The Baader Planetarium FCFR-2 Red Long-Pass Filter has a bandpass at 610 nm and is designed for 2" eyepieces. It has 98% transmission efficiency and is superb for bright planet and cometary details. It can be placed anywhere in the optical path without introducing astigmatism or other defects. Baader Planetarium astronomy filters by far supersede the mediocre quality of common "flame polished" color filters. 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 planetary surface detail, much more important than a horde of different filters of mediocre execution. The 7-layer MC-coatings not only provide a brighter image, they prevent the softening effect of scattered light. Scattered light kills exactly those subtle features in planetary detail that you are longing to see. Painstaking attention to coatings and perfect optical figuring are simple reasons why the image contrast through these premium filters is so rock-solid.