  Large mosaic multiCCD camera is the key instrument for modern digital sky
survey. DECam is an extremely red sensitive 520 Megapixel camera designed for
the incoming Dark Energy Survey (DES). It is consist of sixty two 4k$\times$2k
and twelve 2k x 2k 250-micron thick fully-depleted CCDs, with a focal plane of
44 cm in diameter and a field of view of 2.2 square degree. It will be attached
to the Blanco 4-meter telescope at CTIO. The DES will cover 5000 square-degrees
of the southern galactic cap in 5 color bands (g, r, i, z, Y) in 5 years
starting from 2011.
  To achieve the science goal of constraining the Dark Energy evolution,
stringent requirements are laid down for the design of DECam. Among them, the
flatness of the focal plane needs to be controlled within a 60-micron envelope
in order to achieve the specified PSF variation limit. It is very challenging
to measure the flatness of the focal plane to such precision when it is placed
in a high vacuum dewar at 173 K. We developed two image based techniques to
measure the flatness of the focal plane. By imaging a regular grid of dots on
the focal plane, the CCD offset along the optical axis is converted to the
variation the grid spacings at different positions on the focal plane. After
extracting the patterns and comparing the change in spacings, we can measure
the flatness to high precision. In method 1, the regular dots are kept in high
sub micron precision and cover the whole focal plane. In method 2, no high
precision for the grid is required. Instead, we use a precise XY stage moves
the pattern across the whole focal plane and comparing the variations of the
spacing when it is imaged by different CCDs. Simulation and real measurements
show that the two methods work very well for our purpose, and are in good
agreement with the direct optical measurements.
