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The Quality of the Spectrophotometry

Obtaining accurate spectrophotometry for fiber spectra is a challenging task due to the small size of the fibers relative to the science targets. The SDSS employs some unique techniques which are described on the algorithms page. There have been substantial improvements to the algorithms which photometrically calibrate the spectra published in DR6 and beyond which are also described in detail on that page.

We have quantified the improvements in the spectrophotometry from DR5 to DR6 and beyond in two ways. The net effect of these changes to the Spectro2d pipeline is a substantial improvement in the spectrophotometric quality of both point and extended sources. We have evaluated this in two ways: by comparing r magnitudes synthesized from the spectra to photo fiber magnitudes, and by comparing eigendecompositions of quasar spectra to the original spectra.

Top 3 rows: the distribution of differences between r-band photometry synthesized from SDSS spectra, and PSF and fiber magnitudes, for stars and galaxies; results are shown for DR6 (left-hand panel) and the previous version of the calibration available in DR5 (right-hand panel). Only objects with PSF magnitude brighter than 19 are shown. The most important difference is the offset of 0.35 magnitudes between the two, due to the change in calibration from fiber to PSF photometry. Each panel includes the mean and standard deviation of the best-fit Gaussian, as well as the number of objects lying beyond 3σ (as a measure of the non-Gaussianity of the tails). Results are shown for r band, but g and i band results are very similar. Bottom 2 rows: as top 3 rows, but for the colours shown.

The median, and 25% and 75% quartiles, of the ratio of observed quasar spectra to eigenspectra fits to them. This quantifies the wavelength dependence of systematic errors in the spectrophotometric calibration. The features at Ca H and K, and at Na D, are probably due to absorption from the interstellar medium.

Last modified: Thu Jun 28 13:54:07 CEST 2007