Though UV filters which have transmission shorter than 300nm are not common for
ground-based telescopes, inquiries on such "UV transmission filter" from astronomers
who are involved in space astronomy are nowadays increasing, e.g. for the ULTRASAT
in Israel or the STAR-X in the United States.

It is technically difficult for any vendors to coat the UV filter that has transmission
below 300nm and blocking ~ 1000nm simultaneously because of the property of UV
coating material.

Asahi succeeded in producing the optical blind UV filter for the PETREL ordered
by Tokyo Institute of Technology in 2021.

[Notice]
Due to abnormal price hike of hafnium (UV coating material), our estimation will be
more increasing compared to before.