The telescope surface S is defined in the database and depends on the effective area of the telescope, equal to the total area decreased by the area of the central obstruction. For NTT telescope S=8.9 m2, for VLT S=51.2 m2, for 2p2 S=3.8 m2. The value of the telescope surface is defined in the Instrument Definition File (IDF) which is an ASCII file containing informations on what the instrument is composed of (specifying the optical path followed from the incident radiation in each of the observation modes allowed from the instrument), the location of the transmission curves of its optical elements, some instrument characteristic constants.
The IDF is constructed according the following picture.
