The Baader Planetarium FCFR-1 Red Long-Pass Filter has a bandpass at 610 nm and is designed for 1.25" eyepieces. It has 98% transmission efficiency and enhances planetary details and works well with solar filters. It can be placed anywhere in the optical path without introducing astigmatism or other defects. A fine optical polish and 2 mm thick glass allows these filters to be inserted anywhere into the optical path without introducing astigmatism or other defects.For instance, they can be used in front of a binocular viewer without degrading the image in any way. Substrates are free of striae and plane-parallel polished to within 30 seconds of arc. This eliminates any double images or ghosting due to wedge errors in the glass. The filter cell features the largest clear aperture of any telescope filter on the market for complete freedom from vignetting at any focal length.The front "crown" incorporates milled notches, which makes handling and threading the filter an easy operation in the dark. All surfaces are treated with a 7-layer Multi-Coating, featuring a mere 0.25% residual reflection. It is the most elaborate coating ever applied onto amateur color filters, resulting in a filter free of any false reflections under any incident angle of light.Light is the hottest commodity for all planetary observation. Especially during high magnification work, whether you have enough light or not is the deciding factor for the recognition of the subtlest planeta