This 2" Eyepiece Filter Baader UBVRI Bessel U Filter, from Alpine Astronomical is a classic photometric filter with transmission according to Bessel and parfocal with all Baader filters of the same size. This daylight-proof filter uses ion-beam coating technology and features a Reflex-Blocker hard coating, which has been planeoptically polished, and has Life-Coat sealed coating edges. It is equally suitable for modern CMOS sensors as well as for older CCD camera technologies.