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Understanding SDSS-III APOGEE Infrared Spectroscopic Data

SDSS-III APOGEE infrared spectra are collected mostly for the primary scientific program, a systematic survey of all parts of the Milky Way using homogeneous selection criteria.

With the APOGEE instrument, 300 spectra (including science targets and calibration) are taken at a time. The main APOGEE survey plans to probe several hundred different locations in the sky; DR10 includes results from 170 fields. For most survey fields, multiple observations are made at different epochs to allow for the identification of binary stars through the detection of changing radial velocities. The combined spectra of the multiple observations are a primary data product.

From the combined spectra, stellar atmospheric parameters are derived using the APOGEE Stellar Parameters and Chemical Abundances Pipeline (ASPCAP). These higher level products are tabulated for each object.

Some selected APOGEE spectra.
Some selected APOGEE spectra (click for a larger image)

The Spectrograph

All previously released SDSS spectroscopic data (Data Releases 1-9), including all SEGUE and SEGUE-2 data, were taken with optical spectrographs. In Data Release 10 we release the first data from a new survey and a new instrument, the APOGEE spectrograph.

APOGEE spectra are quite different from previous SDSS spectra: They are obtained in 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 derivation of more accurate radial velocities and more precise information about the chemical compositions as well as the physical conditions prevailing in the atmospheres of the survey stars.

For more information on the APOGEE instrument, 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. Some other data have also been collected for a set of ancillary science programs. 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 plate, 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.

For APOGEE, almost all fields 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 focal plane to the pseudoslit of the spectrograph. Thus, each spectrum is also referenced by the number of the fiber (fiberID) 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 a given fiberID on different MJDs may correspond to different objects on the sky. The visit spectra for individual objects are available in apVisit FITS files.

In addition to the individual visit spectra, the APOGEE software pipeline coadds the spectra for each particular star coming from different visits to the same field; this produces 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 themselves 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

After the reduction pipeline produces combined spectra, a separate pipeline, the APOGEE Stellar Parameters and Chemical Abundances Pipeline (ASPCAP) analyzes them to derive stellar atmospheric parameters (and, eventually, individual chemical abundances; this portion of the pipeline has not been implemented for DR10 results). For more information, see the description of the ASPCAP pipeline , and, if you are interested in using the stellar parameters, the page on Using stellar parameters .