Telescope Reviews: Color in the Triffid Nebula

The exact color that is seen with most planetaries is usually called a sort of "sea green" (bluish with a hint of green or greenish-blue). The OIII lines (4959 and 5007 angstroms) sit right at the point of the visual spectrum where blue and green seem to blur together, so it is not surprising that different people describe the hue differently. Below, I have tried to duplicate that color in a drawing I did of NGC 7009. The H-Beta line (4861 angtroms) appears somewhat more bluish than green, and it is generally not one of the stronger emission lines in planetaries, so the bluish-green or greenish-blue of the OIII emission is the color most often reported. It is a quite commonly reported hue in the observational literature for the past century, and, as I have easily seen it even without a filter, I see no reason to doubt accounts of that color being seen on some of the brighter emission and planetary nebulae. Indeed, those who can't see even this blue-green or green-blue color in M42 probably won't see much color in any deep-sky object.

The reds are another story, as most people can't see red at the lower light levels present in many nebulae. Still, I can see a little of it even without a filter. One example is the "Raspberry Nebula", the planetary IC 418 in Lepus. Even in my 10 inch, I can occasionally see the faint reddish tinge on its outer edges that give the object its name (in the 14 inch, I can see the reddish edge fairly easily). The middle of the nebula is more bluish in color, but at least at lower powers, I can get the reddish fringe. Another nebula which shows some faint red is the tiny planetary "Campbell's Hydrogen Star" (PNG 64.7 + 5.0) in Cygnus. In my 9.25 inch SCT, it shows a distinct reddish-orange hue, making this tiny planetary fairly easy to identify.

On filters, back around 1990, a friend of mine and I had our two 10 inch Newtonians out at our club's observing site (an old decommissioned Atlas missile silo south of Lincoln, Nebr.) and were using our "new" Lumicon OIII filters. M42 was visible in the east, so I put my scope on it and put in the OIII filter. I was a little shocked at what I saw, as there were some definite faint reds showing up in some parts (47x). My friend put his 10 inch on it and put in his OIII, but he only saw the bluish-green of the OIII line contribution. We took our filters out and compared them. My OIII had a huge "red leak" secondary passband, while my friend's did not. Once we got to swapping filters, my friend now saw the reds using my filter. This is what first convinced me that the faint reds that are sometimes reported are real for those who have eyes sensitive enough to detect color at low light levels. Eventually, Jack Marling of Lumicon found out about what we saw, and we sent him both our filters for analysis (I suggested he leave in the red passband, but later OIII filters eliminated it). I also recall one night in deep twilight when I had my "read leak" Lumicon OIII filter in my 10 inch and put the scope on M8. I was shocked to see almost the entire nebula appear distinctly reddish, with the exception of the very brightest part around the "hourglass nebula" and the very faintest outer tendrils. My new 2" Lumicon OIII does not have any red transmission, so with that filter, I no longer see any red in M42, but thankfully, I have the DGM Optics NPB filter. Indeed, after performing my M42 "experiment" with my 14 inch, I came home and was delighted to find that the areas that I saw red in M42 with the NPB were the same ones that showed reddish or orangish hues on my huge David Malin color print of the sword of Orion which hangs in my living room.

With the Trifid, the object has a mix of OIII, H-Beta, and continuum (reflection) emission. For that reason, I generally will use either a broad-band LPR filter like the Orion Skyglow or Lumicon Deep-sky, or no filter at all. The NPB and OIII filters help the object, but they tend to shrink the main Trifid portion a little, as well as really dimming the adjacent reflection nebulosity (especially the OIII filter). Thus, at least under a dark sky, the broad-band filters generally provide the best view of the Trifid. Interestingly enough, the main Trifid section appears the very largest in extent in the Lumicon H-Beta filter, although again, I like a broad-band filter for it the most. The broad-band filters generally just give a faint "sky blue" hue to nebulae bright enough to show color, but on the Trifid, I don't know if it will be bright enough to trigger any color hues even in my eyes. Still, I will take a good look the next time I get a chance to see the nebula. Clear skies to you.