Calibrated object lists

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:

• tsObj*.fits containing the object lists themselves. There is one file per field. The files are binary fits tables with one row per object.

  Notes

  • The primary header of tsObj files contains some parameters applying to the entire field, such as the seeing (but also see the PSF parameters in the tsField file below).
  • The reported magnitudes are corrected for atmospheric extinction (compare converting counts to magnitudes), but not for Galactic extinction. The Galactic extinction for each object as derived from Schlegel, Finkbeiner, and Davis (1998) dust maps is reported as reddening. The target selection algorithms use magnitudes corrected for both kinds of extinction.

  How to read tsObj files includes an illustrative SM script to filter tsObj files and explanations of which flags to check.

• tsField*.fits files, with one row of parameters relevant to the entire field. This includes the average PSF profile, e.g.

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 m_r=12 to m_r=19 in half-magnitude steps, with a one-to-one mapping from m_r 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.