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Nebula Filters Cut Sky Glow and Let You See More with Less

Nebula Filters Cut Sky Glow and Let You See More with Less

EQUIPMENT REVIEW filters cut sky glow and let you see more with less. /// BY PHIL HARRINGTON : WILLIAM WILLIAM : ZUBACK

Secret weapons IMAGES FILTER ALL

It seems inevitable. No sooner do you of inorganic coatings with known optical get a new than you begin to wonder what to buy characteristics on thin glass blanks. When certain strike specifi c next. Maybe some filters? Sounds like a good idea, but which layers, the layers refl ect the light rather than ones? Amateurs use many different filters, but the most passing it through. intriguing — and least understood — are light-pollution Setting the record straight reduction (LPR) filters and nebula filters. Th ere are misconceptions about these Because these nebulae are composed fi lters. Some people mistakenly believe LPR How filters work largely of , most of their visible- fi lters reduce all forms of . Can we actually fi lter out light pollution? light emissions occur in the far-red and Th is seems like a natural assumption given Yes, because many outdoor lighting sources blue ends of the , but they the fi lter’s name, but it’s just not the case. do not shine evenly across the visible emit greenish light from doubly LPR fi lters block certain wavelengths, spectrum. Instead, they emit at ionized as well. but they do little to reduce the only a few distinct wavelengths. For Most nebula fi lters muffl e impact of car headlights, instance, the common high-pressure all wavelengths except directed onto buildings, and sodium-vapor streetlight used throughout those associated with other fi xtures using incandes- North America radiates principally in ionized hydrogen and cent bulbs, which (unfortu- yellow wavelengths. Mercury-vapor lamps . nately for the astronomer) shine radiate and blue wavelengths. LPR Because they block all but a at all visible wavelengths. fi lters suppress those light wavelengths, small part of the spectrum, these Rather than the term “light- while allowing others through. fi lters are more correctly called “narrow- pollution reduction fi lter,” it would be “Nebula fi lters” have a diff erent mission. band fi lters.” better to use their proper name: “broad- Th ey are designed for the sole purpose of Other nebula fi lters suppress all visible band fi lter.” Th at’s because they fi lter out a observing emission and planetary nebulae. light except for a specifi c , or wide range of wavelengths. Similarly, Fluorescence causes ionized in these line, that they allow to pass through. “Line nebula fi lters should be referred to as either objects to glow. fi lters” come in two varieties: Oxygen III “narrowband” or “line” fi lters. (abbreviated OIII) and Hydrogen-beta Another common fallacy is that these (Hβ). As we will see, some narrowband and fi lters make deep-sky objects look brighter. line fi lters are more appropriate for certain Not so. Filters decrease all light at all objects than others. wavelengths, but they transmit more of the All three fi lter types are made essentially desireable wavelengths. As a result, the the same way: Within a chamber, a background sky and fi eld darken more machine deposits precise, ultra-thin layers than the target. Th is boosts the contrast, making objects easier to spot. NEBULA IMAGE: JASON WARE JASON IMAGE: NEBULA

© 2009 Kalmbach Publishing Co. This material may not be reproduced in any form  astronomy ⁄⁄⁄ august  without permission from the publisher. www.Astronomy.com ⁄⁄⁄ HOW FILTERS WORK ON SELECTED OBJECTS

Company Type B33 NGC 2371-2 M76 NGC 1514 M42 M1 M37 M81 No filter 0 2 3 0 3 3 4 3 Astronomik CLS B 0 2 4 1 3 2 5 4 UHC N 0 3 4 2 5 4 3 2 O-III O 1 3 4 3 5 4 3 2 H-Beta H 2 1 1 1 4 2 2 1 Baader UHC-S N 0 3 4 1 4 4 3 2 OIII O 1 3 5 3 4 4 3 2 Neodymium B 0 2 3 2 3 3 4 3 Celestron UHC-LPR N 0 3 4 1 4 4 3 2 DGM Optics VHT B 0 2 4 1 4 4 4 3 NPB N 1 4 5 3 5 5 3 2 IDAS LPS-P B 0 2 3 1 3 3 3 3 Lumicon Deep Sky B 0 2 3 1 3 3 4 4 UHC N 0 3 4 3 5 3 3 2 O-III O 0 4 5 2 5 4 2 2 H-Beta H 2 1 1 0 5 1 1 1 Meade 908B B 0 3 3 2 3 3 4 4 908N N 0 3 4 3 4 4 3 2 908X O 0 4 5 3 5 4 3 2 SkyGlow B 0 2 3 2 4 4 4 4 UltraBlock N 0 4 4 3 5 4 2 2 Tele Vue Nebustar N 0 3 4 3 4 5 3 2 O-III O 0 4 5 3 5 4 2 2 Thousand Oaks LP-1 B 0 2 3 1 4 4 4 4 LP-2 N 0 3 4 2 4 3 3 2 LP-3 O 0 4 5 3 5 4 3 1 LP-4 H 2 1 1 0 4 2 1 1

I rated the view of each object on a 0–5 scale adapted from my book Watch as follows: 0 — object invisible; 1 — visible only with great difficulty; 2 — visible with moderate difficulty; 3 — visible with little difficulty; 4 — visible easily; and 5 — wow! B = Broadband; H = Hβ ; N = Narrowband; O = OIII and line fi lters. Although many manufac- turn on a red light to identify the fi lter and turers make both 1¼ " and 2" sizes, I write down my results. restricted my tests to 1¼ " models only to include all manufacturers. Filtering through the results A visual examination showed Not surprisingly, the broadband fi lters did all the fi lters were made to high best against objects such as star clusters and optical standards. Coatings , which radiate a wide range of appeared uniform, and the wavelengths. While a fi lter seldom showed machined metal housings were more detail than an unfi ltered view or either anodized or painted. Each revealed an otherwise invisible target, was labeled for easy identifi cation broadband fi lters improved the visual under dim lighting, and each screwed into aesthetics by darkening the background. my threaded eyepiece barrels smoothly. Of the seven broadband fi lters I tested, Most were black, which could make them the Orion SkyGlow delivered the best diffi cult to fi nd at night if misplaced. Only results. It added just enough contrast to Some amateurs believe these fi lters dim the four Th ousand Oaks fi lters were make an object stand out nicely without the view so much they can’t be used with anodized with diff erent colors. sacrifi cing too much image brightness. I small . Th at’s not true. Any Th e winter were riding also judged the DGM Optics Very High telescope can benefi t from these fi lters. I high during these tests, which gave me a Th roughput (VHT), Meade 908B, and even have used identical fi lters taped to great choice of deep-sky Th ousand Oaks LP-1 fi lters eff ective. the eyepieces of to detect objects to observe. All Images remained bright and clear through faint celestial objects that were observing sessions all three test telescopes: my 4-inch refrac- invisible otherwise. were conducted from tor, and 8- and 18-inch refl ectors. Finally, there’s the myth that my light-polluted Th e VHT did better on nebulae, while fi lters are only for urban and backyard, where I the others were better on star clusters and suburban observing — that typically can see galaxies. Rounding out the pack were the they’re not needed for magnitude 5 stars with Astronomik CLS, followed by the Baader observing out in the my naked eyes. Sky glow is Neodymium and Lumicon Deep Sky fi lters country. Although severe in some directions, but I in a tie, and fi nally the IDAS LPS-P. broadband fi lters make still get a good view overhead. Of the nine narrowband fi lters, I judged their biggest impact Th ese conditions let me see DGM Optics’ Narrow Pass Band (NPB) under severe light how well the fi lters work against fi lter the best on emission and planetary pollution, narrowband moderate light pollution. nebulae. What sold me were the views I had and line fi lters enhance the I tried each fi lter on showpiece of the Nebula, the Little Dumbbell view of emission and planetary objects like the (M42) and (M76), NGC 1514, and especially M42. nebulae no matter where you are. the (M1), as well as on some diffi cult targets, including the Horsehead Testing Nebula (B33). I also checked several To fi nd out just how well today’s most planetary nebulae as well as a few open popular fi lters perform, I gathered more clusters and galaxies. than two dozen broadband, narrowband, To judge how well the fi lters performed, I laid them on a table, mixing up their Phil Harrington is the author of the observing order so I couldn’t recognize one from guidebook Star Watch, published by John Wiley another during the test. Only aft er I & Sons. observed an object through a fi lter did I

 astronomy ⁄⁄⁄ august  ⁄⁄⁄ CONTACT INFORMATION

Astronomik DGM Optics Orion Telescopes and c/o astro-shop P.O. Box 120 Binoculars Eiffestraße 426 Westminster, MA 01473 P.O. Box 1815 D-20537 Hamburg, [t] 978.874.2985 Santa Cruz, CA 95061 Germany [w] www.dgmoptics.com [t] 800.447.1001 [t] 49.40511.4348 [w] www.telescope.com [w] www.astronomik.com IDAS c/o Hutech Corporation Tele Vue Optics Baader 23505 Crenshaw Blvd. #225 32 Elkay Dr. Zur Sternwarte Torrance, CA 90505 Chester, NY 10918 D-82291 Mammendorf, [t] 877.289.2674 [t] 845.469.4551 Germany [w] www.sciencecenter.net/ [w] www.televue.com Th e faint tendrils of M42 looked as [t] 49.8145.8802 hutech though they were stretching outward like [w] www.baader- Thousand Oaks Optical fi ngers grasping at nearby stars. I also was planetarium.de Lumicon P. O. Box 4813 impressed with the images delivered by 750 Easy Street Thousand Oaks, CA 91359 Orion’s UltraBlock and Tele Vue’s Nebustar Celestron International Simi Valley, CA 93065 [t] 805.491.3642 Bandmate fi lters. 2835 Columbia Street [t] 805.520.0047 [w] www.thousandoaks Views through Meade’s 908N, Lumicon’s Torrance, CA 90503 [w] www.lumicon.com optical.com UHC, and the Astronomik UHC fi lters also [t] 310.328.9560 were pleasing although not quite as [w] www.celestron.com Meade Instruments contrasty as the others. Readers looking for 6001 Oak Canyon Irvine, CA 92620 a narrowband fi lter would be satisfi ed with [t] 800.626.3233 any of these. I found the Baader UHC-S, [w] www.meade.com the Celestron UHC-LPR, and Th ousand Oaks LP-2 less eff ective, however. Results were exceptionally close in the OIII round of testing. I wasn’t disappointed band fi lter to your equipment arsenal. If with any contender, but found the views you enjoy searching for tough planetaries through the Meade 908X were the best. I or challenging emission nebulae, then enjoyed the details revealed in M42, as well Hβ fi lter rose above the others, but only by either a narrowband or OIII fi lter is a must- as some otherwise invisible features in the narrowest measure. Th rough my 18- have. For us deep-sky diehards who already several planetary nebulae. Likewise, the inch refl ector, all three Hβ fi lters revealed own the others, an Hβ fi lter will prove Astronomik, Baader, Tele Vue, and the dim silhouette of the handy, even though it likely will see less Th ousand Oaks fi lters each showed some against the subtle backdrop glow of IC 434 than other fi lters. subtleties in objects that eluded detection when there wasn’t a hint of this object Once you start using fi lters, you’ll be in the other fi lters, save for the 908X (and, through the others. All three also showed amazed at what you’ve been missing. It’s the in some cases, the DGM NPB). Th e fi ner structure in M43 — the tiny append- same pleasant experi- Lumicon fi lter darkened the fi eld of view age hanging off the northern edge of the ence as putting on more than the rest. Orion Nebula — than any of the OIII or your “shades” on a Finally, I tested three Hβ fi lters. Th ey narrowband fi lters, and even added new sunny day. X produce positive results with only a handful dimensions to M42 itself. of emission nebulae. Here, the Astronomik All tested fi lters increased contrast between deep-sky objects and the back- ground sky. If you live under a dome of light pollution, consider adding a broad-

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