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Baader Fringe-Killer Filter

22 March 2004

Minus-violet filters have been in high demand recently due to the resurgent popularity of achromatic refractors - particularly those with fast focal ratios like the NexStar 80, NexStar 102 and Advanced Series C6-R.  Achromatic refractors, especially fast focal ratio models, suffer from significant amounts of "chromatic aberration" - false color.  This is generally experienced as a violet halo around bright objects, particularly planets, bright stars and the Moon.  The halo is caused by the fact that not all the wavelengths of light come to focus at the same point.  In essence, the lens in a refractor is acting like a prism.  Although it is possible to ignore the halo, the real problem is that the unfocused violet and blue light is spread all across the object you are viewing, robbing it of detail.  Thus, fast refractors seldom live up to their theoretical resolution limit.

Fringe-Killer at a Glance

What is it? A minus violet filter to reduce chromatic aberration in achromatic refractors.
Price: $59 for 1.25" and $87 for 2"
Available from: Alpine Astro ( in North America or directly from Baader ( in Europe

  • Improves detail of almost all objects viewed through fast achromatic refractors
  • Very high optical quality allows filter to be used anywhere in the optical path
  • Dielectric coatings are very durable
  • Additional blocking of infrared light makes this a good general filter for web cam and digital camera imaging
  • I need two :-)  one for permanent installation in the front end of my refractor diagonal, the other for use when imaging

The most common "cure" for this is to simply block the violet and blue light waves.  The problem is, most minus-violet filters introduce their own resolution-robbing aberrations and they typically block significant "good" light, further reducing detail.  Enter the Baader Fringe-Killer.

The light spectrum above shows most of the important characteristics of the Fringe-Killer filter.  Almost all violet light (light below wavelengths of 450nm) is completely blocked, while blue light (450nm - 480nm) is attenuated by 50%.  Just as importantly, all other light is transmitted at higher than 95% efficiency - nearly as good as coated white optical glass.  Baader also incorporated full infrared blocking (650nm and up) to make this a perfect filter for web cam and digital camera imaging.  In short, the light transmission characteristics of this filter are precisely what you would want for the task at hand.

To insure the filter does not introduce any optical aberrations, Baader's Fringe-Killer is a precision ground, fully coated, optical filter - as are all the Baader filters.  As noted in a recent review in Sky and Telescope magazine, this allows Baader filters to be stacked (using multiple filters at the same time) and even placed far in front of the focal plane without degrading the image - unlike competing models.  In addition to starting with precision optical glass, Baader uses their unique coating techniques to insure the glass remains stress-free and perfectly flat.  This involves nearly 50 dielectric coats on each side of the glass.  Additionally, these dielectric coatings are very tough, allowing for cleaning without damaging the coatings.  I have a full set of similarly coated Baader color filters and can attest to their durability through the 4 years I have been using them.

One last note on the uniqueness of this filter.  Baader suggests that the Fringe-Killer combined with their Red filter is a good Hydrogen-Alpha filter.   This combination has a 95% transmission rate at the H-Alpha wavelength and yet costs very little.  H-Alpha filters are great for imaging emission nebula in great detail, though a smaller pass-band would produce better results.

The Fringe-Killer in Use

I used a NexStar 80 GT with a NexStar 3.6mm Plossl eyepiece (111x) for all tests.  Mainly I was interested in false-color reduction on bright objects and improved detail on planetary views.  Seeing conditions were very good, bordering on excellent, on the night when most of these notes were made.  Here are the objects I viewed:

  • w/o filter - Sirius displayed a very strong violet halo.  Focus was touchy.
  • with filter - still exhibited a violet halo, but greatly reduced.  Focus was much easier.  This fact - easier, sharper focus - was true on all objects, but I won't repeat it below.
  • w/o filter - 1 diffraction ring usually visible around the in-focus, Airy disk.
  • with filter - 1 diffraction ring always visible, usually 2 and sometimes a third faint diffraction ring was actually visible.
  • w/o filter - very strong violet halo.  The edges of the planet (about half-phase) were indistinct.
  • with filter - a violet halo was still visible, but much reduced.  The edges of the planet were sharply in focus.
  • w/o filter - a slight violet halo was visible.  I could make out a hint of the Cassini Division at the extreme edges of the rings during moments of excellent seeing.  The surface of the planet showed no detail.
  • with filter - the halo was gone.  I could easily see the Cassini Division at the extremes and occasionally even on the part of the rings in front of the planet.  The planet surface showed one band during moments of best seeing.  The color of the planet was golden-brown.  I was simply amazed at the detail this little short-tube 80mm refractor was showing!
  • w/o filter - strong violet halo.  I could see two distinct bands and faint darkening in the north polar region.
  • with filter - the halo was nearly imperceptible.  More importantly, the South Equatorial Band (SEB) and North Equatorial Band (NEB) were very distinct with texture and a dark, reddish-brown color on the NEB.  Both polar regions showed distinct darkening and at times the north polar region even showed a sharp border.  Again, I was amazed at the views.

In short, this filter transformed the views through this f/5 achromatic refractor into what I would expect to see through an f/12 achromatic refractor.  A fast achromatic refractor will never be the best choice for planetary views, but Baader's Fringe-Killer filter can certainly make it a good choice.  If you've been looking for something to improve your achromatic refractor - this is it!

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Michael Swanson
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