FLS-SDSS Image Mosaic Properties
- 5-band ugriz visual image mosaics composed of hundreds
of SDSS drift-scan fields
- sets of four 1.75 deg × 1.25 deg mosaics, north-up
- each mosaic is 15912×11364
- 0.396 arcsec × 0.396 arcsec square pixels
(at fiducial point RA=259.5 deg, Dec=59.5 deg J2000)
- correct, FITS-compliant WCS astrometry headers
- the four mosaics “snap together” into one immense
(31824×22728) mosaic with no overlaps or edge distortion
issues
- calibrated in intensity units of
[nMgy (0.396 arcsec)-2], where 1 nMgy
(“nano maggie”) in flux is equivalent to 22.5 mag
(AB system)
NB: If a source in one of the mosaics is measured to have
total flux N in nMgy, its magnitude m is then
m = [22.5 mag] − 2.5 log10 N .
The above files are unwieldy, so we have made some lower-resolution,
single-field mosaics with the following properties:
- same 5 bands
- individual 3.5 deg × 2.5 deg mosaics, north-up
- each mosaic is 10608×7576
- 1.188 arcsec × 1.188 arcsec square pixels
(at fiducial point RA=259.5 deg, Dec=59.5 deg J2000)
- correct, FITS-compliant WCS astrometry headers
- calibrated in intensity units of
[nMgy (1.188 arcsec)-2]
NB: Unlike the high resolution images, these
lower-resolution images are not band-limited and should be used
with caution.
There are some defects in these mosaics, which need to be worked
out, worked around, or avoided by scientific users:
- sky noise variance is not constant but varies inversely with
the number of overlapping SDSS fields
(1 or 2 for most of the image area); in particular there are
diagonal “stripes” of low noise in the images
where SDSS camera columns overlap
- sky background level is not exactly zero everywhere; use
local sky estimation when photometering sources
- bright stars are saturated and have bleed trails
- the PSF is variable across the fields
- filter reflections in the SDSS cameras cause bright stars
to show nearby flares in sky brightness
- some cosmic rays have slipped through
- small CCD readout defects near extremely bright sources
- many bright single-band satellite trails