Sky Residual Correction

The Sloan Digital Sky Survey (SDSS) is a fiber fed multi-object spectrograph. One of the primary reasons SDSS is fiber fed is for the multiplex advantage. The original SDSS spectrographs would simultaneously observe 640 objects, and after the Baryon-Acoustic Oscillation Spectrographic Survey (BOSS) upgrade SDSS observed 1,000 objects simultaneously. While the fibers do offer a significant multiplex advantage they introduce instrumental systematic errors into the data. The SDSS sky subtraction process, the removal of atmospheric airglow emission from astronomical spectra, is imperfect. SDSS dedicates fibers to astronomically blank patches of sky, termed sky fibers, to measure the atmospheric emission present in the science spectra. If the subtraction process was perfect the spectrum of a sky subtracted sky fiber would be zero across the bandpass, but as seen below this is not the case. The largest residual subtraction errors occur at the wavelengths of the bright OH emission lines. The residual subtraction errors are correlated across wavelengths, and consequently are well characterized by principal component analysis (PCA). Using the sky subtracted sky fibers as a training set PCA creates a basis by which to correct the sky subtraction systematic errors. The video below progresses through the sky residual correction process, and shows significant improvement at the locations of the OH emission lines.

The spectrum below is the same quasar from the sky subtraction page, https://sites.bu.edu/murdock-hart/sky-subtraction/. As the sky residual correction is applied the residual features in the quasar spectrum are reduced.

Below is another example of the sky residual correction process applied to a luminous red galaxy (LRG) spectrum.

For more detailed information on the sky residual correction process see my manuscript Sky Residual Correction, https://doi.org/10.3847/1538-3881/ab1a35.