Algorithms - Spectrophotometry
Because the SDSS spectra are obtained through 3-arcsecond fibers
during non-photometric observing conditions, special techniques must
be employed to spectrophotometrically calibrate the data. There have
been three substantial improvements to the algorithms which
photometrically calibrate the spectra (1) improved matching of
observed standard stars to models; (2) tying the spectrophotometry
directly to the r-band fiber magnitudes measured by the most recent
version of the photometric pipeline; and (3) no longer using the
"smear" exposures. A separate spectrophotometric
quality page describes how we quantify these improvements.
Analysis of spectroscopic standard stars
On each spectroscopic plate, 16 objects are targeted as
spectroscopic standards. These objects are color-selected to be F8
subdwarfs, similar in spectral type to the SDSS primary standard BD+17
4708.
The color selection of the
SDSS standard stars. Red points represent stars selected as
spectroscopic standards. (Most are flux standards; the very blue stars
in the right hand plot are"hot standards"used for telluric absorption
correction.)
The flux calibration of the spectra is handled by the Spectro2d
pipeline. It is performed separately for each of the 2 spectrographs,
hence each half-plate has its
own calibration. In the EDR and DR1 Spectro2d calibration
pipelines, fluxing was achieved by assuming that the mean
spectrum of the stars on each half-plate was equivalent to a synthetic
composite F8 subdwarf spectrum from Pickles
(1998). In the reductions included in DR2, the spectrum of each
standard star is spectrally typed by comparing with a grid of
theoretical spectra generated from Kurucz model atmospheres (Kurucz
1992) using the spectral synthesis code SPECTRUM (Gray
& Corbally 1994; Gray,
Graham, & Hoyt 2001). The flux calibration vector is derived
from the average ratio of each star (after correcting for Galactic
reddening) and its best-fit model. Since the red and blue halves of
the spectra are imaged onto separate CCDs, separate red and blue flux
calibration vectors are produced. These will resemble the throughput
curves under photometric conditions. Finally, the red and blue halves
of each spectrum on each exposure are multiplied by the appropriate
flux calibration vector. The spectra are then combined with bad pixel
rejection and rebinned to a constant dispersion.
Throughput curves for the
red and blue channels on the two SDSS spectrographs.
Note about galactic extinction correction
The EDR and DR1 data nominally corrected for galactic extinction.
The spectrophotometry in DR2 is vastly improved compared to DR1, but
the final calibrated DR2 spectra are not corrected for
foreground Galactic reddening (a relatively small effect; the median
E(B-V) over the survey is 0.034). This may be changed in
future data releases. Users of spectra should note, though, that the
fractional improvement in spectrophotometry is much greater than the
extinction correction itself.
Improved Comparison to Fiber Magnitudes
The second update in the pipeline is relatively minor: We now
compute the absolute calibration by tying the r-band fluxes of the
standard star spectra to the fiber magnitudes output by the latest
version of the photometric pipeline. The latest version now corrects
fiber magnitudes to a constant seeing of 2", and includes the
contribution of flux from overlapping objects in the fiber aperture;
these changes greatly improve the overall data consistency.
Smears
The third update to the spectroscopic pipeline is that we no longer
use the "smear" observations in our calibration. As the EDR
paper describes, "smear" observations are low
signal-to-noise ratio (S/N) spectroscopic exposures made through an
effective 5.5" by 9" aperture, aligned with the parallactic
angle. Smears were designed to account for object light excluded from
the 3" fiber due to seeing, atmospheric refraction and object
extent. However, extensive experiments comparing photometry and
spectrophotometry calibrated with and without smear observations have
shown that the smear correction provides improvements only for point
sources (stars and quasars) with very high S/N. For extended sources
(galaxies) the spectrum obtained in the 3" fiber aperture is
calibrated to have the total flux and spectral shape of the light in
the smear aperture. This is undesirable, for example, if the fiber
samples the bulge of a galaxy, but the smear aperture includes much of
its disk: 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 DR2 reductions, this trend is gone. Finally,
smear exposures were not carried out for one reason or another for
roughly 1/3 of the plates in DR2. For this reason, we do not
apply the smear correction to the data in DR2.
To the extent that all point sources are centered in the fibers in
the same way as are the standards, our flux calibration scheme
corrects the spectra for losses due to atmospheric refraction without the use of
smears. Extended sources are likely to be slightly over-corrected
for atmospheric refraction. However, most galaxies are quite
centrally concentrated and more closely resemble point sources than
uniform extended sources. In the mean, this overcorrection makes the
g-r color of the galaxy spectra too red by ~1%.
Last modified: Mon Mar 15 00:30:20 CST 2004
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