Apertures having references edges are situated to define a sample irradiation zone and a shielded zone. The sample irradiation zone includes a portion proximate the shielded zone that is conjugate to a detector. A sample is scanned into the sample irradiation zone from the shielded zone so that the sample can remain unexposed until situated properly with respect to the detector for imaging. Irradiation exposure of the sample is reduced, permitting superior imaging.