Baader Planetarium 2" Fringe Killer Filter - Chromatic Aberration Control for Achromatic Refractors
Achromatic refractors are among the most popular and affordable telescopes for amateur astronomy - and for good reason. They offer excellent mechanical reliability, long optical life, and consistently good performance across a wide range of targets. Their one well-known limitation is chromatic aberration: the tendency of a simple two-element achromatic lens to focus different wavelengths of light at slightly different distances, producing the characteristic blue-violet fringing around bright objects such as the Moon, planets, and bright stars. The Baader Planetarium Fringe Killer filter addresses this limitation directly - suppressing the unwanted colour fringes optically, at a fraction of the cost of upgrading to an apochromatic or ED refractor.
How the Fringe Killer Works
Chromatic aberration in achromatic refractors is most pronounced at the blue and violet end of the visible spectrum. The short wavelengths between approximately 400 nm and 480 nm are brought to a focus noticeably closer to the objective lens than the green and red wavelengths, which is why bright objects appear surrounded by a blue or purple halo rather than a clean, sharp edge. The Fringe Killer filter uses a precisely controlled transmission profile that reduces light transmission in the 450-480 nm range - the specific blue wavelengths responsible for most of the visible fringing - while maintaining approximately 95% transmission across the remainder of the visible spectrum up to 700 nm.
The effect is a dramatic reduction in the blue-violet fringing around bright objects, with natural colour balance largely preserved across the rest of the spectrum. The Moon loses its distracting purple halo and shows sharper, more contrasty detail. Planets appear cleaner and better defined. Bright stars resolve to tighter, whiter points rather than bloated blue-fringed discs. The overall impression is of an instrument with noticeably better optical correction than its specification would suggest.
Optical Construction - Nearly 100 Coating Layers
Achieving a precise and stable transmission profile across the visible spectrum requires sophisticated coating technology. The Baader Fringe Killer filter uses a stack of nearly 100 individual coating layers deposited onto flat-parallel optical surfaces. This multi-layer interference coating is what defines the filter's transmission curve with the accuracy needed to selectively suppress only the problematic blue wavelengths without affecting the rest of the visible spectrum. The flat-parallel surfaces - a standard that Baader Planetarium applies across its filter range - ensure that placing the filter in the optical path introduces no image shift, no astigmatism, and no degradation of optical quality at any magnification.
The optical glass and coating quality meet the standards expected of a premium Baader Planetarium product - a manufacturer whose filters are used by amateur and professional astronomers worldwide and whose coatings are renowned for their precision, durability, and consistency across production batches.
A Cost-Effective Alternative to Apochromatic Optics
The performance improvement that the Fringe Killer delivers to an achromatic refractor is remarkable when set against its cost. A genuine apochromatic or ED refractor of comparable aperture - one that corrects chromatic aberration through the optical design of the objective itself - typically costs twenty times or more than an equivalent achromat plus the Fringe Killer filter. For observers who own a good quality achromat and want to push its performance closer to apochromatic standards without replacing the instrument, the Fringe Killer is one of the most cost-effective upgrades available in amateur astronomy.
The filter is particularly valuable for Moon and planetary observation with small to medium achromatic refractors in the 60-150 mm aperture range, where chromatic aberration is most noticeable and where the improvement from the Fringe Killer is most immediately visible. At higher magnifications - precisely where colour fringing is most distracting - the difference between filtered and unfiltered views is dramatic.
Additional Filter Combinations
H-alpha Filter Combination With a 610 nm Red Filter
The Baader Fringe Killer has a useful secondary application that makes it particularly attractive to astrophotographers and solar observers. When combined with a Baader 610 nm red longpass filter, the two filters together create an effective H-alpha passband approximately 70 nm wide, with transmission of approximately 95% in the H-alpha region around 656 nm. This combination provides a high-transmission, affordable entry into H-alpha solar and nebula observation and photography - at a cost significantly lower than a dedicated narrowband H-alpha filter of comparable transmission.
Enhanced Moon and Planetary Contrast With Neodymium Filter
Combined with the Baader Neodymium Moon and Skyglow filter, the Fringe Killer creates a particularly effective combination for lunar and planetary observation. The Neodymium filter suppresses yellow-orange sky glow and enhances the contrast of fine surface detail, while the Fringe Killer eliminates the blue fringing that would otherwise degrade the sharpness of the combined image. The result is outstanding contrast and definition on lunar craters, mountain ranges, and rilles, as well as on planetary surface features - an exceptional combination for observers who want to extract the maximum performance from their achromatic refractor.
Practical Details
The Fringe Killer is available in 2" format, fitting directly into any 2" eyepiece, star diagonal, or filter holder. The 2" size provides unobstructed coverage of the full field of view at low and medium magnifications without any vignetting, making it equally suitable for wide-field observation and high-magnification planetary work. The filter can be used in combination with any eyepiece barrel diameter by placing it in a 2" star diagonal or 2" to 1.25" adapter ahead of the eyepiece.
Technical Specifications
- Filter type: blue-suppression / fringe elimination filter for achromatic refractors
- Barrel size: 2"
- Reduced transmission: 450-480 nm (blue-violet fringing wavelengths)
- Transmission across remaining visible spectrum: approx. 95% (up to 700 nm)
- Coating layers: nearly 100 multi-layer interference coating
- Optical surfaces: flat-parallel (planoptical standard)
- Primary use: Moon and planetary observation with achromatic refractors
- Combination use 1: + Baader 610 nm filter - creates approx. 70 nm H-alpha passband at 95% transmission
- Combination use 2: + Baader Neodymium Moon and Skyglow filter - exceptional lunar and planetary contrast
- Manufacturer: Baader Planetarium (Germany)
