Pierced focal plane mirror

The focal plane assembly at the Cassegrain adapter has two sets of fibers (one each for the HE and HR modes). Each set consists of two 3 arc-second apertures separated by 1.86 arc-minutes, nominally in the E-W direction (their physical separation is 16.7 mm). This picture (thanks to Sandrine Perruchot) shows the tilted, concave pick-up mirror, located at the focal plane of the telescope, which holds the A and B fiber entrances for one set and, at the same time, allows the field to be imaged onto the acquisition and guiding camera.