Baader Planetarium Anti-Fringing

The FringeFilters Killer, Semi-APO and Contrast-Booster Compared

By Gary Parkerson

I’ve tested three quality achromatic excellent refractors, each exhibiting less Booster. I also had the Baader refractors extensively in recent years: (1) chromatic aberration than I judge ac- Neodymium & Skyglow filter on Hands On Optics’ Astro Telescopes ceptable in achromats of their respective hand to stack with the Fringe Killer, a AT102F11, the same 102-mm f/11 apertures and focal ratios – even the fast strategy I’d learned as a loyal reader of “Super Planetary” refractor covered in AT1523. Sky & Telescope. the March-April 2011 issue of ATT; (2) Nevertheless, each produced enough an Astro Telescopes AT1523, Hands On color fringing at high magnification on Thomas Dobbins Intro- Optics’ 152-mm f/5.9 rich-field refrac- brightest objects to warrant anti-fring- duces Me to Stacking tor covered in the September-October ing filters. While working with the For the past 10 years, I’ve used the 2012 issue; and (3) a classic 6-inch f/15 AT102F11, I took the opportunity to Baader Moon & Skyglow filter more long-focus refractor with Jaegers objec- test each of Baader Planetarium’s current than any other in my nightly casual tive that I helped my local astronomy trio of anti-fringing filters: the Fringe viewing. It tames the mild light pollu- club refurbished several years ago. All are Killer, the Semi-APO and the Contrast- tion of my favorite observing site with-

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out adding an objectionably-unnatural hue, darkening the background sky per- ceptively without unduly dimming tar- gets. Bottom line: It’s my favorite all-purpose filter. I was introduced to the Moon & Skyglow filter by Thomas A. Dobbins’ article, Curing the Refractor Blues, in the April 2004 issue of Sky & Telescope. Dob- Image 1 - Spectrum of the Fringe Killer above the full solar spectrum. bins tested a number of minus-violet fil- ters with a 6-inch f/8 achromat, including the stacked combination of the Fringe Killer and Contrast-Booster filters, and it was that pairing that most impressed me. His image of sunlight glinting off of an electrical insulator – the classic daytime bright-, point- source analog – shot through those stacked Baader filters was closest in over- all hue to his sans-filter image, but with- out the violet fringe. I was so impressed with Dobbins’ report of the Fringe Image 2 - Baader Semi-APO spectrum above the complete solar spectrum. Killer/Neodymium combination, that I bought both. In contrast (pun intended), Dob- bins’ image of the insulator shot through the 6-inch f/8 and Baader’s Contrast- Booster showed excellent cancellation of the violet fringing, but at the cost of an overall yellowish hue that I judged from Dobbins’ report to be more objection- able than the fringing it cancelled. Hands-on testing of the Contrast- Booster would subsequently change my mind, but more on that later. Image 3 - Spectrum of the Baader Neodymium Moon & Skyglow filter above the solar For the next five or six years, I used spectrum. the Baader Neodymium filter almost nightly. It’s not only the most benign light-pollution filter I’ve used, but also allows me to perceive greater detail and in the bright planets, as well as the Moon. Of course, I also combined it with the Baader Fringe Killer whenever viewing brightest objects through a qual- ity achromat. Then Baader introduced its Semi- APO filter, described as combining a stronger version of the Fringe Killer Image 4 - Contrast-Booster spectrum above the solar spectrum. coatings with its Neodymium substrate. The stacked Fringe Killer and

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Neodymium filters worked well, but Neodymium substrate of the Moon & Grant’s imaging rig, and asked that he share stacking them is just one more thing Skyglow filter, combining the benefits of his impressions using the Fringe Killer, I’d rather not have to do, so the Semi- the stacked Fringe Killer-Moon & Sky- Contrast-Booster and Semi-APO filters. I’d APO sounded like a real winner. I called glow filters without the hassles already tested the Semi-APO extensively Bob Luffel of Alpine Astronomical, of threading two filters together. Its and concluded it was my favorite of the Baader’s U.S. distributor, and requested transmission spectrum is shown in three. Its performance reminded me of the one, along with fresh samples of Image 2, again compared to the solar stacked Fringe Killer/Neodymium combi- the Fringe Killer and Moon & Skyglow. spectrum. Image 3 is shows the trans- nation that had satisfied me since 2004, so Although I’d long ago rejected the mission spectrum of the Moon & Sky- that was no surprise. Contrast-Booster based upon Thomas glow above the Sun’s. Austin is an astrophotographer and is Dobbins’ 2004 report, I figured I so focused on that activity that he rarely might as well compare it to the others Baader Contrast-Booster looks through a telescope. I reasoned that while at it, if only to confirm their Alpine Astro describes the Contrast- his mind’s eye would, unlike mine, not be superiority. Booster as “… an aggressive filter, de- prejudiced by expectations generated by signed to deliver the maximum years of through-the-eyepiece viewing. But Baader Fringe Killer sharpness and contrast possible from an I still expected him to agree that the Baader The Fringe Killer is the least aggres- achromat. The result is complete re- Semi-APO produced the most pleasing sive of Baader’s anti-fringing options, moval of the violet haze, with a some- views. He studied Sirius sans filter and formulated to attenuate the most dis- what dimmer and yellower image. The through each of the three Baader anti-fring- tracting defocused wavelengths while re- Contrast-Booster also incorporates addi- ing filters – back and forth, from one to an- taining maximum image brightness and tional RGB filtration for further en- other – and judged all successful in overall color fidelity. My observations hancing color contrasts.” That it canceling the violet fringe to a satisfying de- confirmed these essential characteristics. completely removes violet fringing is a gree, but kept coming back to the Con- Although not as color-neutral as the bold claim, and one I read with initial trast-Booster, which he finally declared the Semi-Apo, it was noticeably more so skepticism – even as I appreciated winner. He repeated the lengthy evaluation than the Contrast Booster. It reduced vi- Alpine’s confirmation that the filter pro- on Jupiter and still felt that the Contrast- olet fringing significantly on bright ob- duced the “dimmer and yellower” image Booster provided the sharpest, most violet- jects, but not entirely. Image 1 shows revealed by Thomas Dobbins’ photos. The free and pleasing views. the spectrum transmitted by the Fringe Contrast-Booster’s transmission spectrum To my eye, the Contrast-Booster did Killer as revealed by an MPI-DADOS is shown above the full solar spectrum in indeed yield the least violet fringing, but spectrograph compared to the solar spec- Image 4. still at the cost of a decidedly yellowish trum below. overall hue. The cooler hues produced by Two Eyes Are Better the Semi-APO filter better fit my precon- Baader Semi-APO Than One ceived notions of ideal views. Austin had The Semi-APO filter overlays a I set up the Astro Telescopes 102-mm no such preconceptions, so reacted solely slightly more aggressive version of the f/11 refractor on a in late November, to the net reduction of fringing. Or so I Fringe Killer coatings onto the 2011, next to ATT Associate Editor Austin thought.

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Image 5 - DSLR image of sunlight glinting from a headlight shot Image 6 - Sunlit headlight through the AT102F11 and Baader’s through the AT102F11 without filter. Fringe Killer anti-fringing filter.

Image 7 - The same headlight through the AT102F11 and the Image 8 - Image captured after adding Baader’s Contrast-Booster. Baader Semi-APO filter. This aggressive filter removed all visible violet fringing.

When Eyes Don’t Agree, university, planning to photograph the sun you to view them instead in ATT’s online Resort to Photography glinting off a street lamp, but the school’s digital version. We shot multiple photos of Sirius dark-sky-friendly shielded the re- Image 5 is typical of the sharpest focus through the 102-mm f/11 achro using an flective glass surfaces from direct sunlight – we could achieve in a single, unprocessed unmodified Canon T2i DLSR, but the re- not so the exposed headlamps of the cars image across the sun-heated asphalt of the sulting images were too inconclusive to set- parked there. We targeted a particularly parking lot, but tells the tale of the chro- tle the debate, so I won’t share them here. clean sample on the far side of the lot and matic aberration exhibited by the 102-mm We met the next day to take daylight im- hoped its owner wouldn’t drive away before achromat on a very bright object without ages through the Baader filters using the we’d completed the project. benefit of an anti-fringing filter. The violet same scope and camera. My guess is that the newsprint interior fringing is minimal considering the bright- We set up in a shaded spot at the pages of this magazine will not do the ac- ness of the reflected sunlight, testament to edge of a large parking lot at our local companying images justice, so I encourage the quality of the little refractor, but it is

62 Astronomy TECHNOLOGY TODAY BAADER PLANETARIUM ANTI-FRINGING FILTERS there. tribute this to the seven-layer antireflection Image 6 was taken after adding the coatings Baader applies to the ultra-smooth Baader Fringe Killer and produced signifi- glass substrate. cant improvement in fringing without in- I shared the Baader filters with a few troducing a distracting degree of unnatural advanced observers, and none noticed fil- hue to the overall image. I now better un- ter-induced reflections. I’ve been accused of derstand why so many observers stick with having a shiny corneas, so perhaps that ex- this option. plains the faint ghosting I experienced at Image 7, taken after adding the Baader rare moments while enjoying these Baader Semi-APO filter, demonstrates filters. The fact that the dim reflections significant improvement in mitigation of moved in concert with movement of my violet fringing – at least to my eyes – and eye may be evidence that it is indeed my does indeed effectively maintain the eye and not the filter-eyepiece combina- natural color balance of the overall scene, tion. The fact that I did not notice these justifying my preference for that filter … occasional reflections when viewing until I saw Image 8, taken after swapping through the same eyepiece sans filters ar- the Baader Contrast-Booster for the Semi- gues that the reflections are filter related. In APO filter. Wow! Do you see any false either case, I don’t mean to make too much color in that image? I don’t. Yes, the overall of it. The reflections I noticed were infre- hue of the image is a bit warmer than that quent and faint, and did not distract mean- captured sans filters or through the Semi- ingfully from my enjoyment of the views. APO, but … wow! The photograph con- the best views I’d managed through it. firms Alpine Astro’s claim of “complete Back to Jupiter Given these experiences, I conclude removal of the violet haze.” The daylight DSLR images we took that the Baader Fringe Killer filter is still through the Contrast-Booster and well worth the price of admission as the Further Reflections on AT102F11 were simply astounding – most benign of the Baader anti-fringing Baader Anti-Fringing Filters enough so to reset my preconceptions of options. If you already have the Aside from the hues the filters add, what constitutes acceptable performance by Neodymium Moon & Skyglow filter, and ranging from negligible to noticeable, I no- an anti-fringing filter. I went back to the 6- you should, then you need only add the ticed internal reflections that were not pres- inch f/15 achromat on the next night of Fringe Killer to achieve correction of violet ent when viewing through the eyepiece steady seeing and enjoyed some of my best fringing similar to that of the excellent alone. These were faint, fleeting and infre- views of Jupiter after adding the Baader Semi-APO filter. Meanwhile, I still like the quent, and only appeared when viewing Contrast-Booster. The seeing let me crank Semi-APOs as the most color-neutral of the the brightest objects, but they did occur the magnification up well past 250x, where three. But, when I want removal of virtually from time to time. They were far dimmer views remained as crisp and contrasty as all visible fringing and maximum contrast than the filter-induced reflections I’ve ex- any of Jupiter I can recall. I knew the old from a quality achromat, the Baader Con- perienced with other products, and I at- long-focus 6-inch was good, but these were trast-Booster is now my go-to solution. SUBSCRIBE NOW!!! BUY A FRIEND A GIFT SUBSCRIPTION! ASTRONOMY TECHNOLOGY TODAY www.astronomytechnologytoday.com

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