University Lowbrow Astronomers


By Doug Scobel
Printed in Reflections:  December, 1995 and June, 1998.

Well, not exactly.  But, if you like to go after faint nebulae, you CAN get your telescope to perform like a 60 inch with a little, $100.00 (or less) accessory called a Nebular filter.  With it you’ll be able to see things through your telescope that you never thought were possible.

What They Do

Without going into a lot of detail, the way these filters work is to block out most wavelengths of light except for those which are commonly emitted by gaseous nebulae.  The net effect is to let the light of the nebula pass through unattenuated, while the background is darkened.  So, although the object is not made any brighter, it is the resulting increase in contrast of the nebula against the background that makes the nebula become much more visible.  For me, the enhancement was so dramatic that when I tried mine for the first time, it felt like suddenly I had quadrupled the aperture of my telescope!

There are actually two classes of filters currently available.  So-called Light Pollution Rejection (LPR) filters are designed to reject wavelengths from certain artificial lights and natural skyglow, and allow the rest to pass through.  LPR filters are sometimes called broadband filters, because they pass a wide range of wavelengths.  Nebular filters, on the other hand, are actually specialized LPR filters, passing only the narrow portion of the visual spectrum that brackets the desired emission lines, and rejecting the rest.  For this reason nebular filters are sometimes called narrowband filters.

One thing to keep in mind is that these filters work better with certain classes of objects than others.  They work the best with most emission and planetary nebulae, and not very well with galaxies and star clusters.  The reason is that emission and planetary nebulae emit light at certain wavelengths, namely hydrogen beta at 486.1 nm, and doubly ionized oxygen (O III) at 495.9 and 500.7 nm.  In this case, it is easy to design a filter that passes these wavelengths and rejects the rest.  However, stars, galaxies, and reflection nebulae emit light pretty much across the entire spectrum, so there are no wavelengths that can be “singled out”, and the contrast gain is modest at best.

How to Use Them

Usually you simply screw the filter into the rear of the eyepiece you are using.  It can get to be a pain unscrewing and rescrewing the filter, though, if you switch eyepieces often.  If you own a Schmidt-Cassegrain telescope, you can get filters sized to screw into the rear cell of your telescope, in front of the star diagonal.  This will let you change eyepieces without replacing the filter.  If you have an eyepiece with enough eye relief, you can even hold the filter between the eyepiece and your eye.  However, with narrow bandpass filters, you have to be careful to keep the filter exactly normal to the optical axis.  If you don’t, the angle will cause the bandpass to “shift” off the desired wavelengths, dimming the nebula.

In any case, I have found it to be important to keep stray light from hitting the filter from the “eyeball” side.  Nebular filters actually act as mirrors, reflecting the rejected wavelengths rather than absorbing them.  So, if any stray light (even skyglow) enters the eye lens of your eyepiece, it will reflect off the filter and back into your eye, reducing contrast.  Use a shroud, or at least cup your hands around the eyepiece to keep all stray light out.  Try it and you’ll see a big difference.

What Kind to Get

If you can afford to buy only one filter, which one would I recommend?  I own Lumicon’s Deep Sky, UHC, and O III filters.  The Deep Sky is a broadband LPR filter, while the UHC is narrowband and the O III is REALLY narrowband (the UHC passes both the Hydrogen Beta and the O III lines; as its name implies, the O III passes only the O III lines).  At relatively dark sites, like Peach Mountain, the one I find myself using the most is the O III.  It consistently provides the best “60 inch” results.  A close second is the UHC.  I have also heard that Orion’s Ultrablock filter is very good, but I have never tried it in a side by side comparison with the O III or UHC.  The filter I seldom use is the Deep Sky.  It generally provides the least noticeable contrast gain, but sometimes helps with galaxies.  Perhaps in a more light polluted environment it would work better due to the artificial skyglow.  If I could only keep one, though, it would be the O III.

Another thing to consider is the kind of viewing you expect to do.  If you intend to spend most of your time observing small, faint planetary nebulae at high magnifications, then you may wish to consider the Lumicon UHC or Orion’s Ultrablock, instead of the O III.  In my experience, the O III’s extremely narrow bandpass lets so little light through that at high power (say, 200x or higher), the view is sometimes too dim and it’s hard to see any detail.  Whenever I go to high power I’ll use the UHC and actually see more.

Observational Comparisons

Here is a sampling of objects that in my experience benefit greatly from the use of Nebular filters.  Note that what I’ve listed here are only the big, bright, showpieces.  There are many more smaller, fainter (mostly planetary) nebulae that benefit greatly from nebular filters as well.

Also, note that my descriptions here are from the moderately light polluted skies of Peach Mountain, using my 13” Dob.  Other sites and/or telescopes may let you see more or less, but the difference between observing with and without a filter should be similar.

Veil Nebula
This supernova remnant in Cygnus is notoriously difficult to observe.  I had seen it before, but it had always been very faint, and with little detail.  But with the O III filter it really comes alive!  The brighter portions, which are barely visible without the filter are now bright and easily seen, even without using averted vision.  Fainter portions throughout the huge complex come into view.  All the filamentary structure seen in photographs is now apparent - it’s now obvious why it is called the Veil.  I’ve considered making a sketch of it, but there’s so much detail that I doubt that I could begin to capture it all.  Just to see this nebula alone might be worth the price of the filter.

Dumbbell Nebula
The bright planetary nebula M27, is impressive enough without a filter.  But, with the O III, it’s even more impressive.  Its entire oval outline is apparent, with the “dumbbell” shape superimposed on top of it.  There are also fine details in the brighter portions.

Lagoon Nebula
Also known as M8, in Sagittarius, this cluster in nebulosity should be an excellent object if observed from a really dark site.  But from Peach, it’s low in the South, which is usually pretty murky.  The nebulosity is pretty much washed out except for the brighter portions.  But with the UHC or O III, the nebulosity expands to twice the diameter that is visible without it.  Details that were hidden now pop out at you.  The dark “lagoon” now is framed by glowing gas.  It’s a fantastic view.

Omega or Swan Nebula
M17, also in Sagittarius, is typically a pretty good sight, as portions of it have a fairly high surface brightness.  With the UHC or O III, though, nebulosity and detail that were previously hidden are now visible.  Like M8, with the filter the nebula appears to be nearly twice as large as without it.  The “swan” shape (it always looks like a “2” to me) is now very bright, and there is structure everywhere.

Trifid Nebula
You say you have trouble seeing the dark lanes that give M20 its popular name?  With the UHC or O III they’re a cinch - very inky black against the surrounding nebula.  The reflection portion of the nebula nearly disappears, though, with the narrowband filters.  To me it is enhanced slightly by using the Deep Sky filter.

Eagle Nebula
M16, in Serpens, is an open cluster embedded in nebulosity.  Without a filter, I’ve only on the darkest of nights noticed any nebulosity at all.  With the O III, however, the nebulosity pops into view, and the eagle shape is easily apparent.

Helix Nebula
NGC 7293, a very large, low surface brightness planetary nebula, is also situated low in the South in Aquarius.  If you can find it without the filter, it just looks like a slight brightening in the background glow.  With the O III, it now shows its true self, clearly nearly circular with a “hollow” appearance.  Some portions of the ring appear to be brighter than others.

Rosette Nebula
This diffuse nebula surrounding the bright open cluster NGC 2244 in Monoceros has always been nearly invisible to me, only a couple bright portions being apparent.  Low magnification and the O III really makes it pop out.  With it the nebulosity surrounds the cluster like a wreath, with lots of structure and detail.

Great Nebula in Orion
You wouldn’t think that the bright nebula M42/M43 should need any help.  But the O III helps bring out the faint outer portions of the nebula.  At low magnification, the entire fan shape is apparent, and details previously hidden can be seen.  It also enhances M43 noticeably, which without a filter is rather unimpressive compared to its neighbor.

Owl Nebula
M97, a relatively low surface brightness planetary nebula in Ursa Major, has never impressed me much until I used the O III on it.  The O III makes it look much brighter, with a much sharper edge.  The “eyes”, though, still elude me.  I can tell that there are brightness variations, but had I never seen a photograph of them I doubt that I would ever describe them as looking like eyes.  Perhaps they are more apparent from a truly dark site.

So, what are you waiting for?

They’re not that expensive, about $80.00 to $100.00 brand new, even less if you can find one used.  Plus, like a barlow lens, you can use it with any of your oculars.  So, for about the price of a good eyepiece, you too can look through that 60 incher you’ve always dreamed about!  Or at least you’ll feel like you are.


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Copyright © 2013, the University Lowbrow Astronomers. (The University Lowbrow Astronomers are an amateur astronomy club based in Ann Arbor, Michigan).
This page originally appeared in Reflections of the University Lowbrow Astronomers (the club newsletter).
This page revised Sunday, March 9, 2014 4:30 PM.
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