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Baader Planetarium Fringe Killer Filter

The Fringe Killer filter is designed for use with moderately priced achromatic refracting telescopes that are usually not corrected to perform well into the violet and deep red portions of the spectrum. This is among a series of filters marketed as "violet blocking". It is a multi-bandpass filter transmitting most light in the visual spectrum while attenuating much of the light in the violet portion of the spectrum. It also blocks light in the infrared region for CCD imaging, doing all of this while passing a high percentage of the critical yellow green portions of the visual spectrum.

Right: Baader Planetarium Fringe Killer 1.25 and 2 inch filters.(44801 bytes).

The filter enhances contrast, making the objects appear sharper and more clear particularly when observing the brightest objects including as the moon, planets and very bright stars. It does this by reducing the violet that is seen as "blue blur" or "haloing" around bright objects, and also by reducing the deep red secondary spectrum that may be perceptible as a crimson halo around bright objects. And as it blocks the violet, unlike most other "minus violet" filters the Fringe Killer causes less of a perceptible yellow color shifting of the image so colors appear more natural to the eye; one could filter the yellow then this would cut image brightness and unless using first rate filters could introduce other problems.

This filter is not at all necessary for use with reflecting or well made apochromatic telescopes. Nor is it beneficial for visual uses with the faint extended objects (nebulae, etc.).

What Does This Filter Do For Me?
This filter is especially designed to improve the views seen through the ubiquitous beginners refracting telescopes. Colors of the spectrum have differing wavelengths. In normal optical glass the red, green and blue rays of light are bent at slightly differing angles as they pass through. This dispersion shows up as color fringing (chromatic aberration) and it tends to reduces sharpness of the image since all three primary colors are not brought to focus at the exact same plane. Achromatic telescopic lenses refract (bend) the light as it passes through the glass, and this magnifies the variation in focus between colors in the extremes of the visual spectrum - the red and blue rays. Therefore, an Achromat can only bring two of the three primary colors to focus. Since most moderate performance telescopes sold are designed for visual use, and since the human eye is less sensitive to deep violet than crimson red, these telescopes are corrected accordingly. So when one looks through an Achromat at a bright star such as Sirius or Vega, or when one looks at the Moon, one will see a violet colored halo appearing to surrounding such objects, and in some achromatic systems this may appear crimson.

The Fringe Killer rejects almost all below wavelengths of 420nm - in the violet portion of the spectrum, while blue light from 420 nm to about 480 nm is attenuated by about fifty percent. The transmission of this filter forms a plateau within the range between 450 and 480 Nm with approximately fifty percent transmission. It is so specialized a design that it permits light to pass across the remaining visual spectrum averaging about ninety five percent.

This reduction of false violet means that an comparatively inexpensive achromatic refracting telescope with a focal ratio of f/6 or f/8 may appear to have as good color correction as that of an f/10 or f/12 achromat.

The Fringe Killer filter permits reasonably made achromatic refractors to be operated at higher magnifications than without the filter when observing brighter objects, since the violet blurring does not become apparent and dramatically reduce resolution until the telescope is used at magnifications well beyond that otherwise possible without the filter. Details along the terminator of the Moon, or a craterlet in Plato may appear just as sharp and clear at 200x with the filter than at 100x without the filter!

Owners of moderate quality achromats of 80 mm and larger will see a more clear, distinct Venus with a perceptibly attenuated violet halo - reduced enough so that it is no longer obnoxious. Study the polar regions, banding and the great storm on Jupiter. See the banding on Saturn with the famous Cassini division in the surrounding rings. However, as pleasing as these views are compared to life without the Fringe Killer, these do not rival the clarity and fidelity of a well made apochromatic telescope; Jupiter for example will always be a bit more yellow in an Achro with any filtering than how it will appear in a similar sizes Apo without filtering,

A star that might show one diffraction ring surrounding the Airy disc, may with the Fringe Killer show two or more fainter rings.

While this filter may at first glance appear similar to the Baader Contrast Booster, the Fringe Killer with it's added elimination of the infrared (700 nm and up) the Fringe Killer enhances contrast further and also sharpens star images when imaging with webcams and CCD cameras; with Fringe Killer the sky background becomes pitch black - and the stars needle sharp!

This filter achieves in the desired pass band, with the highest transmission rate of all so far available color fringing filter. This brings with it sufficiently light throughput so that high magnification enlargements to not dim the object beyond useful limits.

There are numerous reports from our customers of innovative applications of these filters, for example:

• stacking the Fringe Killer with a Baader Moon & SkyGlow neodymium filter produces a view of the moon and planets with even less yellow tinting of the image than without the Moon & Skyglow filter. The Neodymium filter prevents much of the yellow cast and presents an image more true to color, at the expense of very little light loss. And the combination shows further improved color contrast.
  

• Since it blocks near-infrared wavelengths from just below 700 nm to beyond 1.1 micron, the Fringe Killer is also useful for CCD imaging. It does particularly well when imaging faint nebula when combined with the Oxygen III narrow band nebula filter.
  

• When used with the RG-610 red filter (as provided in the Baader Color Filter Set) this produces a narrow pass band filter with only one 70 nm wide window, and high ninety-five percent transmission - at an unbelievably a favorable price. This combination used on a CCD system with the neodymium Moon & Skyglow filter results in a balanced, breath-taking, high-contrast revealing fine distinct moon and planet details.

The Fringe Killer available in diameters of 1.25 (31.7 mm) and 2 inch (50.8 mm).

How Are These Constructed?
The filter is made with a disc of selected substrate of optical glass that has been ground to be plane parallel and then polished until very smooth. The glass undergoes a vacuum deposition multicoating process where approximately one hundred (100) microns-thin layers of various elements are applied one upon another in a precise order and thicknesses to produce the desired bandpass.

The element then undergoes hard multicoating process consisting of nearly fifty layers on each side to reduce undesired reflections to 0.25%. This improves light transmission and reduce possibility of ghosting or reflected images.

The Fringe Killer coating is from a technical viewpoint, much more difficult to manufacture than even the most narrow band nebula filter.

The filter element is housed in a machined aluminum cell, retained by a thread in retaining ring. The filter name, size and part number are in silver, and this mirror like print when combined with the fine print make this information difficult to read under dim red lighting. Company Seven recommends our customers keep these filters in well organized accessory cases, so that you know which filter is which. It helps to become familiar with how the elements appear too.

Right: Baader Fringe Killer filter transmission curves in nanometers, with the transmission (permeability) given in per cent. The area under the curve is the spectral region which the filter transmits (107,572 bytes).
Click on image to see enlarged view (146,613 bytes).

Features Of The Baader Filters: