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 fpM*.fits image masks These are the full binary mask files provided by the imaging pipeline; we also provide an alternate set of processed mask files, described next, which may be more useful or convenient for your science application.
• SDSS provides a .csv (comma-separated variable) version of the image masks as well. 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.
• 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)

Caveats

• Overestimation of sky levels near bright galaxies

  Because of scattered light (see the EDR 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, and as was already reported in the DR4 paper and (Mandelbaum et al. 2005), 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).

  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 Sersic half-light radius determined using the mean observed relation between these quantities for Main sample galaxies with exponential and de Vaucouleurs profiles. Axis ratios of 0.5 and 1 were used, with random position angle for the non-circular simulated galaxies. The results in the r band are shown in the Figure, showing the difference between the input magnitude and the model magnitude returned by the SDSS photometric pipeline, as a function of magnitude.

  Also shown is the fractional error in the scale size r_e. The biases are significant to r=16 for late-type galaxies, and to r=17.5 for early-type galaxies. Also shown is the results of a separate analysis by by Hyde & Bernardi (unpublished) who fit deVaucouleurs models to SDSS images of extended elliptical galaxies, using their own sky subtraction algorithm, which is less likely to overestimate the sky level near extended sources. Their results are quite consistent with the simulations.

  

  Upper panel: The error in the r band model magnitude of simulated galaxies with an n=1 (exponential) profile (blue hexagons) and an n=4 (de Vaucouleurs) profile (red crosses) as determined by the photometric pipeline, as a function of magnitude. Fifteen galaxies are simulated at each magnitude for each profile. Also shown are the analogous results from Hyde & Bernardi (unpublished) for three early-type galaxy samples: 54 nearby (z<0.03) early-type galaxies from the ENEAR catalog (da Costa et al. 2000) in black; 280 brightest cluster galaxies from the C4 catalog (Miller et al. 2005) in green; and 9000 early-type galaxies from the Bernardi et al. (2003a) analysis in magenta. Lower panel: The fractional error in the scale size r_e as a function of magnitude from the simulations and the Hyde & Bernardi analysis.

• 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 fpM*.fits. These files can be found on the Data Archive Server in the objcs subdirectory of each run/rerun directory (e.g., http://das.sdss.org/DR6/data/imaging/1740/40/objcs/1/. See how to read fpM*.fits image masks.

• 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.