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
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