Off-axis viewing of radiation emission by long wiggler sources

When high-brilliance radiation is needed for experiments, insertion device sources are generally viewed on-axis. Off-axis emission of flux can be prodigious especially from wiggler sources having large emission fans. The on-axis and off-axis radiation emission characteristics from insertion device sources have been calculated extensively and are well known, but experimental verifications of some characteristics, particularly those associated with off-axis emission, are relatively few. Here measurements of the flux spectrum and apparent source size are described, as a function of horizontal emission angle, from the former X25 hybrid wiggler at the National Synchrotron Light Source (NSLS).