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Nebula Filters Cut Sky Glow and Let You See More with Less EQUIPMENT REVIEW Nebula filters cut sky glow and let you see more with less. /// BY PHIL HARRINGTON : WILLIAM WILLIAM : ZUBACK ASTRONOMY Secret weapons IMAGES FILTER ALL It seems inevitable. No sooner do you of inorganic coatings with known optical get a new telescope than you begin to wonder what to buy characteristics on thin glass blanks. When certain light wavelengths 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 hydrogen, most of their visible- fi lters reduce all forms of light pollution. 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 visible spectrum, 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 radiation at ionized oxygen 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, lights 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 doubly ionized oxygen. nately for the astronomer) shine radiate green 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 wavelength, or wide range of wavelengths. Similarly, Fluorescence causes ionized atoms 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 vacuum chamber, a background sky and fi eld stars 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 Orion 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 Star 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 galaxies, 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 telescopes. Th at’s not true. Any Th e winter constellations 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 binoculars 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 Crab Nebula, the Little Dumbbell view of emission and planetary objects like the Orion Nebula (M42) and (M76), NGC 1514, and especially M42. nebulae no matter where you are. the Crab Nebula (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.
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