Dichroic beam splitter is currently getting important for the state-of-the-art
spectrograph such as KCRM / Keck, GMACS / GMT, WFOS / TMT, or for
multi-channel CCD cameras such as 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 wavefront error, throughput, or available wavelength range.

Essential requirements for astronomical dichroic beam splitter are as follows.

a) High reflection in (often broad) band
b) High transmission in (often broad) band
c) Sharp transition from reflection to transmission or the opposite
d) Minimized flatness error after coating
e) Minimized transmitted wavefront error after coating

Both a) and b) contribute the high overall throughput of the instrument.

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

Both d) and e) contribute the image quality of the instrument.

There are few vendors in the world that can achieve all the requirements.
Asahi is one of potential vendors to provide such dichroic beam splitters.

We provided the KCRM 670mm x 265mm, the world's largest dichroic
beam splitter in 2020.
The dichroic met the challenging specifications requested by Caltech.