Summary

The primary data products from APOGEE are high-resolution (R~20,000), infrared-wavelength (1.5 - 1.8 micron) spectra of stars in the Milky Way.  The raw data is processed using the APOGEE Data Reduction Pipeline which produces Individual visit spectra from each night and combines them into a co-added reduced spectrum. The combined spectra are then analyzed in the APOGEE Stellar Parameters and Abundances Pipeline (ASPCAP), producing a comprehensive set of stellar parameters including temperature, surface gravity, and chemical compositions for more than 20 species of elements. Some examples of APOGEE data are shown below in Figure 1, including an HR diagram made from the allStar catalog and three example spectra of different types of stars.

 In general, one APOGEE observation (one row in the search results) corresponds to one APOGEE target, and will contain between 2-50 files corresponding to that star. The full list of products can be viewed in the "Available SDSS Data Products at MAST" table on the Data Access at MAST page of this manual. but a summary of the APOGEE products is shown in this table here:

Data Products at MAST

The SDSS APOGEE observations in MAST have the following naming scheme:

sdss_apogee_[TELESCOPE]-[FIELD]-[APOGEE_ID]

where TELESCOPE is the telescope used for observation ('apo25m', 'apo1m', or 'lco25m'), FIELD is the name of the field, and APOGEE_ID is the name of the target from the allStar catalog. The APOGEE_ID is usually the 2MASS identifier for each target, with some exceptions (for example, the asteroid "Vesta" or Cepheid variable star "RS_Ori"). An example APOGEE observation name in MAST would be, "sdss_apogee_apo25m_n7789_2m23593133+5738262". Observation names are all lowercase. Each observation represents one star, and contains the reduced combined spectrum, the individual visit spectra, and the output from the analysis pipeline for that star.

For several of the data products listed below, files with the prefix "ap-" were obtained using the APO SDSS-2.5m telescope in the northern hemisphere, and files with the prefix "as-" were obtained using the LCO Irénée du Pont 2.5m Telescope in the southern hemisphere. The file structure is otherwise identical.

More documentation on for all APOGEE data products can be found on the SDSS Using APOGEE Spectra page.

apStar files

• apStar-dr17-[STAR].fits
• asStar-dr17-[STAR].fits

The SDSS apStar files are the combined, RV-corrected spectrum from all nights of observation for a single star. Depending on the observation, anywhere between 1 to 97 single visit spectra (apVisit files) were co-added to make the apStar spectrum. These combined spectra are generated by resampling individual visits onto a common, logarithmically-spaced wavelength scale after removing each visit's derived radial velocity.  The wavelength scale is recorded in the header in standard FITS keywords, which users can access to build wavelength arrays corresponding to the stored fluxes. The apStar files contain a summary header, the combined flux array and uncertainties, radial velocity corrected and resampled visit spectra and uncertainties, pixel bit mask arrays, sky and telluric flux and uncertainties, and radial velocity data. The SDSS Data Model for the apStar files can be found here describing the file contents in more detail.

apVisit files

• apVisit-[PLATE]-[MJD]-[FIBER].fits
• asVisit-[PLATE]-[MJD]-[FIBER].fits

The apVisit files contain the individual visit spectrum for a given star from one night of observation before resampling and removal of radial velocities. The wavelength information is included as a separate wavelength array, and the spectra from each of the three ccd chips are stored in different rows in the image extensions. The apVisit files contain a summary header, the flux array and associated uncertainties, the pixel bit mask arrays, wavelength arrays, sky flux and uncertainties, telluric flux and uncertainties, and coefficients for the wavelength solution and line spread function for the relevant fiber number. The SDSS Data Model for the apVisit files can be found here.

aspcapStar files

• aspcapStar-dr17-[STAR].fits

The aspcapStar files contain the output from APOGEE analysis pipeline (ASPCAP), which includes the pseudo-continuum normalized combined spectrum and the measured parameter estimates from ASPCAP.  Like the apStar files, the spectra have been corrected for radial velocity and resampled to vacuum wavelengths. The aspcapStar files contain a summary header, the pseudo-continuum normalized combined flux and uncertainties, the best-fit model spectrum matching the best fit parameters form ASPCAP, and an ASPCAP data table containing the stellar parameter and chemical abundance measurements for each star. The SDSS Data Model for the aspcapStar files can be found here.

Catalog Files

There are currently two APOGEE catalog files available at MAST, the "allStar" and "allVisit" catalogs. 

allStar

• allStar-dr17-synspec_rev1.fits

The allStar catalog contains stellar parameters, radial velocities, chemical compositions, and observation metadata for each star observed in APOGEE. The allStar catalog contains all output information from the ASPCAP spectral analysis pipeline, including stellar parameters, chemical abundances, and uncertainties for all objects. Each row in the allStar file corresponds to one combined spectrum (apStar and ascapStar files), but some individual targets may appear more than once because ASPCAP processing is done per APOGEE field and some stars were observed in multiple fields (these are actually useful since they allow us to estimate empirical uncertainties from the scatter in measurements from different spectra). The allStar file does not contain the observed spectra or the model spectral fits, which are stored in the aspcapStar files.  View the SDSS data model for the allStar file here.

allVisit

• allVisit-dr17-synspec_rev1.fits

The allVisit catalog contains targeting information, photometry, and observation summaries for each individual visit observed in APOGEE. Each row in the allVisit catalog corresponds to one apVvisit file, and contains various summary information related to each observation, including dates, plate numbers, fiber position, and targeting information for each object. View the SDSS data model for the allVisit file here.

For Further Reading...

• APOGEE Survey Documentation
• Using APOGEE Spectra
• APOGEE Data Access
• APOGEE data on the SDSS Science Archive Server (SAS)