The Orion Nebula, M42 and M43
12/22/2013 4:50 PM
There's really no two ways about it: The Orion Nebula is one of the best objects to see in the sky, and THE best nebula to observe. But how it is seen is dependent on many different factors, starting with your own eyes.
Fill my eyes.. with that double vision
The retina of the human eye has two types of cells: Cones, and rods. Cones are concentrated at the back/center portion of the eye. This makes sense, given that during daylight hours is when we see the most color, and also have plenty of light to be able to see both in the center of our field of vision, and peripherally. This is called photopic vision.
But rods are the cells our eyes use at night. They pick up lower light levels, called scotopic vision but there's a catch: They also don't perceive color. So much of what we see in the 'dark' (technically, very low light conditions) is thanks to our rods. They are in the central portion of our retina too, but there are many more of them than cones in the outer reaches of the eye, so that our peripheral vision is actually enhanced in darker conditions. This also makes sense for evolutionary purposes - humans would want to be able to see movement in the dark, and either move towards it (hunting) or get away from it (hunted).
What all this means for visual astronomy observing is that when we look at objects through a telescope, we are often perceiving light at lower levels, except for the Moon, most planets, and brighter stars. When looking at extended, dimmer objects, our rods are what perceive light vs. dark. This is why averted vision is a popular way for amateurs to "see more" at the eyepiece.
Brighter is better
But the Orion Nebula, the Lagoon Nebula (M8), the Dumbbell Nebula (M27) and many planetary nebula are exceptions to this. They tend to be bright objects, and though photographs pick up the reds, oranges and pinks in these objects, our eyes peak in the green part of the spectrum. So when it is dark, but there is sufficient light to see some color, our eyes work somewhere between photopic and scotopic, called "mesopic" vision. So we can (sometimes!) see green, blue-green, blues, and occasionally in larger scopes and with younger eyes and with the brightest sections of these objects, oranges and/or reds. Now, the colors are subtle, but they are real, and there.
Aperture, aperture, aperture
More aperture of a given telescope gathers more photons coming from M42, so the image at the eyepiece will be brighter, which increases the likelihood of seeing color. This is where a "light bucket" such as a Dobsonian telescope really helps.
My understanding of the way perception of color is enhanced in objects like the Orion nebula is when the sky has less humidity and dust in it. The more transparent the sky, the more likely it is to see color. So don't just look one night and decide that color can't be observed. My best night of seeing color was an exceptionally clear night: I saw not only green and blue, but orange and a hint of rose/red. I do NOT see those colors every time in my 10" Dobsonian telescope. So look often, and particularly when Orion is at the meridian (highest point in the sky due south), to get above as much "thicker" atmosphere as possible.
For those who never see color, there's not much that can help. Your eyes just may not have enough ability to see color at night. But for others, try this: Shine a flashlight into your eyes, or look directly into a streetlamp, or car headlights, or walk inside and look at a lamp - then go back out and look at M42 again. Your cones are now activated. So you MAY see more color now... or not. If nothing else, it is worth a try.
Charles Messier certainly didn't miss M42, but he thought that the small, comma-shaped bit of nebulosity just north of the larger one was a separate object. In his small, narrow-field telescopes of the period, he couldn't have known that M43 is part of the larger gas and dust cloud in this region. So be sure to look for this "separate, but part of the whole" but of nebula that has its own designation thanks to a lane of obscuring dust in our line of sight.
Check out the video below for where to see this fabulous winter-time object, and if possible, use more aperture on it and try NOT to dark adapt. See what happens, you just might see a bit of color.
And don't miss what you can see in Orion's Sword beyond the nebulae here: The open cluster NGC1981 awaits, as do the double and multiple stars of Orion's sword.