The Baader Planetarium FCFS-2 Premium Six Piece Color Filter Set includes three bandpass filters (Green, Bright Blue & Dark Blue) and three long-pass filters (Red, Orange, Yellow) and is designed for 2" eyepieces. It provides the largest clear aperture on the market, discouraging vignetting at any focal length. 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 complete set of Baader color filters comprises three long-pass and three genuine bandpass filters, which act similar to a nebula filter, so that only a small portion of the visual spectrum is transmitted to emphasize certain planetary surface detail.
Transmission efficiencies for the three long-pass filters (Yellow, Orange and Red) peaks at 98%, while the efficiency for the three bandpass filters peaks at almost 70%, unheard of for a simple colored glass filter. Each filter cell has a male as well as a female thread to allow stacking for narrower passbands or to combine with the Baader UV/IR-Cut filter. The combination of the Red and IR-Cut filter, for instance, results in an extremely narrow H-Alpha pass-band filter.
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.