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Home > Reviews > Accessories > Filters > Baader Planetarium Fringe Killer

Baader Planetarium Fringe Killer
By Clayton Cramer - 8/30/2004

Why I Love Refractors

The Fringe Killer Filter
Inexpensive achromatic refractors have flooded the market these last few years, offering 5” and even 6” refractors for well under $1000--even with a go-to mount included. Like a lot of amateurs, I’ve been tempted by these deals. My experience has been that at least on the Moon, planets, and other fairly bright objects, a really well made refractor will outperform most commercially made reflectors with twice the aperture.

My first step into the world of refractors was a Televue Ranger. It would often show more detail on Saturn and Jupiter than many 8” reflectors, not because refractors obey a different set of optical laws, but because so many reflectors are poorly collimated, and have poorly made mirrors. When I sold my Ranger recently, it was rather like having to give away my cat when I went off to college--some things are so amazing and so wonderful it is hard to say goodbye. I have seen the Ring Nebula with that Ranger--using averted vision. I have, under extraordinarily still and clear skies, gone up to 200x on Saturn--and the image was just reaching that magic point that I call “the fuzz point,” where more magnification just gives a bigger fuzzy image.

Refractors, unfortunately, have two big problems, relative to reflectors. One of them has been cost--although the flood of cheap achromats has changed the equation a bit. The other is color.

All The Colors of The Rainbow-But Mostly Violet & Yellow

The great deficiency of refractors, from the beginning of the telescopic age, has been chromatic aberration. Rays of light go through a lens, and by the laws of physics, different frequencies are bent at different rates. The red light comes to a slightly different focal point than the violet light--and like Humpty Dumpty, all the king’s horses and all the king’s men, can’t bring those rays back together again.

Chromatic aberration causes two problems, one obvious, one subtle. The obvious problem is that bright objects have very noticeable color fringes. Venus and Jupiter will be surrounded by a glorious violet halo, sometimes extending several diameters around the planet. The Moon will show a violet fringe on one limb, and a yellow fringe on the other.

These color fringes are distracting and aesthetically unpleasant, but that’s not the worst of the problem. Because the different parts of the spectrum reach focus at slightly different points, there is not one focal point, but rather, a range of focal points. As you increase magnification, the distance between red’s focal point and violet’s focal point gets larger and larger; you never get a really crisp focus. Even if you get a crisp focus with red, violet light will be unfocused, drowning out the red part of the image, or vice versa. This problem gets worse as you reduce the focal ratio, increase the diameter of the lens, or increase magnification. Indeed, the problem increases with the square of the increase in diameter--which is part of why amateur refractors, until a few years ago, were traditionally 60mm to 80mm aperture. [J.R. Haviland, “The Refracting Telescope—Principles of Design and Construction,” in Albert G. Ingalls, ed., Amateur Telescope Making Advanced (Kingsport, Tenn.: Kingsport Press, Inc., 1965), 222-23.]

Over the centuries, astronomers and opticians have come up with different techniques to get around this problem. One solution was to use very long focal ratios. As refractor tubes became increasingly ungainly, seventeenth century astronomers such as Christaan Huygens dispensed with the tube completely, putting the objective at the top of a pole, and using a system of pulleys to aim it at the eyepiece.[Ralph Lorenz and Jacqueline Mitton, Lifting Titan’s Veil: Exploring the Giant Moon of Saturn (Cambridge, U.K.: Cambridge University Press, 2002), 5-6.] In the eighteenth century, a London optician named John Dollond came up with the combination of flint and crown glass lens to create the modern achromatic refractor--better than a single type of glass, but the images were still a bit colorful.

For the last few decades, manufacturers have made refractors using Extra-low Dispersion (or ED) glass, fluorite, and other exotic materials, with the goal of producing “apochromats,” from the Greek prefix “apo” for “away from” or “beyond.” Some of the refractors that use ED glass, such as my Televue Ranger, are extremely good. At the highest magnification, my Ranger would show color on Jupiter and Saturn--not a lot, but enough that you could appreciate a true apochromat. Some manufacturers call these ED glass refractors “semiapos”; others (usually apochromat owners) prefer to call them “very well corrected achromats.”

If money were no object, the decision about whether to buy an achromat or an apochromat would be easy. Unfortunately, apochromats aren’t cheap; figure that a 4” apochromat will cost at least twice or three times as much as an achromat the same size. The difference becomes even more dramatic as you get above 4”. As much as I enjoy the view through an apochromat, I just could not talk myself into spending $4000 to $6000 for a 5” refractor. What were the alternatives?

I recently purchased a 127mm f/9 refractor. Photon Instruments of Mesa, Arizona imported 150 of these achromats, made in China to Photon’s specifications, and sold them for $650 plus shipping. When last I checked, they had four left--and no prospects of getting anymore of similar quality, at a similar price. Consequently, this isn’t really a review of the refractor. It is a review of my attempts to solve the problem of chromatic aberration.

I tested two very different products to correct chromatic aberration: the subject of this test is the Baader Planetarium Fringe Killer, a 1.25” filter that screws into your eyepiece. (It is also available as a 2" filter.)

Testing Methodology

In some ideal world, I would have all the optical testing gear to unleash tables of graphs, showing you how the Fringe Killer performed across multiple frequencies, relative to the original scope. Instead, I can only tell you what I can see--and in some senses, this is the most important test. The human eye and brain do not respond identically to different wavelengths of light. The human eye is far more sensitive to the red and green parts of the spectrum than to blue, and even in blue, the eye’s sensitivity peak is quite narrow. What I see matters more than what a light meter sees, because humans read this test--light meters don’t.

I performed all of the tests in this article on one evening. To make sure that I was comparing equivalent viewing conditions, having picked a celestial object, I would observe it through several eyepieces, installing the Fringe Killer after I had used each eyepiece.








eyepiecemagnification (with 1147mm f/9 refractor)
Orion Ultrascopic 35mm33x
Omcon 18mm orthoscopic64x
University Optics 12.5mm orthoscopic92x
Orion 9mm orthoscopic127x
Celestron 6mm orthoscopic191x
University Optics 4mm orthoscopic287x


I observed from my suburban Boise backyard on the night of March 23, 2004. The sky was clear, with very little moisture, but some turbulence. While my location was not perfectly dark or perfectly calm, it approximates what most refractor users will experience. My targets were an 8% illuminated Moon, Venus, Saturn, and Jupiter. (The Moon was on the horizon, casting little light, by the time I observed Saturn and Jupiter.)

I should add that I have taken this telescope to darker, more hospitable skies before, without the Fringe Killer. It performed considerably better in the uncorrected form than the results described below. Consider the following results indicative of the relative benefits of the Fringe Killer, not the best possible results for this refractor.

The Baader Planetarium Fringe Killer is a filter reduces the violet end of the spectrum by about 50%. This not only reduces the esthetically objectionable violet haloes, it also means that you are no longer trying to focus on quite so broad a range of focal points. A filter that aggressively removes part of the spectrum should not only reduce those violet haloes, it should also improve detail, because you now have a smaller range of focal points from which to choose.

Against Blue Sky, Showing Color Change
The downside of this filtering approach is that it changes the color of objects, turning white objects a bit yellow, and bluish objects a bit closer to white. It also reduces the total light reaching your eyepiece. (Some other filters that improve color fringing do so with an extreme cutoff of the violet end of the spectrum, causing predictably more severe effects on total light throughput and color shift than the Fringe Killer does.) Of course, the celestial bodies where chromatic aberration is most severe are the really bright ones. Just remove the Fringe Killer from the eyepiece when you look at faint objects.

The Moon

My first test subject was the Moon--a subject bright enough that chromatic aberration is readily apparent. I started with the 35mm Ultrascopic. Low power eyepieces are generally least affected by chromatic aberration, and this showed in the relative performance. There was a slight, not objectionable color fringe visible in the naked scope, with only slightly less color using the Fringe Killer. Unsurprisingly, because of the low magnification, and the advantages of the orthoscopic design, the 18mm Omcon orthoscopic showed even less color fringing at the limb than the 35mm. The Fringe Killer showed a very small, but visible improvement over the uncorrected scope.

Raising the power revealed the advantages of the Fringe Killer over the plain jane refractor. At 9mm, the Fringe Killer allowed me to pick out detail on the floor of a small crater near the terminator--a crater floor that was featureless with just the uncorrected refractor. At 6mm, the uncorrected scope had reached its useful limits under these viewing conditions. The image had turned fuzzy, and the violet fringe on both the limb and edges of craters was strong enough to impair detail. With the Fringe Killer, however, 6mm produced an image about as sharp as I had seen with the unfiltered 9mm eyepiece; I had not reached the fuzzy limit yet. Those details on the crater floor that had been visible but indistinct now seemed to be craters. With the Fringe Killer, 4mm was now beyond the fuzz limit. I could resolve no more detail than at 6mm, and the shadows along the terminator that should have been black were now purple.

Venus

Venus is among the most demanding planets for a telescope. It is most visible when the sky is not yet completely dark, and the echoes of the day’s heat add turbulence. It is very bright. Even when you have resolved it, the best that you can hope for is a featureless mass of clouds. Aggravating whatever problems the telescope might have, even very good eyepieces often stumble because of their own chromatic aberration and internal reflections.

Venus was between 40° and 35° above the horizon. The 35mm Ultrascopic eyepiece with the standard telescope showed a violet halo around Venus, perhaps half the diameter of the planet. This problem became markedly worse with the 18mm, making it difficult to get a crisp focus with any of the higher-powered eyepieces.

The Fringe Killer made a world of difference; at 18mm, the violet fringe was visible, but not distracting. As I went to 9mm and 6mm, the fringe became larger, although still not interfering with the image. Only at 4mm with the Fringe Killer would Venus no longer come to a crisp focus.

Saturn

Saturn was about 10° past the meridian as I started my testing. Saturn, not being very bright, is not one of the more difficult targets with respect to chromatic aberration. Its rings, however, do present some opportunities for testing resolution of fine detail.

Without the Fringe Killer, I could see slight color fringes on Saturn with a 12.5mm orthoscopic, and they were certainly visible (although not terribly offensive) at 9mm. Cassini’s Division was barely visible at the ansae (the portion of the rings to either side of the planet) with the 9mm, and I could see a temperate zone cloud band on the planet. Moving up to 6mm was just above the fuzz limit, with no more detail visible under these conditions, although Cassini’s Division seemed a little more visible than at 9mm. The 4mm eyepiece was well above the fuzz limit, with Cassini’s Division now completely lost.

With the Fringe Killer, I could find no color in any eyepiece. The Fringe Killer made no apparent improvement in detail over the uncorrected scope until I reached 6mm. At this point, Cassini’s Division was still not visible all the way around the planet, but it was more visible with the Fringe Killer than without it. Moving up to 4mm, the Fringe Killer was just reaching the fuzz limit. Cassini’s Division was still not visible all the way around the planet, although you could clearly see that it was much darker at the ansae than in front and behind the planet.

Jupiter

Jupiter was most of the way to the zenith by the time I turned the scope towards it. Here the Fringe Killer demonstrated its value most impressively. An unfiltered 18mm showed very apparent color fringing, with the violet halo becoming increasingly distracting as I increased the power. The Fringe Killer with the 18mm orthoscopic did not completely eliminate the fringing, but reduced it to a barely noticeable level. In both cases, the level of detail was the same: two dark bands, with polar darkening.

Moving up to the 9mm eyepiece, the Fringe Killer turned a violet halo 1/5 diameter of the planet into a barely noticeable fringe. While the amount of detail was still the same--I could see three dark bands across the planet--the Fringe Killer certainly enhanced the contrast of the polar darkening.

At 6mm, the unfiltered eyepiece showed a violet fringe about 1/3 Jupiter’s diameter. The Fringe Killer reduced this to 1/5 Jupiter’s diameter, and the color was certainly less intense. I was unable to see any more detail than at 9mm. The move up to 4mm exceeded the fuzz limit for the scope alone, and using the Fringe Killer. There was no more detail visible than at 6mm, and the violet halo was lost in the general confusion of the image.

The Fringe Killer isn't going to keep apochromat makers up at night, worrying about their sales, but it is a cost effective way to take the bargain basement achromatic refractor and get a bit better image and a bit more power. Compared to more effective (and much more expensive) alternatives to be discussed in a later review, Fringe Killer is painless to install and use. It screws into your existing eyepieces, assuming that your eyepieces have threaded barrels. It is portable to any telescope, and perhaps even some of the ED refractors might get some benefit from it.

Clayton E. Cramer is a software engineer in Boise, Idaho. http://www.claytoncramer.com


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