The Baader Bessel-U filter is a precision-grade photometric bandpass filter with a central wavelength window between 320 - 400nm. The UBVRI system was developed in the 1950s for the 0.9m McDonald Observatory telescope and initially consisted of a Johnson UBV filter set, later extended by Cousins to include the R and I filters for the red region of the spectrum. The filters of the extended UBVRI system according to Johnson and Cousins include: U – Ultraviolet, with a transmission window between about 320 and 400 nm. B – Blue, with a transmission window between about 400 and 500 nm. V – Visible, with a transmission window between about 500 and 700 nm. R – Red, with a transmission window between about 550 and 800 nm. I – Infrared, with a transmission window between about 700 nm and 900 nm. According to the state of the art at that time, the filters had different thicknesses and transmittance values as well as no sharply defined edges (if any). Their spectrum therefore corresponds to a curve instead of a plateau. In 1990, M. S. Bessel looked into the subject in order to find a filter combination for the new CCD cameras that were becoming increasingly widespread at the time. These Bessel-UBVRI filters are still standard today and are also the most widely used in amateur circles such as the AAVSO. The new generation of Baader Planetarium UBVRI filters, introduced at the end of 2021, is a modern filter set that is both fully compatible with the characteristics of the original Bessel filters and at the same time meets modern demands for contemporary filters: The photometric filters now have the same thicknesses as all other Baader filters. They fit into any commercially available filter wheel, for which the original filters were often simply too thick and could not be fixed securely. They are parfocal with all other Baader filters of the same size and now also available in sizes up to 100x100mm - large enough even for modern, professional sensors, or optimal for easy handling in the common filter mounts for amateur telescopes. The coating of all photometric filters is manufactured in the same technology as the CMOS-optimized narrowband and LRGB filters. Like all Baader CMOS-Optimized filters, it is designed to work very well with modern highly-sensitive astronomy cameras and telescopes with focal ratios between f/15 to f/1.8. Baader's CMOS-optimized filters have excellent halo and reflection control, increased contrast and even more narrow passbands than previous generations. These filters set the new standard for monochrome astrophotography with CMOS sensors. Baader CMOS-optimized filters - advantages Very high contrast due to steep edges of the coating Reflex-Blocker coatings to minimize reflections caused by correctors, reducers and other optical elements Standardized thickness for maximum possible parfocality - less need to refocus due to different filter thicknesses Blackened edges with side markings in the form of a black outer rim on the telescope side Each filter is individually fine-polished and coated with sealed coating edge, preventing moisture ingress Life-Coat for hardness and age-resistance, making the filters able to withstand tougher conditions for longer For more information on Baader's new generation of CMOS-optimized Bessel filters, check out this blog post! Specifications Transmission range Transmission according to Bessel specification Central wavelength UBVRI-U Suitable for f/ratio f/15 to f/1.8 AR-coating Reflex-Blocker hard coated, planeoptically polished Filter thickness 1.25", 2", 31 mm, 36 mm 2 mm (without cell), 6 mm with cell Filter thickness 50.4 mm, 50 x 50 mm, 65 x 65 mm, 100 x 100 mm 3 mm Filter size 1.25", 31 mm, 36 mm, 2", 50.4 mm round, 50 x 50 mm square, 65 x 65 mm square, 100 x 100 mm square
