Rich Field Telescopes and Nebula Filters

In a previous article I discuss low power observing through a RFT and its flavors.

Here I amplify my discussion about nebula filters and their usage.


I earlier pointed out that the "maximum apparent surface brightness" of a RFT and low powers is vital to the use of nebula filters, because the filters themselves are blocking most of the light, both from the background (good) and from most target objects (not good):

A nebula filter may transmit a passband of under 10nm. Your dark-adapted eye has a natural passband of around 100nm, though its efficiency drops off at the passband edges. So the filter is cutting the background (and continuum sources like stars) by a factor of ten or so. Now you do not see the "dark grey background", you see a black (or very close to it) background. So the "increase the power makes both object and background darker" is no longer true, because the background is close to black. And the nebula filter will cut the brightness of the target too, because the target will not be radiating all of its light at one wavelength: it may be a mix of nebula and continuum-sources and even if it purely nebular, there is more than one nebula line. So, using nebula filters costs you light and cuts the need for higher powers to darken the background: a perfect use for a RFT.

Nebula Filters

I am a big user of modern nebula filters. I use them with my telescopes. I use them with my binoculars. I have been known to use them naked-eye: no optics, just a filter. I own over a dozen two-inch filters. (I own multiple instances of filters because I use them with binoculars.)

Using nebula filters with a "filter slide" makes it easy to switch between filters, and especially from a filter to no-filter. This makes using the filters much less combersome and fiddly. You can see a filter-slide in use with an RFT in the photos of one of my RFT refractors.

Filter Selection

The "regular" filters that I use are the Lumicon "deep sky", "ultra high contrast", O-III, and H-Beta filters. I use the UHC and the O-III most of the time. The Deep Sky is more a general anti-light-pollution filter than a nebular filter. The H-Beta is for a few special targets that are well-known "H-Beta objects": California Nebula, Horsehead, ...

I also own two narrow (4nm) Custom Scientific filters, one at 500.7nm (O-III) and one at 510nm (no lines there). This enables an interesting observing game: shifting rapidly back and forth between them with a filter-slide. Stars and other continuum-sources do not change at all as you switch, but an object that is largly O-III will dim or even vanish in the 510nm filter. This "blinking" will be the subject of a future web-page.

Nebular Objects for a Rich Field Telescope

In the old days (pre-filters), most nebular objects in the sky were un-interesting visually. Oh, you had the Orion Nebula, and you had the Lagoon, but most of the objects that made great color photographs were barely visible. The Rosette? Looks like a star cluster with some haze. The Veil? Yeah, I can see it, but it does not impress. Fainter objects? Forget about it.

Nebula filters changed that. Now objects like the Rosette and the Veil can be very nice and I can see much detail and the high surface-brigntness objects like M42 and M8 are simply spectacular. And many many objects are within reach.

Where to begin? You can start with the larger nebular objects that you know about. Then move on to fainter objects. Sky-charts are an obvious source. I used the Sharpless Catalog: I scanned it for objects that are a) fairly large and b) not in his lowest brightness category and created an observing list. The Sharpless article "A Catalog of H-II Regions" of 10 September 1959 is available via the NASA ADS system. I have posted a copy of the ADS PDF file here. And the Sharpless-object pages at Galaxy Map are a valuable resource.

Wide-field astro-photographs are another resource. Early on, I had never heard of the big bubble of Sh2-27 surrounding zeta Ophiuchus. I was looking at a wide-field narrow-band image taken by Richard Crisp that covered much of Sagittarius and Ophiuchus and saw this bubble away from the Milky Way and said to myself "Wait, what the heck is that?" And I was pleasantly surprised to discover that a nebular object I'd never heard of was visible in an RFT -- indeed best in filter-equipped binoculars.

Some of My Favorite Nebular Objects

This is just a sampler to "whet your appetite". Contact me if you would like to correspond about nebular targets in detail: I have many observations recorded. This list is arranged by RA:

  • "Pacman nebula" NGC 281 in Cass, 1.5 deg E of alpha Cas. RA 0h05m Dec +56d38m. 35' in diameter. Not as bright as you'd expect from photos. Can you see the "mouth"?
  • California Nebula (Pegasus) NGC 1499, Sh2-220. 145' x 40' (!) Illuminated by nearby Xi Persei (M=4). RA 4h00m Dec +36d37m. Requires dark skies. You want a H-Beta filter. Trust me. Nothing without filter, a smudge in O-III, H-Beta: Oh wow!
  • The Orion Nebula. It is OK to look at eye-candy. With a O-III and good skies, M42 reveals all sorts of detail.
  • Sh2-264 at 39=lambda Ori 6.5 deg diameter. RA 5h35m Dec +9d56m. This is another bubble, like the zeta Oph one. Do not believe the small nebula that is drawn in Sky Atlas 2000. You are looking for a big bubble. The easiest way to spot it is to look for the south-west edge, about halfway from lambda Ori to Bellatrix. After learning what to look for, in a dark sky I can see the south part of this bubble with 10x42 binoculars and no filter! (The north part is harder, because the edge is less edge-y.) Easiest is the south-west and west edges with a small RFT and UHC or O-III filter.
  • Rosette Nebula NGC 2237, 2238, 2239, 2246. With cluster NGC 2244. Sh2-275 RA 6h32m Dec +5 deg About one degree in diameter. The Rosette isn't as bright as you might expect from photos, but it is very pretty in a dark sky.
  • Sh2-27 A giant red bubble surrounds zeta Oph at RA 16h37m Dec -10.33'. It is eight degrees in diameter! Check out this little-known object with binoculars and filters under a dark sky.
  • NGC 6559 and IC 4685 in Sag. 1.5 deg east of Lagoon about 1/2 degree across. RA 18h10m Dec -24d07m. After you feast on the Lagoon, can you see this nebulosity, covering most of a "Z" of stars?
  • Omega Nebula M17, Sh2-45. Ra 18h21m Dec -16d11m. Also called the "Swan Nebula". Central bright region 20x15' I love this guy. First, the bright region (which I liken to a "2") is good eye-candy. Second, there is much detail over the whole region that rewards study.
  • M27 The Dumbbell Nebula RA 20h00m Dec +22d43m 8' by 5' in size. Small for this list, but a wonderful object that responds superbly to filters. Almost all of its light is in nebular lines. I can switch between an "O-III" filter and a "green continuum" filter at 510nm that doesn't pass any nubular lines and the Dumbbell goes from a fine object to "nothing there".
  • Cygnus Loop, Veil Nebula NGC6992-6995. Center about at RA 20h52m Dec +31 2.5 deg diameter. I couldn't leave this guy out.
  • Helix Nebula in Aquarius. NGC 7293. RA 22h30m Dec -20d48m 16' diameter. Big and in a empty region of the sky. Study the ends and see how much detail you can see -- compare with a photo afterward.
  • Acknowledgement: I thank Chris Beckett for questioning my earlier statement that Sh2-27 was a SN remnant: I don't know where I got that idea, but it doesn't seem supported by facts :-O



    I would be happy to correspond about observing with interested individuals.

    I can be reached via email as