The total efficiency

The total efficiency E is the resulting product of the efficiencies of all the optical components present along the optical path, starting from the atmosphere, the telescope reflecting surfaces, any filter or lens or optical element, the detector.

The efficiencies curves are based on engineering data and will be adjusted as soon as observational data will be available to reflect the actual instrument performance in observing configuration.

Usually each optical element has its own transmission curve expressing the efficiency as a function of the wavelength and it is stored in the corresponding file in a two column format.

A residual factor which is built on the fly from the program which manages the ETC input/output is the dependency of the efficiency from the slit loss.