Calibrated object lists
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About SDSS object lists
The calibrated object lists reports positions, fluxes, and shapes
of all objects detected at >5 sigma on the survey
images. Photometry is reported on the natural system of the APO 2.5m survey telescope (a
system which includes 1.3 airmasses at APO; see description of photometric flux calibration)
in asinh magnitudes.
Getting and using object lists
You need to look at the object
flags in the object lists to obtain meaningful
results.
Calibrated object lists are stored in two file types in the Data Archive Server:
The fpAtlas*.fits
files contain "postage-stamp"
images, the set of pixels determined to belong to each object.
See how to read an atlas
image.
The data access page contains various
query forms to search the object lists by coordinates, magnitude,
color etc., and to retrieve data from the archive. In particular, the
Catalog Archive Server
provides a fast search capability for object lists and spectroscopic
parameters as well as pointers to the files in the Data Archive Server. The Imaging
Query Server query form is dedicated to the search of the imaging
database.
Caveats
Missing high proper-motion stars in SDSS DR6 and before
A comparison of SDSS catalogs has shown that high proper motion
stars from Sebastien Lepine's database (SUPERBLINK) are not registered
as high proper motion stars in the DR6. For those stars, the ProperMotions
table lists pm=0.0. The reason their motion is not
registered in DR6 is because of the incompleteness of the USNO-B
catalog, from which the DR6 proper motions are derived. Areas where
the imcompleteness is particularly severe include regions where there
are bad SERC-I or POSS-II N plates (open squares, list from
J. Munn).
If one tries to select off nearby stars with e.g. a
pm<0.75 mas/yr proper motion cutoff, then the sample
will be contaminated with these "pm=0" high
proper motion stars. The plot shows the stars with
pm>100 mas/yr, which are relatively rare, but I suspect
that a similar fraction of 10 mas/yr < pm < 100
mas/yr stars will be similarly unregistered in the DR6, which can add
up a lot of foreground contaminants.
Top panel: high proper motion stars from the
Superblink survey that are missing in DR6. Bottom panel: High proper
motion stars recovered by SDSS.
At the moment, the only mitigation strategy is to avoid the regions
where contamination will be most severe.
Incomplete and/or inaccurate photometry at low galactic latitudes
Much of the data in SEGUE and DRsup is imaging at low
Galactic latitude |b| < 25 degrees, and as such, there are highly
crowded fields, and regions of high extinction. These data were
processed with the standard SDSS photo pipelines. Since these
pipelines were not designed to work in such crowded regions, the
quality of the photometry in these areas is not guaranteed to be
accurate to the SDSS quoted limits of 2% in color and r magnitude, nor
is each and every crowded frame fully deblended; i.e. many fields are
incompletely cataloged.
Overestimation of sky levels in the vicinity of bright
objects
Because of scattered light (see the Early Data Release paper [Stoughton
et al. 2002]), the background sky in the SDSS images is
non-uniform on arc-minute scales. The photometric pipeline determines
the median sky value within each 100" square on a grid with
50" spacing, and bilinearly interpolates this sky value to each
pixel. This biases the sky bright near large extended galaxies. As was already reported
in the Data
Release 4 paper and in
Mandelbaum et al. 2005, this effect causes a systematic decrease in the number density
of faint objects near bright galaxies. In addition, it also strongly
affects the photometry of the bright galaxies themselves, as has been
reported by Lauer et
al. (2007), Bernardi et
al. (2007), and Lisker et
al. (2007). This effect was overestimated in Data Release 6 but has been corrected for
Data Release 7.
We have quantified this effect by adding simulated galaxies with
exponential or de Vaucouleurs profiles to SDSS images. The simulated
galaxies ranged from apparent magnitude mr=12 to
mr=19 in half-magnitude steps, with a one-to-one
mapping from mr to Sérsic half-light radius
determined using the mean observed relation between these quantities
for Main sample galaxies with exponential and deVaucouleurs profiles.
Axis ratios of 0.5 and 1 were used, with random
position angle for the non-circular simulated galaxies.
Results in the r band are shown in the figure below, which
shows the difference between the input magnitude and the model magnitude
returned by the SDSS photometric pipeline as a function of r magnitude for
galaxies with both exponential and deVaucouleurs profiles. (Figure and text from
DR7 paper*)
Difference between measured model and true r-band magnitudes of a
series of simulated galaxies with Sérsic index of 1 (disk galaxies; upper
panel) and 4 (elliptical galaxies; lower panel).
Isophotal radii in DR3-DR7 are given in pixels, not arcseconds
The isophotal radii of objects are supposed to be reported in
arcseconds, as they were in earlier data releases. Due to a
programming error, all isophotal radii are given in pixels in
DR3-DR7. To obtain the isophotal radii in arcseconds, scale by the
pixel size of 0.396 arcseconds.
The bug is present in both the tsObj files in the DAS and the
photoObj and derived tables in the CAS.
SDSS and AB magnitudes
The SDSS photometry is intended to be on the AB system. However,
this is known not to be exactly true. See Conversion from SDSS to
AB magnitudes in the Flux calibration section of the Algorithm
descriptions.
Sky brightness values are extinction-corrected
The various measures of sky brightness reported in the
tsField files are corrected for atmospheric extinction in
the same way as calibrated object magnitudes in tsObj
files. To do a correct conversion from
magnitudes to counts and vice versa, you need to treat object and
sky magnitudes in the same way.
Sky brightness values are given in maggies, not magnitudes
The various measures of sky brightness reported in the
tsField files are given in maggies/square arcsecond (as
in tsObj files), but the fits headers incorrectly give
magnitudes/square arcseconds as units. Only tsField
files in the TARGET version of runs 94, 125, 1033 and
1056 still have the numbers in magnitudes.
Object counts
The nobjects etc. entries in tsField
files (field table in the CAS database) are currently
meaningless.
A few runs processed with slightly older version of photo
As described in the DR2 paper, mis-estimates of the sky background
in the postage stamps of stars used for PSF determination occasionally
coupled with the PSF determination itself in early versions of the
photometric pipeline. We were able to suppress this behavior by
explicitly forcing the sky-subtracted images to zero at their edges.
This revised code was run on most of the imaging runs included in DR2
and DR3, but not quite all of them. In every case that was run with
the old code, a comparison of PSF and aperture photometry of stars
confirmed that there was no significant contribution to the PSF from
wrongly estimated sky, but the user should be aware that these runs have
not been reduced by the identical version of the pipeline. The run/reruns
in question are: 1239/40, 1336/40, 1339/40, 1356/40, 1359/40, 1659/40, 1889/40,
2075/40, 2076/40, 2248/40, 2305/40, 2328/40, 2335/40, 2662/40, 2738/40,
3538/40, 3704/40, 3723/40, 3894/40, 3905/40, 3909/40, 3910/40, 3919/40,
3927/30, 3325/41, and 3838/41. These will be reprocessed and replaced
in the archive for a future data release.
Red leak to the u filter and very red objects
The u filter has a natural red leak around 7100 Å
which is supposed to be blocked by an interference coating. However,
under the vacuum in the camera, the wavelength cutoff of the
interference coating has shifted redward (see the discussion in the
EDR paper), allowing some of this red leak through. The extent of
this contamination is different for each camera column. It is not
completely clear if the effect is deterministic; there is some
evidence that it is variable from one run to another with very similar
conditions in a given camera column. Roughly speaking, however, this
is a 0.02 magnitude effect in the u magnitudes for mid-K
stars (and galaxies of similar color), increasing to 0.06 magnitude
for M0 stars (r-i ~ 0.5), 0.2 magnitude at r-i ~
1.2, and 0.3 magnitude at r-i = 1.5. There is a large
dispersion in the red leak for the redder stars, caused by three
effects:
- The differences in the detailed red
leak response from column to column, beating with the complex red
spectra of these objects.
- The almost certain time variability of the red leak.
- The red-leak images on the u chips are out of focus and are
not centered at the same place as the u image because of
lateral color in the optics and differential refraction - this means
that the fraction of the red-leak flux recovered by the PSF fitting
depends on the amount of centroid displacement.
To make matters even more complicated, this is a detector
effect. This means that it is not the real i and
z which drive the excess, but the instrumental colors
(i.e., including the effects of atmospheric extinction), so the leak
is worse at high airmass, when the true ultraviolet flux is heavily
absorbed but the infrared flux is relatively unaffected. Given these
complications, we cannot recommend a specific correction to the
u-band magnitudes of red stars, and warn the user of these
data about over-interpreting results on colors involving the
u band for stars later than K.
u-band sky determination
There is a slight and only recently recognized downward bias in the
determination of the sky level in the photometry, at the level of
roughly 0.1 DN per pixel. This is apparent if one compares
large-aperture and PSF photometry of faint stars; the bias is of order
29 mag arcsec-2 in r. This, together with
scattered light problems in the u band, can cause of order
10% errors in the u band Petrosian fluxes of large
galaxies.
Astrometry bug fixed since DR2
Astrometry for each object is referred to the reference frame of
the r-band images. DR1 had a bug in the reported right
ascension and declination (and all other celestial coordinates, such
as l and b) for those rare sources that are not detected in the
r band (for example, cool brown dwarfs and z > 5.7
quasars). This bug has been fixed in DR2 and beyond and the positions
of z-band only detections are now correct.
Deblending of bright galaxies significantly improved since DR2
The behavior of the deblender of overlapping images has been
further improved for DR2 and beyond; these changes are most important for
bright galaxies of large angular extent (> 1'). In the EDR, and to
a lesser extent in the DR1, bright galaxies were occasionally
"shredded" by the deblender, i.e., interpreted as two or
more objects and taken apart. With improvements in the code that
finds the center of large galaxies in the presence of superposed
stars, and the deblending of stars superposed on galaxies, this
shredding now rarely happens. Indeed, inspections of several hundred
NGC galaxies shows that the deblend is correct in 95% of the cases;
most of the exceptions are irregular galaxies of various sorts.
Last modified $Date: 2008/10/31 18:00:40 $ (UT).
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