This filter is designed for photometric UV photography in the Bessell U band.
Bessel filters for the digital age: In 1990, M. S. Bessel set himself the task of finding a filter combination for the CCD cameras that were becoming increasingly popular at the time. These Bessel UBVRI filters are still the standard today and are also the most widely used in amateur circles such as the AAVSO. The V filter in particular has proven itself as an inexpensive introduction to photometry: it corresponds approximately to visual brightness; since only one measured value is used, it is ideal for learning the techniques.
