on the sky from the Two Micron All-Sky Survey (2MASS). The combined spectra are a primary data product of the APOGEE survey. As described in the data access description, the combined spectra are provided in apStar files in FITS format. The image to the right shows examples of some typical APOGEE spectra for stars of different spectral types.

Because of the nature of infrared observing, various instrumental features, and the still developing state of the reduction pipeline, there are important things to be aware of when looking at the APOGEE spectra. For more information, read the page on Using APOGEE Spectra, as well as the information about the APOGEE pipeline.

APOGEE Stellar Parameters and Chemical Abundances

After the reduction pipeline produces combined spectra, a separate pipeline, the APOGEE Stellar Parameters and Chemical Abundances Pipeline (ASPCAP) analyzes them in a two step process to derive stellar atmospheric parameters and individual chemical abundances. For more information on this process, see the description of the ASPCAP pipeline. If you are interested in using the stellar parameters or individual chemical abundances, see the pages on Using Stellar Parameters  and Using Chemical Abundances.