An important optical element to consider during preparation of any OASIS observations in the f/20 mode is the AOB beamsplitter. The beamsplitter divides the light coming from the telescope in two paths: one going to the instrument and the other to the wavefront sensors for the AO correction. There are several beamsplitters available now for the AOB, depending of the spectral range useful for your observations. It is possible to change manually beamsplitters during an observing night. This operation is done by the Observing Assistant (not the interface one, the real one!) when requested but about 15 minutes on the sky will be lost; planning your observations is essential to minimize this lost time.

The beamsplitters available for OASIS observations are given below. In the transmission curve plots, the transmission refers to the light path going to OASIS, the reflectance being the fraction of light going to the wavefront sensors.

• Beamsplitter V
• Beamsplitter R
• Beamsplitter I
• Beamsplitter 50-50
• Beamsplitter 85/15
• Beamsplitter 15/85

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