An imaging method includes imaging a scene using an imaging system, which includes an array of radiation sensing elements, including first sensing elements with symmetrical angular responses and second sensing elements with asymmetrical angular responses, interspersed among the first sensing elements, and optics configured to focus radiation from the scene onto the array. The method further includes processing first signals output by the first sensing elements in order to identify one or more areas of uniform irradiance on the array, and processing second signals output by the second sensing elements that are located in the identified areas, in order to detect a misalignment of the optics with the array.