Eyeballing Big Game Nebulas
Most of the time, nebulas (or more properly, nebulae) fit comfortably in the field of any eyepiece with a modestly large field of view. But some of these fluorescing gas clouds are so huge that they require the very widest field eyepiece at our disposal, and we still may not be able to see the whole thing in a single field of view. These large objects also have low surface brightness, so the secret to finding nebulae that span a large swath is to have a lot of dark sky around them for the sake of contrast. One solution is to use either a small rich-field telescope or binoculars.
An alternative is to have no magnification at all; that is, use just your naked eye, plus a narrow-band nebula filter, such as a UHC or Orion UltraBlock (the bandpass of each is almost identical). In the following examples, I was using a 2" format UltraBlock filter simply held up to my eye. There's nothing magic about the UltraBlock. My Lumicon UHC filter works very well too, except that it's in the 11/4" format, and I prefer the larger 2" filter for naked eye observing because it allows you to easily move your eye around a larger area of sky to locate the target. Moreover, in some cases you may have to use averted vision to catch the nebula, and this is not quite as easy with a 11/4" filter.North America Nebula (NGC 7000) in Cygnus - This has always been regarded as one of the easiest of the large nebulae, even visible naked eye without a filter in a very dark sky. Using a telescope even in a dark sky, I've always had to scan back and forth across the field until one of the brighter edges becomes visible. Usually it's the area around the nebula's "Gulf of Mexico". It's not a difficult object, but despite using a wide field eyepiece, contrast with the background sky is not great enough for it to pop right out at you. That's pretty much the case with all these objects. Yet I found the North America Nebula, and neighboring Pelican Nebula, to be relatively easy targets in 9x60 binoculars from my back yard when they were high overhead. I've also seen them by just looking through the UltraBlock filter.
California Nebula (NGC 1499) in Perseus - This is sometimes a difficult object in a telescope because of its low surface brightness and large size (140 x 40 arc minutes). But naked eye using just an UltraBlock filter from my back yard it showed up fairly well as a large oblong glow, even one night with a six-day-old moon nearby.
IC 348 in Perseus - This one surprised me because I came across it by accident one night. At first I thought it was the California, but then realized it was in the wrong spot. It too is very large and of oblong shape, though not so large as the nearby California nebula. Both are in the same naked eye field of view.
Rosette Nebula (NGC 2237, 2238 & 2246) in Monoceros - The only time I was able to get this nebula in a single telescope field was with a low-power, homemade copier lens refractor. The surface brightness is uneven, and it's normal to see only faint disjointed patches. This impression is evidenced by the three NGC designations - early observers thought the patches were discrete nebulae. With the naked eye plus a filter it appears as a circular hazy patch. Although classified as a diffuse nebula, it's a supernova remnant, radiating strongly in the doubly ionized oxygen band. With this in mind, I tried a 2" Baader OIII filter, but its narrower passband darkened the sky so much that I could see no sign of the Rosette.
I've also tried the naked eye method on the Veil Nebula in Cygnus with no luck. The strands of the Veil are thin and sinuous, and without benefit of magnification I suspect a larger mass is needed in order for it to be visible. I'd be interested to hear if anyone has ever seen it naked eye.
Of course, a dark sky is better, but don't be deterred if you're flooded by light pollution. A narrow-band nebula filter can diminish a lot of skyglow. To get the most advantage, cup your hands around the filter to protect your eyes from any ground lights or skyglow. One trick I read about is to look through a cardboard toilet paper tube while holding the filter on the outer end. And as mentioned earlier, try averted vision by looking slightly to one side of where the object lies. Normally it's hard to make out exact shapes; large nebulae tend to appear only as faintly glowing patches.
In a very dark sky, many nebulae can be seen without using any filter at all. And of course, the low magnification and wide field of binoculars makes them excellent instruments for studying these objects. But just for the fun of it, try the naked eye plus filter method in your local skies if you crave some deep sky observing without the bother of hauling out a telescope.Published in the August 2007 issue of the NightTimes