Modelling the fringing of the FORS2 CCD

Abstract

Thinned CCD detectors display fringing which arises from the interference of multiply reflected light in the layers of the CCD. If the layer construction - the thicknesses and refractive indexes of the layers - is known, then the observed fringing can be accurately modelled and used to correct imaging and spectroscopic data for its effects. In practice the specifications on the actual deposited layer thicknesses may not be known to sufficient accuracy to predict the fringe behaviour. Thus calibration data, in the form of monochromatic flat fields, is required and can be modelled using the technique outlined by Malamuth et al. 2003, which has been applied to ACS CCDs. Initial tests to model the observed fringing of the FORS2 MIT CCD are described. A set of six monochromatic flat fields was generated by shining light from a tunable monochromator into the FORS2 instrument calibration unit. Peak-to-peak fringe amplitude of 7-8% was measured at a wavelength of 970 nm. In order to model the CCD structure, a series of monochromatic flats at closely spaced wavelengths is required, which would have been very time-consuming to acquire with the monochromator. An alternative strategy is outlined. The fringing characteristics of CCDs should be systematically calibrated while still in the test laboratory. © 2008 Springer-Verlag Berlin Heidelberg.