Standards Calibration

APOGEE Standards

The standard stars for APOGEE observations are used primarily to correct telluric absorption by Earth’s atmosphere. They are selected in a process very similar to that used for APOGEE-1 and APOGEE-2 observations, which aimed to select hot, blue stars (with few absorption lines of their own) distributed as evenly as possible across the field of view to allow for spatially-dependent telluric corrections (see Zasowski et al. 2013 and Zasowski et al. 2017).

The carton ops_std_apogee contains these calibration stars, which are drawn from the 2MASS PSC catalog. They are restricted to have magnitude 7 < H < 11, color -0.25 < (J-K_s) < +0.5, and magnitude uncertainties (JERR, HERR, KERR) of ≤ 0.1 mag. In addition, we applied the requirements that the 2MASS read flag be equal to 1 or 2, the quality flag for J, H, and K_s be equal to A or B, the galaxy contamination flag be equal to 0, the confusion flag be equal to 000, the extkey ID (linking to the 2MASS Extended Source Catalog) be Null, and the star lie at least 6 arcsec from its nearest 2MASS neighbor. From this subset of potential standard stars, the five bluest sources are chosen from each HEALPix nside=128 (0.21 deg² pix^-1) as telluric standard stars.

BOSS Standards

Spectrophotometric standard stars are used by the BOSS pipeline to calibrate the absolute and relative throughput of the instrument during science observations. See the BOSS Pipeline pages for more details.

The strategy for selecting BOSS standards in SDSS-V builds upon past experience from the SDSS-III/IV BOSS and eBOSS projects. Selecting standards for the SDSS-V program presents some new challenges: i) we need to select standard stars outside the footprint of the SDSS photometric imaging catalog, and ii) we need to reliably select standards along lines of sight that may be heavily reddened by Galactic dust. In order to satisfy i) we have exploited wider area photometric/astrometric information from PanSTARRS1, legacysurvey.org/dr8 and Gaia DR2. In order to mitigate the complications of extinction, we have utilized the 3D reddening information provided in the TIC v8 catalog (Stassun et al., 2019).

A list of BOSS standard cartons released as part of DR18 (targeting generation “v0.5.3”)

• ops_std_eboss
• ops_std_boss_lsdr8
• ops_std_boss_ps1dr2
• ops_std_boss_gdr2
• ops_std_boss_tic
• ops_std_boss
• ops_std_boss_red

Selection method per carton

ops_std_eboss

This carton is identical to the spectrophotometric standards used by the eBOSS survey (Dawson et al., 2016). These standards lie in the magnitude range 16 < r_psfmag < 18 and so are most suitable for use in dark time.

ops_std_boss_ps1dr2, ops_std_boss_lsdr8, ops_std_boss_gdr2

These standard cartons are based on Panstarrs1-dr2, legacysurvey.org/dr8 and Gaia DR2 photometry respectively. For each carton, we use the empirically determined location of eBOSS standards within the dereddened color space of the new photometric system to train our new selection locus. Additional cuts on data quality and in the parallax vs magnitude plane (via Gaia DR2) are applied to further clean outliers from the sample. Magnitude limits are applied such that these standards are appropriate for BOSS dark-time observations (roughly 16 < r_psfmag < 18, or 16 < G < 18). The ops_std_boss_gdr2 carton is limited to Galactic latitudes |b|>10 degrees.

We estimate SDSS griz PSF magnitudes for each standard star target – transferring their native photometry into the SDSS system via color transforms derived from the ops_std_eboss sample.

ops_std_boss_tic, ops_std_boss, ops_std_boss_red

These additional BOSS standard cartons were developed to facilitate all-sky observations during bright time, as well as at low Galactic latitudes. These cartons fill in the regions of the sky that are not covered by the BOSS standard cartons described above, and they include brighter standards, to mitigate the potential impact of cross-talk from brighter science targets in the design. The ops_std_boss_tic carton consists of likely F stars, selected via T_eff and surface gravity cuts applied to TIC-V8 stellar parameters (Stassun et al., 2019). To provide uniform coverage over the sky, targets are sorted into a NSIDE=128 HEALPix grid (0.21 deg²/pix), and the 10 highest-gravity stars in each HEALPpix are retained for the final carton. The ops_std_boss carton similarly targets F stars, but selected via cuts on Gaia DR2 parallax, color, and absolute magnitude. Finally, the ops_std_boss_red carton is intended to provide standards even in the most heavily extincted sections of the Galactic midplane. This carton uses cuts on observed and dereddened Gaia+2MASS colors, as well as parallax, proper motion and reduced proper motion criteria to reduce contamination from non-F stars whose observed colors can be dereddened into the relevant areas of color-magnitude space.