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 (www.alpineastro.com)
in North America
or directly from Baader (www.baader-planetarium.de)
Improves detail of almost all objects viewed through fast
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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