Nonuniformity of CCDs and the effects of spatial undersampling

CCDs are widely used in astronomy for spectroscopy, photometry, astrometry, and many other applications. This array sensor has a periodic structure which defines individual picture elements (pixels). The stability and excellent overall response of this type of detector are well known. Less widely described is the fact that the internal structure of the array gives rise to intrapixel variations in responsivity. These response modulations are particularly relevant when the data is spatially undersampled; a situation that is not entirely uncommon in certain instruments. We have made detailed measurements of the response variations within CCDs, using an experimental arrangement that gave a 2 micron resolution. Optical response has been determined at all sample points within a pixel, at selected wavelengths in the range 400 to 900 nm. Measurements are presented for front-illuminated thick (EEV) devices and backside- illuminated thin (Tek) CCDs. Models of the internal structure have been constructed and used to calculate theoretical response data; these have been compared with the experimental results. An example of an extracted (FOS) spectrum which demonstrates these undersampling effects is discussed.