From Majewski et al. 2017, Figure 14: Portion of a raw 2D APOGEE image from observations of a bulge field. The horizontal stripes correspond to individual stellar spectra. Vertical bright bands correspond to airglow features at the same rest wavelength in each spectrum, whereas absorption features at the same horizontal position from spectrum to spectrum correspond to telluric absorption features. Also obvious are variations in the expression of stellar atmospheric absorption features from star to star, evidenced by their varying strengths due to temperature and chemical composition differences, as well as changing relative positions due to Doppler shifts. Fiber assignments were managed by color-coding fiber holes in the plugplates (see Figure 13) and the fiber optic jackets at the telescope end to correspond to stars in each field sorted into three brightness groups (bright, medium, faint). These fibers were sorted at the spectrograph slit head into a repeating pattern of faint−medium−bright−bright−medium−faint to minimize the contamination of any given spectrum by the PSF wings of a much brighter spectrum in an adjacent fiber. This management scheme gives rise to the brightness modulation pattern apparent in this image.
