The 65mm square, unmounted Baader 3.5/4nm f/3 Ultra-High-Speed Filter Set from Alpine Astronomical features three precision-grade filters recommended for extremely fast optical systems from f/3.4 to f/2.3. The set includes H-alpha, O-III, and S-II filters optimized for use with CMOS sensor cameras and suitable for CCD sensors.
3.5/4nm half width (to harmonize exposure time of H-alpha with O-III / S-II), recommended for extremely fast optical systems from f/3.4 to f/1.8
Specially designed for filter requirements of highly opened optics, such as the Celestron RASA astrograph
Enables high contrast even with the strongest light pollution to minimize the size of stars and make nebula details accessible which is useful under a Bortle 9/8/7/(6) sky
Reflex-Blocker hard-coated and optically polished, with Life-Coat sealed coating edges
Blackened edges all around, with a filter front indicator in the form of a black outer edge on the front
Optimized for CMOS sensor cameras and suitable for CCD sensors
FWHM on each filter category is carefully designed to allow for 1:1:1 exposures, matched for typical CMOS quantum efficiency and s/n ratio
Identical filter thickness to existing standards, with attention given to parfocality in the design