analog-to-digital unit (ADU)
Fr.: unité analogue-numérique
A number that represents a → charge-coupled device (CCD)'s output and is proportional to the → electron charge created by the → photons, plus the constant → bias offset. The relationship between the ADUs generated and the number of electrons acquired on the CCD is defined by the → CCD gain. Intensities given in ADUs provide a convenient method for comparing images and data generated by different cameras. Also referred to as → count and digital number. In most cases, the analog signal is digitalized by an analog-to-digital (A/D) converter and fed into a computer where further manipulation and analysis are done on what the detector originally produced from the star's photons (Howell, S.B., Handbook of CCD Astronomy, Cambridge Univ. Press, 2000).