EQUIPMENT REVIEW

Nebula filters cut sky glow and let you see more with less.

ALL FILTER IMAGES ASTRONOMY: WILLIAM ZUBACK

/// BY PHIL HARRINGTON

Secret weapons
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 specific
next. Maybe some filters? Sounds like a good idea, but which layers, the layers reflect 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. There are misconceptions about these
Because these nebulae are composed filters. Some people mistakenly believe LPR
How filters work largely of hydrogen, most of their visible- filters reduce all forms of light pollution.
Can we actually filter out light pollution? light emissions occur in the far-red and This seems like a natural assumption given
Yes, because many outdoor lighting sources blue ends of the visible spectrum, but they the filter’s name, but it’s just not the case.
do not shine evenly across the visible emit greenish light from doubly LPR filters 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 filters muffle 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 fixtures 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.
filters suppress those light wavelengths, small part of the spectrum, these Rather than the term “light-
while allowing others through. filters are more correctly called “narrow- pollution reduction filter,” it would be
“Nebula filters” have a different mission. band filters.” better to use their proper name: “broad-
They are designed for the sole purpose of Other nebula filters suppress all visible band filter.” That’s because they filter out a
observing emission and planetary nebulae. light except for a specific wavelength, or wide range of wavelengths. Similarly,
Fluorescence causes ionized atoms in these line, that they allow to pass through. “Line nebula filters should be referred to as either
objects to glow. filters” come in two varieties: Oxygen III “narrowband” or “line” filters.
(abbreviated OIII) and Hydrogen-beta Another common fallacy is that these
(Hβ). As we will see, some narrowband and filters make deep-sky objects look brighter.
line filters 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 filter types are made essentially desireable wavelengths. As a result, the
the same way: Within a vacuum chamber, a background sky and field stars darken more
machine deposits precise, ultra-thin layers than the target. This boosts the contrast,
making objects easier to spot.
NEBULA IMAGE: JASON WARE

© 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 filters. Although many manufac- turn on a red light to identify the filter 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 filters did
all the filters 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 filter seldom showed
machined metal housings were more detail than an unfiltered view or
either anodized or painted. Each revealed an otherwise invisible target,
was labeled for easy identification broadband filters improved the visual
under dim lighting, and each screwed into aesthetics by darkening the background.
my threaded eyepiece barrels smoothly. Of the seven broadband filters I tested,
Most were black, which could make them the Orion SkyGlow delivered the best
difficult to find at night if misplaced. Only results. It added just enough contrast to
Some amateurs believe these filters dim the four Thousand Oaks filters were make an object stand out nicely without
the view so much they can’t be used with anodized with different colors. sacrificing too much image brightness. I
small telescopes. That’s not true. Any The winter constellations were riding also judged the DGM Optics Very High
telescope can benefit from these filters. I high during these tests, which gave me a Throughput (VHT), Meade 908B, and
even have used identical filters taped to great choice of deep-sky Thousand Oaks LP-1 filters effective.
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 reflectors.
Finally, there’s the myth that my light-polluted The VHT did better on nebulae, while
filters 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 filters
country. Although severe in some directions, but I in a tie, and finally the IDAS LPS-P.
broadband filters make still get a good view overhead. Of the nine narrowband filters, I judged
their biggest impact These conditions let me see DGM Optics’ Narrow Pass Band (NPB)
under severe light how well the filters work against filter the best on emission and planetary
pollution, narrowband moderate light pollution. nebulae. What sold me were the views I had
and line filters enhance the I tried each filter 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
difficult targets, including the Horsehead
Testing Nebula (B33). I also checked several
To find out just how well today’s most planetary nebulae as well as a few open
popular filters perform, I gathered more clusters and galaxies.
than two dozen broadband, narrowband, To judge how well the filters 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 after I
& Sons. observed an object through a filter 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 Planetarium 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
The faint tendrils of M42 looked as [t] 49.8145.8802 hutech
though they were stretching outward like [w] www.baader- Thousand Oaks Optical
fingers 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 filters. 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 filters also [t] 310.328.9560
were pleasing although not quite as [w] www.celestron.com Meade Instruments
6001 Oak Canyon
contrasty as the others. Readers looking for
Irvine, CA 92620
a narrowband filter would be satisfied with
[t] 800.626.3233
any of these. I found the Baader UHC-S, [w] www.meade.com
the Celestron UHC-LPR, and Thousand
Oaks LP-2 less effective, however.
Results were exceptionally close in the
OIII round of testing. I wasn’t disappointed band filter 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 filter rose above the others, but only by either a narrowband or OIII filter is a must-
as some otherwise invisible features in the narrowest measure. Through my 18- have. For us deep-sky diehards who already
several planetary nebulae. Likewise, the inch reflector, all three Hβ filters revealed own the others, an Hβ filter will prove
Astronomik, Baader, Tele Vue, and the dim silhouette of the Horsehead Nebula handy, even though it likely will see less
Thousand Oaks filters each showed some against the subtle backdrop glow of IC 434 starlight than other filters.
subtleties in objects that eluded detection when there wasn’t a hint of this object Once you start using filters, you’ll be
in the other filters, 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). The finer structure in M43 — the tiny append- same pleasant experi-
Lumicon filter darkened the field 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 filters. They narrowband filters, 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 filters 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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