Quasar Spectrophotometric Calibration


Daniel Margala
University of California, Irvine


An ancillary target program undertaken to improve the spectrophotometric calibration of BOSS quasars, by targeting standard stars with similar focal plane offsets to BOSS quasar observations

Finding Targets

An object whose ANCILLARY_TARGET2 value includes one or more of the bitmasks in the following table was targeted for spectroscopy as part of this ancillary target program. See SDSS bitmasks to learn how to use these values to identify objects in this ancillary target program.

Program (bit name) Bit number Target Description Number of Spectra Number of Unique Primary Objects
QSO_STD 20 A standard star targeted to improve BOSS spectrophotometric calibration 1,782 1,456


As described in Dawson et al. (2013) and Pâris et al. (2014), the fibers assigned to BOSS CORE and BONUS quasar targets (Ross et al. 2012) were offset in the focal plane to optimize throughput in the blue part of the spectrum, to better observe the Lyman-alpha forest. Because the standard stars were not observed with this same offset, the spectrophotometric calibration of these quasar targets is systematically incorrect.

Target Selection

The QSO_STD ancillary target flag denotes an additional sample of spectrophotometric standard stars, from 10 to 25 per plate spread evenly across the focal plane, that were drilled to follow the same offsets in the focal plane as the BOSS quasar targets. These objects were chosen using the same algorithm as for normal spectroscopic standard stars in BOSS, as explained in Dawson et al. (2013).


Dawson, K.S. et al., 2013, AJ, 145, 10

Pâris, I., et al. 2014, A&A, 563, A54

Ross, N. P., et al. 2012, ApJS, 199, 3