A computer includes a processor and memory, the memory can store instructions executable by the processor to display a first calibration symbol to appear at a first location of a virtual image plane viewable from a measured location of the eyes or head of an operator and display a second calibration symbol to appear at a second location of the virtual image plane according to an offset distance from the first location. The instructions additionally can additionally be to adjust the offset distance in response to adjusting the second location by receiving input from the operator to align the first calibration symbol with the second calibration symbol.