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.
• 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)

New for DR7: Runs and Stripe 82 Databases

The DR7 Catalog Archive Server contains the imaging RUNS database, separate from the DR7 (Legacy + SEGUE) database. This database contains 530 imaging scans, some of which are not present in the main DR7 database, and thousands imaging fields from the beginnings and ends of DR7 strip scans which did not resolve into a complete rectangular stripe on the sky.

The DR7 Catalog Archive Server contains the Stripe82 database, distinct from the DR7 and RUNS databases, containing all imaging from SDSS stripe 82 along the Celestial Equator in the Southern Galactic Cap (Stripe 82). Subsets of the 303 individual stripe 82 scans were made public in previous data releases, but with DR7, we now make a coaddition (the Stripe 82 coadd) of about 47 South strip and 55 North strip scans into a scan in which objects are detected and cataloged about 2 magnitudes deeper than in any individual stripe82 scan. It includes a total of 303 runs (plus 2 coadd runs), covering any given piece of the approx. 270 deg² area approximately 80 times.

All 303 runs covering Stripe 82 are part of the Stripe 82 database, which can be queried to search for variability (and propermotion, with a bit more effort). Only about one-quarter of the Stripe 82 scans were obtained in photometric conditions, the rest were taken under variable clouds and often poorer than normal seeing. For those runs which are non-photometric, an approximate calibration, using the photometric frames as reference, is derived and available in the CAS stripe82 database, for objects with RA > 310 or RA < 59 degrees.

Caveats

• Overestimation of sky levels near bright galaxies

  Because of scattered light (see the Early Data Release 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. As was already reported in the Data Release 4 paper and in Mandelbaum et al. 2005, this effect 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). This effect was overestimated in Data Release 6 but has been corrected for Data Release 7.

  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 Sérsic half-light radius determined using the mean observed relation between these quantities for Main sample galaxies with exponential and deVaucouleurs profiles. Axis ratios of 0.5 and 1 were used, with random position angle for the non-circular simulated galaxies.

  Results in the r band are shown in the figure below, which shows the difference between the input magnitude and the model magnitude returned by the SDSS photometric pipeline as a function of r magnitude for galaxies with both exponential and deVaucouleurs profiles. (Figure and text from DR7 paper*)

  

  Difference between measured model and true r-band magnitudes of a series of simulated galaxies with Sérsic index of 1 (disk galaxies; upper panel) and 4 (elliptical galaxies; lower panel).

  ---

• 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/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.

• u3 half-bad for runs 2188,2189,2190

  In early 2001, the u band CCD of CamCol 3 suffered from an electronics problem in its 'left' amplifier, resulting in missing u band data in half the u-band CCD in CamCol 3 (only) for runs 2188, 2189, 2190 (only). The problem was then corrected. This data remains missing. The problem can be seen by downloading a u3 fpC or drC corrected frame from the DAS from one of these runs. There is no work around other than not using the u-band data in this region.

*Text and figures on this page come from an author-created, un-copyedited version of the SDSS Data Release 7 paper, an article submitted to Astrophysical Journal Supplements. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. A preprint of the DR7 paper is available from the arXiv preprint server.