New Reductions of SEGUE Imaging Data and Crowded Fields
As was noted in the DR6 paper, the SDSS imaging pipeline
(photo) was designed to analyze data at high Galactic
latitudes, and is not optimized to handle very crowded fields. The
Legacy survey is restricted to high latitudes, and photo performs
adequately throughout the Legacy footprint. However, at lower
latitudes, when the density of stars brighter than r=21 grows above
5000 deg-2, the pipeline is known to fail,
as it is unable to find sufficiently isolated stars to measure an
accurate PSF, and the deblender does poorly with overly crowded
images. Many of the SEGUE scans probe these low latitudes, and we
therefore adapted an alternative stellar photometry code
called PSPhot developed by the Pan-STARRS team
(Kaiser et al. 2002;
to be used for these runs. In brief, we first
run this code, and then run photo using the list of objects
detected by PSPhot as input to help photo's object
finder in crowded regions. This approach thus provides two sets of
photometry at low latitudes.
Like e.g., DAOPHOT (
PSPhot begins with the assumption
that every object is unresolved, and therefore does a better job than
photo in crowded stellar regions. It uses an analytical
model based on Gaussians to describe the basic PSF shape, with
parameters which may vary across the field of the image to follow the
PSF variations. It also uses a pixel-based representation of the
residuals between the PSF objects and the analytical model, which is
also allowed to vary across each field. Candidate PSF stars are
selected from the collection of bright objects in the frame by
searching for a tight clump in the distribution of second moments.
After rejecting outliers, the PSF fit parameters are used to constrain
the spatial variations in the PSF model.
Unlike photo, PSPhot processes each frame separately
(without any requirement of continuity of PSF estimation across frame
boundaries), and each filter separately (without any requirement that
the list of objects between the separate filters agree). The pipeline
outputs positions and PSF magnitudes (and errors) for each detected
object; the results are found in the PsObjAll table in the CAS.
The resulting photometry is then matched between filters
using a 1" matching radius. While the estimated PSF
errors output by photo include a term from the uncertainty in
the PSF fitting, this component is not included in the errors
reported by PSPhot.
We then run photo, asking it to carry out photometry at the position
of each object detected by PSPhot, in addition to the positions of
objects photo itself detects. This allows photo to do a much better
job of distinguishing individual objects in crowded regions.
In addition, the pipeline is fine-tuned to less aggressively look for
overlap between adjacent objects, and not to give up as soon as it
does at high latitude when faced with deblending large numbers of
objects. We describe below how the photometry directly out of PSPhot,
and that from photo, compare.
The SDSS PSF photometry had an offset applied to it to make it agree
with aperture photometry of bright stars within a radius of
7.43''; this large aperture photometry was in fact
what was used by ubercalibration to tie all the photometry
(Padmanabhan et al. 2008).
In crowded regions, finding
sufficiently isolated stars to measure aperture photometry becomes
difficult. PSPhot photometry was forced
to agree with these large-aperture magnitudes for bright stars; this
was done in practice by determining, for each CCD in the imaging
camera for each run, the
average aperture correction needed to put the two on the
same system, using stars at Galactic latitude |b| > 15º, where
crowding effects are less severe.
If any part of a SEGUE imaging run extended to |b| < 25º, the
entire run was processed through the photo and PSPhot code. This
sample includes most (but not all) of the SEGUE imaging runs.
These PSPhot+photo processed runs, designated with rerun=648 in the DR7
CAS and DAS, are declared the Best reduction of these SEGUE
runs. There is also an inferior Target version of these SEGUE
runs which was used to design SEGUE spectroscopic plates; it is based
on photo alone, as the PSPhot code was unavailable at the time the
plates were designed. The Target reductions have rerun=40, and
are segregated to the SEGUETARGDR7 database.
This processing revealed a problem with photo. In crowded
regions, one cannot find sufficiently isolated stars to measure
counts through such a large aperture, and in practice, the code
corrected PSF magnitudes to an aperture photometry radius of 3.00
instead, whenever any part of a given run dipped below
|b| = 8º. Thus the aperture correction was underestimated, typically
by 0.03--0.06 mag, depending on the seeing. This was not a problem
for any of the Legacy imaging scans, but is very much an issue for
the SEGUE runs. Fortunately, there is a strong correlation, in a
given detector, between the aperture correction from a 3.00
aperture to a 7.43 aperture (as measured on high-latitude fields),
and the seeing. We therefore applied this correction after the fact
to the photo PSF, de Vaucouleurs, exponential, and model
magnitudes for all SEGUE runs affected by this problem. This was
carried out before ubercalibration, so these runs are
photometrically calibrated in a consistent way.
Comparison of photo and PSPhot Photometry
The quality of the photometry produced by PSPhot, and by photo with
the PSPhot-detected objects as input,
was evaluated by comparing the
magnitudes computed by the two methods. Within each field, we calculated the
median of the difference of PSF magnitudes for stars with
14 < u,g,r,i,z < 20. This median difference had an rms of 0.014
mag. Fields with a difference greater than 0.08 mag are suspect, and
further investigation is needed to determine which of the two
pipelines might be at fault. We followed
McGehee et al. (2005)
measure reddening-free colors of the same stars that track the stellar
Qgri = (g - r) - Egri(r - i),
Qriz = (r - i) - Eriz(i - z),
where Egri = 1.582 and Eriz = 0.987.
These are normalized to equal zero at high Galactic latitude (note
that these colors do not include the u band).
Median Qgri and
Qriz colors in each field were computed for
objects identified as stars in each field, and satisfying magnitude
and color cuts as follows: 14<(u,g,r,i,z)<20, 0.5<(u-g)<1.9,
0.0<(g-r)<1.2, -0.2<(r-i)<0.8, and -0.2<(i-z)<0.6. The Q-parameters
were found to be lower by up to 0.1 mag at low Galactic latitudes; to
remove this effect, we fit a model of a constant plus Lorentzian to
the median Q values as a function of Galactic latitude, and subtracted
it. The distributions of the Qgri and
Qriz values for both photo
and PSPhot are compared as density plots here:
The distribution of median Qgri and
Qriz parameters measuring the position of
the stellar locus within each field for the photo (left)
and PSPhot (right) photometric pipelines; zero values are
indicative of uniform photometry. Within the Galactic plane (lower
panels), the PSPhot values are more concentrated, but contain a
higher number of systematic departures from the main locus.
The PSPhot code in fact gives a tighter locus at high latitudes
as well (upper panels). Histogram equalization of the gray-scale was
used to emphasize low density regions.
From the reddenining free color equations, photometric
errors in a single filter manifest themselves differently: δ g as
a shift in Qgri, δ r as a line with
= -1/(1+Egri) = -0.35, δ i as a line
-1.07, and δ z as a shift in Qriz.
The photo data in a given field was flagged as bad when
either |Qgri| or
|Qriz| > 0.12 mag (≥5σ) as
measured from photo magnitudes, and similarly for
the PSPhot outputs. Of course, a field could be flagged as bad
in both sets of outputs. By this criterion, about 2% of the fields
processed with PSPhot were flagged bad based on
the photo outputs, and 3.6% were bad based on PSPhot
photometry. The vast majority of the flagged fields are within
15º of the Galactic plane, and essentially all the fields in
which the median difference between photo and PSPhot
photometry was greater than 0.08 mag in a given band were flagged as
bad by the Q criteria. This flag and the
Qgri and Qriz
quantities themselves can be found in the fieldQA table in the
Although more fields are flagged based on the PSPhot outputs,
the PSPhot scatter in
the ditribution plots is tighter at
both high and low Galactic latitudes than for photo.
The PSPhot stellar photometry is therefore preferred for
studies of the stellar locus (we have not fully assessed its
robustness to outliers), but comes with the caveat that fields flagged
bad should be identified in the fieldQA table and be culled.
An alternative check of SDSS photometry in dense stellar fields was carried
An et al. (2008),
who reduced the SDSS imaging
data for crowded open and globular cluster fields using the
DAOPHOT/ALLFRAME suite of programs (
At a stellar density of ∼400 stars deg-2 with r<20, they found
∼2% rms variations in the difference between photo and
DAOPHOT magnitudes in the scanning direction in all five bandpasses
(see their Figure 3). The systematic structures
are likely due to imperfect modeling of the PSFs in photo, given that DAOPHOT
magnitudes exhibit no such large variations with respect to aperture photometry.
In other words, the PSF variations were too rapid for the photo pipeline
to follow over a time scale covered approximately by one field (≈10'
or ≈54 seconds in time).
An et al. (2008)
further examined the accuracy of photo magnitudes in
semi-crowded fields using three open clusters in their sample.
Stellar densities in these fields were as much as ∼10 times
higher than those in the high Galactic latitude
fields, but photo recovered ∼80-90% of stellar objects
in the DAOPHOT/ALLFRAME catalog.
An et al. (2008)
found that these fields have
only marginally stronger spatial variations in photo magnitudes than
those at lower stellar densities.
*Text and figures on this page come from an author-created, un-copyedited
version of the SDSS Data Release 7 paper, an article submitted
to Astrophysical Journal Supplements. IOP Publishing Ltd is not responsible for any errors
or omissions in this version of the manuscript or any version derived from it. A preprint of the
DR7 paper is available from the arXiv preprint server.
Last modified: Tue Apr 15 12:00:12 CDT 2003