Spatial Variations of Galaxy Number Counts in the Sloan Digital Sky Survey. I. Extinction, Large-Scale Structure, and Photometric Homogeneity

We study the spatial variation of galaxy number counts using five-band photometric images from the Sloan Digital Sky Survey (SDSS). The spatial variation of this sample of 4.3 million galaxies collected from 2200 deg2 can be understood as the combination of Galactic extinction and large-scale clustering. With the use of the reddening map of Schlegel, Finkbeiner, & Davis, the standard extinction law is verified for the color bands from u to z within 5% in the region of small extinction values, E(B-V) < 0.15. The residual spatial variation of the number counts suggests that the error in the global calibration for SDSS photometry is smaller than 0.02 mag.