SDSSmosaic allows you to create science image mosaics of SDSS frames in 3 easy steps, described below (see Usage).

--Download for DR8 or Download for DR7

--System requirements:

SDSSmosaic requires a standard IRAF installation (including STSDAS and tables packages); python (including urllib); SExtractor and Swarp by E. Bertin (can be obtained at

The following shell commands must be available in the standard path as well: rm, gawk, ln, gunzip (DR7) or bunzip2 (DR8)


SDSSmosaic is installed as an external IRAF package.

$ cd <inst_dir>

$ tar -zxvf <path_to_file>/SDSSmosaic.tar.gz

Add the following lines in your IRAF

set     SDSSmosaic="<inst_dir>/SDSSmosaic/"

task    $SDSSmosaic="SDSSmosaic$"

From the IRAF cl prompt just type SDSSmosaic to load the package.


SDSSmosaic works in 3 steps:

1) SDSS fields belonging to the same scan (same run and camcol) are stitched together, accurate astrometry is computed and the photometric calibration is written into the header.

2) Background is subratcted from each scan by fitting a plane surface (allowing for linear gradient along the scan direction, constant in the perpendicular direction) outside a circle centered on the source.

3) Stripes are combined in a mosaic using Swarp

Steps 1) and 2) are done with two IRAF procedures, mkSDSSstripes and invcirc_bkgfit respectively, while step 3) is implemented in a simple bash script.

1) mkSDSSstripes takes as arguments:

RA, dec: the center of the mosaic field

dist: the maximum distance from the center of the mosaic of the center of a SDSS field to be considered. Remember SDSS fields are approx. 13.5 arcmin diagonal.

fpC "corrected" frames and photometric informations are queried and downloaded from the SDSS web site for all 5 bands, and combined in stripes with names fpCc*.fits.

An accurate astrometric solution is recomputed based on SDSS astrometry and the WCS info in each stripe image is updated.

2) invcirc_bkgfit takes as arguments:

images: the list of images from which the background has to be subtracted

ra, dec: the center of the circle to be excluded from the fit

radius: the radius of the circle to be excluded from the fit in arcmin

The sky-subtracted images are output with the suffix _skyfit

3) mkSDSSmosaic.bash is a short bash script that calls Swarp to mosaic all stripes (i.e. all fpCc-*-${band}_skyfit.fits files in the current directory).

$ ./mkSDSSmosaic.bash filename_root RA dec Xsize[pix] Ysize[pix]

The script must be customized to point to the correct swarpdir in the package directory.

Note that the final mosaic contains the photometric zero point (P_ZP keyword) in mag per second of exposure time (EXPTIME keyword).

The package is developed by Stefano Zibetti (2009-..., MPIA-Heidelberg, DARK, INAF-OAArcetri) and is distributed by the author on demand. If you received this package from a third party, please notify the author (zibetti at If you make use of this package for any publication, please refer to the paper by Zibetti, Charlot and Rix (2009, MNRAS, 400, 1181; arXiv:0904.4252) for a description of the method and add the following acknowledgement: "SDSS science image mosaics are produced using the package SDSSmosaic developed by Stefano Zibetti."

Do not hesitate to contact the author in case you have problems and/or suggestions.

Please note that SDSS database and image formats have changed between DR7 and DR8: please install the respective package, according to your needs.