An embodiment of the present disclosure provides a charged particle-optical apparatus for scanning a sample with charged pat tick's and detecting signal charged particles emitted from the sample, the charged particle-optical apparatus comprising: a charged particle-optical device configured to direct a charged particle beam onto a field of view of the wimple; a detector comprising a plurality of detector elements configured to detect signal charged particles emitted from respective locations within the field of view, so as to generate scan data; and a controller configured to compensate for variation across the field of view of a proportion of signal charged particles emitted from the locations that are incident on the respective detector elements, such that an effect of the variation on the scan data is reduced. An embodiment of the present disclosure provides a method of operating a charged particle-optical apparatus for scanning a sample with charged particles and detecting signal charged particles emitted from the sample, the method comprising: a charged particle-optical device of the charged particle-optical apparatus directing a charged particle beam onto a field of view of the sample: a plurality of detector elements of a detector of the charged particle-optical apparatus detecting signal charged particles emitted from respective locutions within the field of view, so as to generate .scan data; and compensating for variation across the field of view of a proportion of signal charged particles emitted from the locations that are incident on the respective detector elements, such that an effect of the variation on the scan data is reduced. An embodiment of the present disclosure provides a method of processing scan data, the method comprising: providing scan data, wherein the scan data has been generated by a charged particle-optical apparatus scanning a field of view of a sample with charged particles and detecting signal charged particles emitted from locations within the field of view with a plurality of respective detector elements of a detector of the charged particle-optical apparatus; and correcting the scan data so as to compensate for variation across the field of view of a proportion of signal charged particles emitted from the locations that are incident on the respective detector elements, such that an effect of the variation on the scan data is reduced. An embodiment of the present disclosure provides a method of determining at least one compensation parameter value for a charged particle-optical apparatus for scanning a sample with charged particles and detecting signal charged particles emitted from the sample, the method comprising: a charged panicle-optical device of the charged particle-optical apparatus directing a charged particle beam onto at least one field of view of the sample, wherein the field of view is substantially featureless; a plurality of detector elements of a detector of the charged particle-optical apparatus detecting signal charged particles emitted from respective locutions within the at least one field of view, so as to generate measurement data; recording the measurement data associated with the locations as a record of variation across the field of view of a proportion of signal charged particles emitted from the locations that are incident on the respective detector elements; and determining at least one compensation parameter value to be applied by a controller of the charged particle-optical apparatus so as to compensate for the variation in a subsequent scan on a sample comprising features. An embodiment of the present disclosure provides a non-transitory computer readable medium that stores instructions for a processor of a controller to carry out a method of operating a charged particle-optical apparatus the method comprising: controlling a charged particle-optical device of the charged particle-optical apparatus to direct a charged particle beam onto a field of view of a sam
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