Stack of 100 DR1 central-region spectra in the rest-frame, sorted by redshift. Shown are the residuals after subtraction of the best-fitting stellar population model with Starlight. Systematic deviations from a idealized model can be seen as vertical stripes (rest-frame mismatches, e.g. imperfect stellar model or emission lines), slanted stripes (observed-frame mismatches, e.g. imperfect sky model) or general blue-to-red features (imperfect relative flux calibration with respect to SDSS).