Images

Getting and using images

The Data Archive Server provides the survey images, called "corrected frames", as fpC*.fits files. See the fpC data model.

The Catalog Archive Server serves 3-color jpeg images generated from the g, r, i images as finding charts, cutouts for object lists and for point-and-click navigation of the sky.

The data access page contains various query forms to get images by coordinates, or to search for objects from the imaging and spectroscopic catalogs by redshift, object magnitude, color etc., and to retrieve the corresponding data from the archive.

There is a separate set of fpAtlas*.fits files, containing the "postage-stamp" images for individual objects from the photometric object lists. See how to read an atlas image.

• We provide a tutorial describing how to retrieve fits images for a set of objects.
• How to reconstruct the PSF at an arbitrary position in a frame
• How to read image masks (These are the full binary mask files provided by the imaging pipeline; we also provide an alternate set of processed mask files, see below, which may be more useful or convenient for your science application.)
• How to apply the photometric calibration to images, which are in counts.
• How to apply the astrometric calibration (beyond the WCS in the fits files)
• SDSS provides image masks listing "bad" regions of the sky (pixels affected by saturation, bright stars, satellite trails, bad seeing, or with locally unacceptably bad data). Look at the image mask creation algorithm to understand their creation, and the image mask instructions for use for details on the file format and location.

Caveats

• Overestimation of sky levels near bright galaxies

  A study of weak lensing in the SDSS imaging data (Mandelbaum et al. 2005) found a systematic, 5% decrease in the number density of faint objects within 90′′ of bright (r < 18) galaxies. This was found to be due to a systematic overestimation of the sky levels in the vicinity of bright objects, which of course will affect all the measured photometric quantities of faint objects, including the classification as stars and galaxies. The brighter the object, the larger the overestimation of sky; indeed, restricting to foreground galaxies brighter than r = 16, the number density of fainter galaxies is 10% below the mean. The effect is due to the way in which the sky levels are estimated in the SDSS; they can be biased upward by the faint outer isophotes of a bright galaxy (see the EDR paper [Stoughton et al. 2002]; see also the discussion in Strauss et al. 2002). Closer than 40′′ to the bright objects, the intrinsic clustering of galaxies makes it difficult to assess the problem. This problem is most relevant for applications that involve correlating positions of bright objects with faint ones, yielding a spurious anti-correlation on the affected scales. We are currently investigating changes to the imaging pipeline to address this problem.

• Bad CCD columns

  Some chips have bad CCD columns which get interpolated over by the photometric pipeline, leading to noticeably correlated noise. The bad columns for each run are currently available in opBC*.par files (which are ASCII parameter files). These files can be found on the Data Archive Server in the logs subdirectory of each run/rerun directory (e.g., http://das.sdss.org/DR3/data/imaging/1740/40/logs/

• 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

  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.