APOGEE Caveats

This page relays a list of known issues in the DR13 APOGEE data release. Note that these are mostly problems or bugs, but some are issues or imperfections that are intrinsic to the data and users should be made aware. For additional discussion of certain issues, see the pages on Using APOGEE spectra, Using ASPCAP stellar parameters, and Using elemental abundances.

Effective temperature offset with metallicity

No calibration was applied to the spectroscopic effective temperatures (in DR13). Yet, it now appears that a metallicity dependent correction is warranted resulting from a comparison with photometric temperatures for low reddening stars. See
Using stellar parameters for more information.

Surface gravities inaccurate at low metallicity

The original correction applied to the spectroscopic surface gravities is likely insufficient at low metallicity ([M/H]<-1.5), leading to probable systematic offsets. Consult Using stellar parameters for more information.

Parameter/abundance issues for stars with non-standard abundance ratios

Since ASPCAP works by varying element families together, inaccuracies can occur if aberrant/non-standard element abundance ratios are present in stars. For example, the ASPCAP parameter derivation is somewhat flawed in second generation globular cluster stars, where non-standard oxygen abundances lead to systematic offsets in effective temperature and gravity, which in turn result in offsets in other quantities.

Persistent effects of persistence

While DR13 implements a rough persistence correction, there are some indications that stars whose spectra fall in the region of the chips with significant persistence are still affected, and that this can be seen in the derived abundances. Users should be wary of abundances for stars flagged as being affected by persistence.

Lingering effects of LSF

Although DR13 ASPCAP attempts to account for LSF variations by splitting the analysis into groups by mean fiber number, there are indications that the mean and scatter in abundances of stars varies at a low level (few hundredths of a dex) with the mean fiber number.

Some targets observed in multiple fields and appear as duplicates

For certain fields which overlap one another, a few stars were inadvertently targeted in both fields. Since spectra are combined only within a field, these stars appear more than once in the combined spectra and summary files/tables. When multiple combined spectra of the same object exist, the lower S/N observations all have a bit set in the EXTRATARG flag that appears in the summary allStar file and in the CAS table. The EXTRATARG bit should allow users to avoid duplicate use of the same target in analysis.

Some stars observed in multiple target classes

Some targets may have been selected independently for different programs within different visits to the same field. When we combine spectra, the target flag that we adopt for the combined spectrum is a bitwise OR of all of the target flags of the individual visit spectra. As a result, there are stellar spectra constructed from multiple visits for which a target flag bit may be set in the combined spectra, but not in all of the visit spectra that were used to construct it.

Ancillary target bit not set for two ancillary programs


Inconsistent IDs

For a few objects, alternate object/star names were used during the reduction. 2MASS identifications have been adopted in the final summary files and associated database tables, but if users want to find the individual star spectrum files (e.g., the apStar or aspcapStar files), they will need to know the star name used during the reduction, which is stored in the REDUCTION_ID tag. This is particularly true for the 1m+APOGEE observations.

Erroneous extinctions for bright stars

For very bright objects, the WISE photometry employed to generate some extinction estimates can have significant problems, which leads to estimated extinctions that may be appreciably in error. This is particularly true for the bright star sample observed with the 1m+APOGEE. Users are cautioned against using the tabulated extinctions for these stars.