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 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.
• Data model link

Caveats

• 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/DR1/data/imaging/1740/21/logs/

• 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.

• u-band sky

  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.