Capacitive focus gauge for LED printer

An imaging apparatus having a concentric support with an at-least-partially cylindrical inner surface for receiving a recording media about an axis of the support, a rotor which is rotatable about an axis, a portion of the rotor and the inner surface of the support forming a gap such that a dimension of the gap changes as the rotor rotates and as an inverse function of the concentricity of the support and the rotor, a write head assembly carried by the rotor to write on recording media received on the at least partially cylindrical inner surface of the support, said imaging apparatus further comprising:

• adjustment apparatus for translating at least one of the support and the rotor to co-align the support axis and the rotor axis;
• apparatus for measuring the concentricity of the support and the rotor while the rotor is rotating, said apparatus for measuring comprising:
• a first electrode carried at the portion of the rotor forming the gap with the inner surface such that a capacitance between the electrode and the inner surface varies as the rotor rotates and as an inverse function of the concentricity of the support and the rotor;
• and
• a detector, including a power source adapted to produce an electrical charge across the gap and an impedance, for measuring a change in the dimension of the gap as the rotor rotates.