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 DR2 and
beyond which are also described in detail on that page.
We have quantified the improvements in the spectrophotometry from
DR1 to DR2 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 the spectra of hot DA white dwarfs to models.
Comparison of synthetic
r magnitudes from the spectra with
magnitudes for stars (left) and galaxies (right). We have included
all objects in DR2/DR3 with S/N per pixel >5.
Comparison of synthetic
g-r, r-i colors synthesized from the spectra with
photo fiber magnitudes. We have included all objects in
DR2/DR3 with S/N per pixel >5.
The first of the two figures above compares r fiber
magnitudes with those synthesized from the spectra of all DR2/DR3
objects with spectral S/N per pixel greater than 5. For point sources
alone, this rms difference is 0.040 magnitudes, a 45% improvement over
DR1. For extended sources, the effect of the smears was to give a
systematic offset between spectroscopic and fiber magnitudes of up to
a magnitude; with the reductions used in DR2 and beyond, this trend is
gone. The slight offset of the mean from zero is a seeing effect. We
cannot make this comparison for extended sources in DR1 because their
fluxes are calibrated to match the total light in the smear aperture
which can be greater than that in the fiber aperture by as much as a
The lower of the two figures above compares the g-r and
r-i colors derived from the spectra and photometry; the
scatter is ~40% lower in DR2 and beyond than in DR1. The few percent
offset of the colors from zero is an indication that there are small
residual errors in our spectrophotometry, perhaps due to errors in the
theoretical models used to calibrate the standard stars, or to offsets between our
photometric system and a true AB system.
To evaluate our spectrophotometry over smaller scales, of order
100Å, we compared the calibrated spectra of a sample of 166 hot DA
white dwarfs drawn from the DR1
White Dwarf Catalog (Kleinman et al. 2004) to theoretical models.
DA white dwarfs are useful for this comparison because they have
simple hydrogen atmospheres that can be accurately modeled (e.g., Finley, Koester, & Basri 1997).
The figure below shows the results of dividing each spectrum by its
best fit model. The median of the curves shows a net residual of order
2% at the bluest wavelengths. This is a major improvement over DR1
where the residuals were of order 15% at 4300Å due to the
mismatch between the observed standard stars and the assumed model.
Comparison of white dwarf spectra and models. The grey lines
represent 166 individual spectra divided by their best fit model. The
heavy red line is the median. The equivalent median residuals in
DR1 were of order 15% at 4300Å; they are now of order a
Last modified: Tue Jul 26 21:57:00 CDT 2005