Dichroic beam splitter is currently getting important for the state-of-the-art
spectrograph such as the KCRM / Keck, the GMACS / GMT, the WFOS / TMT,
the DESI / Mayall, the PFS / Subaru, or for multi-channel CCD cameras such as
the HiPERCAM / GTC to take images in different colours simultaneously.

Dichroic beam splitter splits the incident beam to two directions, eventually
gives an influence to the performance of at least two or more cameras in
terms of throughput, available wavelength range, or imaging quality.

Essential requirements for astronomical dichroic beam splitter are as follows.

1) High reflection in (often broad) band
2) High transmission in (often broad) band
3) Sharp transition from reflection to transmission or the opposite
4) Minimized flatness error after coating
5) Minimized transmitted wavefront error after coating

Both 1) and 2) contribute the high overall throughput of the instrument.

3) contributes to minimize unavailable wavelength range.
The key technique is to reduce the separation of p and s-polarization at
the specified AOI.

Both 4) and 5) contribute the image quality significantly.

There are few vendors in the world that can achieve all the requirements.
But Asahi is the vendor to fulfill these requirements.

We provided the RSS Red Arm 297mm x 196mm dichroic beam splitter
to SAAO (South African Astronomical Observatory) in 2024.