Title: The JCMT Transient Survey: Data Reduction and Calibration Methods,
Abstract: Though there has been a significant amount of work investigating the early
stages of low-mass star formation in recent years, the evolution of the mass
assembly rate onto the central protostar remains largely unconstrained.
Examining in depth the variation in this rate is critical to understanding the
physics of star formation. Instabilities in the outer and inner circumstellar
disk can lead to episodic outbursts. Observing these brightness variations at
infrared or submillimetre wavelengths sets constraints on the current accretion
models. The JCMT Transient Survey is a three-year project dedicated to studying
the continuum variability of deeply embedded protostars in eight nearby
star-forming regions at a one month cadence. We use the SCUBA-2 instrument to
simultaneously observe these regions at wavelengths of 450 $\mu$m and 850
$\mu$m. In this paper, we present the data reduction techniques, image
alignment procedures, and relative flux calibration methods for 850 $\mu$m
data. We compare the properties and locations of bright, compact emission
sources fitted with Gaussians over time. Doing so, we achieve a spatial
alignment of better than 1" between the repeated observations and an
uncertainty of 2-3\% in the relative peak brightness of significant, localised
emission. This combination of imaging performance is unprecedented in
ground-based, single dish submillimetre observations. Finally, we identify a
few sources that show possible and confirmed brightness variations. These
sources will be closely monitored and presented in further detail in additional
studies throughout the duration of the survey.