  Tomography of the solar corona can provide cruicial constraints for models of
the low corona, unique information on changes in coronal structure and rotation
rates, and a valuable boundary condition for models of the heliospheric solar
wind. This is the first of a series of three papers which aim to create a set
of maps of the coronal density over an extended period (1996-present). The
papers will describe the data processing and calibration (this paper), the
tomography method (\paperii) and resulting atlas of coronal electron density at
a height of 5\Rs\ between years 1996-2014 (\paperiii). This first paper
presents a detailed description of data processing and calibration for the
Large-Angle and Spectrometric Coronagraph (LASCO) C2 instrument onboard the
Solar and Heliospheric Observatory (SOHO) and the COR2 instruments of the Sun
Earth Connection Coronal and Heliospheric Investigation (SECCHI) package aboard
the Solar Terrestial Relations Observatory (STEREO) A \& B spacecraft. The
methodology includes noise suppression, background subtraction, separation of
large dynamic events, conversion of total brightness to K-coronal brightness
and simple functions for crosscalibration between C2/LASCO and COR2/SECCHI.
Comparison of the brightness of stars between LASCO C2 total and polarized
brightness (\pB) observations provide in-flight calibration factors for the
\pB\ observations, resulting in considerable improved agreement between C2 and
COR2 A, and elimination of curious artifacts in the C2 \pB\ images. The
crosscalibration between LASCO C2 and the STEREO coronagraphs allows, for the
first time, the potential use of multi-spacecraft coronagraph data for
tomography and for CME analysis.
