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Calibrated object lists

Up: Data Products Sections: Images - Object lists - Spectra - Tiling

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.

Proper motions are not correct

This affects the 'propermotions' table in the CAS for DR6:

This catalog was published in the 
paper "An Improved Proper-Motion Catalog Combining USNO-B And
The Sloan Digital Sky Survey", Munn et al. 2004, AJ, 127, 3034.  The
proper motions are contained in the "ProperMotions" table in the CAS.

The bug itself involves an incorrect sign when interpreting residuals
in the USNO-B catalog; for those interested the details are given
below.  The results of the bug are as follows:

1) The proper motions in declination were unaffected by the bug, and
    are thus correct.

2) The proper motions in right ascension suffer from smoothly varying
    stematic errors of as much as 3 mas/yr, though more
    typically ~1-2 mas/year.  The systematic error is less than
    1 mas/year over 20% of the DR6 sky, and less than 2 mas/year over
    80% of the DR6 sky.

3) For objects which had a non-zero right ascension proper motion in
    USNO-B, the differences between the right ascension proper motions
    when correctly and incorrectly determined, after removing the
    systematic error, has a dispersion of only about 1 mas/year.  This
    is insignificant, since the known statistical errors in the proper
    motions are about 3.5 mas/year per coordinate.

4) In USNO-B, objects which had a proper motion consistent with 0 to
    within a few sigma were set to have a proper motion of exactly 0.
    For these objects, the dispersion between the correct and incorrect
    right ascension proper motions, after removing the systematic
    errors, is about 5 mas/year.  These motions are simply wrong, though
    their incorrect values are still consistent with zero, and thus
    selecting objects to have zero proper motion within some number of
    sigma should still mostly work.

Thus, for users working with samples of stars chosen to have
significantly detected proper motions, the impact of the bug should
be small.  The bug adds a systematic error of ~2 mas/year in right
ascension to objects with statistical errors of about 3.5 mas/year.
However, for users working closer to the statistical error limit, or
averaging over many stars to determine small mean motions, the
additional errors are clearly significant.  In particular, the proper
motions in right ascension of objects which were set to have an exactly
zero proper motion in USNO-B are unreliable (though still correctly
consistent with zero).  The use of the catalog in its current form for
Galactic structure work is severly impacted.

A corrected version of the catalog has been generated.  This will be
used to load the CAS for the next (DR7) data release. 

Please contact us if you require access to the corrected proper motions
table prior to the DR7 release.

Overestimation of sky levels in the vicinity of bright objects

Because of scattered light (see the EDR 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, and as was already reported in the DR4 paper and (Mandelbaum et al. 2005), 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).

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 Sersic half-light radius determined using the mean observed relation between these quantities for Main sample galaxies with exponential and de Vaucouleurs profiles. Axis ratios of 0.5 and 1 were used, with random position angle for the non-circular simulated galaxies. The results in the r band are shown in the Figure, showing the difference between the input magnitude and the model magnitude returned by the SDSS photometric pipeline, as a function of magnitude.

Also shown is the fractional error in the scale size re. The biases are significant to r=16 for late-type galaxies, and to r=17.5 for early-type galaxies. Also shown is the results of a separate analysis by by Hyde & Bernardi (unpublished) who fit deVaucouleurs models to SDSS images of extended elliptical galaxies, using their own sky subtraction algorithm, which is less likely to overestimate the sky level near extended sources. Their results are quite consistent with the simulations.

Sky misestimation near bright galaxies

Upper panel: The error in the r band model magnitude of simulated galaxies with an n=1 (exponential) profile (blue hexagons) and an n=4 (de Vaucouleurs) profile (red crosses) as determined by the photometric pipeline, as a function of magnitude. Fifteen galaxies are simulated at each magnitude for each profile. Also shown are the analogous results from Hyde & Bernardi (unpublished) for three early-type galaxy samples: 54 nearby (z<0.03) early-type galaxies from the ENEAR catalog (da Costa et al. 2000) in black; 280 brightest cluster galaxies from the C4 catalog (Miller et al. 2005) in green; and 9000 early-type galaxies from the Bernardi et al. (2003a) analysis in magenta. Lower panel: The fractional error in the scale size re as a function of magnitude from the simulations and the Hyde & Bernardi analysis.

Isophotal radii in DR3-DR6 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-DR6. 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: 2008/02/14 21:38:38 $ (UT).