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

Isophotal radii in DR2 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 DR2. 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. It will be fixed in the next data release.

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.

Object counts

The nobjects etc. entries in tsField files (field table in the CAS database) are currently meaningless.

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.

Sky determination

There is a slight and only recently recognized downward bias in the determination of the sky level in the phot ometry, 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 in 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 the positions of z-band only detections are now correct.

Deblending of bright galaxies significantly improved in DR2

The behavior of the deblender of overlapping images has been further improved since the DR1; 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.