Computation of etendue without a telescope

[oczoske]

The etendue is computed in OpticsManager from the on-sky solid angle covered by a pixel and the (purported) area of the entrance pupil. The latter is chosen by OpticsManager.area as the maximum area of all surfaces in the optical train. Usually, that would be the primary mirror, fine. However, when the telescope is missing, e.g. when using internal calibration units, the etendue needs to be computed differently. For one thing, only surfaces in either a pupil plane or a focal plane can be used.

• pupil plane: we need the area of the pupil plane element and the solid angle that the beam corresponding to on subtends in that plane (the pixel scale in that plane)
• focal plane: we need the solid angle that the following (or a downstream) pupil element subtends when seen from the focal plane (can be obtained when the effective focal ratio is known), and the size of a pixel when projected into the focal plane (from pixel scale and the plate scale).

A computation for the METIS WCU is given in Sect. 2.5.1 of E-REP-MPIA-1203 (which gives the WCU essentially the same etendue as for the ELT).