  We report a design and implementation of proton radiography with an in situ
reference X-ray image of a mesh to precisely measure non-uniform magnetic
fields in expanding plasmas at the OMEGA and OMEGA EP laser facilities. The
technique has been developed with proton and x-ray sources generated from both
directly-driven capsule implosions and short pulse laser-solid interactions.
The accuracy of the measurement depends on the contrast of both the proton and
x-ray images. We present numerical and analytic studies to optimize the image
contrast using a variety of mesh materials and grid spacing. Our results show a
clear enhancement of the image contrast by a factor of 4-6 using a high Z mesh
with large grid spacing. This would lead to a further factor of two improvement
in accuracy of the magnetic field measurement.
