Calibrated object lists

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:

• 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.

Caveats

Primary caveats for DR1 beta

The following points apply to the current DR1 beta, produced with version 5_3 of the photometric pipeline. The next data release will contain data which have been reduced with version 5_4, which fixes the bug in 5_3 which leads to these caveats. See About DR1.

• For photometry of resolved sources, one should use Petrosian magnitudes, especially at the bright end.
• For photometry of unresolved sources, one should use PSF magnitudes.
• Colors derived from model magnitudes are almost completely insensitive to the bug. Model magnitudes remain the optimal quantities to use for the colors of extended objects, especially at the faint end.
• The scale sizes derived from the model fits are systematically wrong. For exponential fits, the effective radii are systematically too large by 0.15" in the present code (almost independent of r_e itself), while for the de Vaucouleurs fits, they are roughly 25% too large (almost independent of r_e itself, for r_e > 2 arcsec). These correction factors depend on seeing to some level.

Caveats: subtleties and minor bugs in DR1

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 magniutdes in the same way.

Object counts

The nobjects etc. entries in tsField files are currently meaningless.

Astrometry

A bug was found in the reported right ascension and declination (and all other celestial coordinates, such as l and b, of course) for those rare sources that are not detected in the r band.  For example, this bug affects extremely red objects (brown dwarfs and high-redshift quasars) that are detected only in the z-band.  The r-band astrometric solution was applied to the centroid position in the band in which the object in question was detected. As there is typically a few arcsecond offset between the bands, this results in a few arcsecond error in the position.  The error can be corrected by examining the fields offsetRA and offsetDec.  First, note which band is flagged CANONICAL_BAND (if this band is the r band, which is the case the majority of the time, there is no error).

To obtain the correct position for these objects,    determine offsetRA/cos(dec) and offsetDec for this band, and subtract these quantities from the nominal ra and dec.

Very red objects

The u magnitudes of very red stars, with spectral types later than about M0 (u-i > 4), are affected by camera-column and imaging-run dependent systematic variations of order 0.1 mag, as determined, e.g., from changes in the stellar locus of these very red stars in color-color diagrams. The cause of these u-band systematics is not fully understood at present, but may be due to a u-band red leak problem which appears to be larger than original design specifications (due to the effects of the dewar vacuum shifting the wavelengths of the interference coatings used to reject red leak for the u filters). This issue of u-band systematics for very red stars is being actively investigated, and we will post our findings at this web site once we have a fuller understanding of the problem.

Position angles

The position angle fields in the DR1 dataset (those processed through photo v5_3 and target v4_5 or lower), have position angles modulo 360 degrees rather than the proper standard of modulo 180 degrees (or in the range [-90,90] degrees). Please convert the position angles to mod 180 or to [-90,90] yourself as needed. This affect the fields: iso_phi, phi_exp, phi_deV (isophotal position angle, exponential disk fit position angle, deVauc. fit position angle, all in each of 5 bands) in the calibrated imaging outputs (tsObj files).

Sky determination

There are known problems in the determination of the background sky at the level of a few hundredths of a DN. This seems tiny, but gives noticeable non-uniformities in the Petrosian photometry in the u band of large (>10'') galaxies. We think we have found a fix, but as of this writing, it is not yet tested.

Deblending of bright galaxies

The SDSS image deblender works completely automatically, and needs to work both for the case of a saturated star superposed on the disk of a face-on spiral galaxy, a small cluster of faint stars, and an asteroid moving next to a galaxy. It is known to occasionally incorrectly deblend galaxies brighter than r=16th magnitude, giving unphysical deblends in a few percent of the galaxies in the UZC (Falco et al, 1999, PASP, 111, 438). At the brightest magnitudes, this fraction probably increases substantially. Further improvements to the software, to be included in DR2, are known to reduce this error rate substantially. Similarly, there are quite a few false negatives: typically poorly deblended saturated stars that manifest themselves as bright galaxies. Again, this should be much improved in DR2, but in the meantime, visual inspection will be needed for any astronomer wishing to put together a complete catalog of bright galaxies, or to obtain photometry for galaxies brighter than r=16, from the SDSS outputs.