Near field effects in off-axis undulator radiation

It is shown that near field effects can be important when off-axis radiation is used from an undulator, even when the source to observer distance is relatively large compared to the undulator length. The effects on the spectral and angular distributions are examined in detail, both by direct integration of the radiation formula and by use of an analytic approximation, and it is shown that the main features of the results can be described in terms of a single new parameter. (orig.)