  The Northern Sky Variability Survey (NSVS) is a temporal record of the sky
over the optical magnitude range from 8 to 15.5. It was conducted in the course
of the first generation Robotic Optical Transient Search Experiment (ROTSE-I)
using a robotic system of four co-mounted unfiltered telephoto lenses equipped
with CCD cameras. The survey was conducted from Los Alamos, NM, and primarily
covers the entire northern sky. Some data in southern fields between
declinations 0 and -38 deg is also available, although with fewer epochs and
noticeably lesser quality. The NSVS contains light curves for approximately 14
million objects. With a one year baseline and typically 100-500 measurements
per object, the NSVS is the most extensive record of stellar variability across
the bright sky available today. In a median field, bright unsaturated stars
attain a point to point photometric scatter of ~0.02 mag and position errors
within 2 arcsec. At Galactic latitudes |b| < 20 deg the data quality is limited
by severe blending due to ~14 arcsec pixel size. We present basic
characteristics of the data set and describe data collection, analysis, and
distribution. All NSVS photometric measurements are available for on-line
public access from the Sky Database for Objects in Time-Domain (SkyDOT;
http://skydot.lanl.gov) at LANL. Copies of the full survey photometry may also
be requested on tape.
