M42 Images (and why space doesn’t look how you think it does)
so apparently it’s Space Day or something and astronomy is one of this blog’s many hats, and we got far enough into our data reduction for our observing project this semester to make some color images, so here! have a bunch of pretty space pictures, and me rambling about them
All of these are images of M42, aka the Orion Nebula, in various combinations of visible wavelengths. They were taken with the CCD camera and 24-inch telescope at the observatory associated with my university, which I’m not naming because I don’t want that information online. All of them have been cleaned up (removing satellite trails, etc) and flux-calibrated, but no additional corrections have been made.
This is, I think, a useful demonstration of how space doesn’t necessarily look like the famous images we get. The colors in a photograph assembled from telescope data, which usually produces a black-and-white image, depend on which wavelength filters were used to produce the component images and which color channels they were mapped to. There’s also a fair bit of artistic license involved as well.
[R - R broadband filter, G - G broadband filter, B - B broadband filter]
This image is (I think) the best approximation of what the nebula would actually look like to the eye- it’s composed of images taken in the 3 broadband color filters and assembled such that the layers correspond to the color associated with that filter. As such, this captures more of the different colors your eye would pick up on if you could actually fly there and see it directly, so is closer to the true color that you would see.
the Hubble Palette - [R - SII, G - Hα, B- OIII]
This is a spectral line image, meaning that it uses narrow-band filters that are designed to capture the emission and recombination lines of certain elements in the nebula. This gives us more insight into the structure of the nebula than the broadband filters do, and because the wavelengths of the spectral lines are known we can make color images with these as well. This image uses a pretty standard configuration called the Hubble Palette, and a lot of the famous visible-light images from the Hubble Space Telescope use this palette as well. However, as might be obvious from how different they look, this is not analogous to what the eye would see. First of all, narrow-band filters block out a lot of the colors- your eye would pick up on many more wavelengths than are shown in the image and so the colors you see would be very different. Second, and more importantly, the Hα hydrogen line that is mapped to green in this image... isn’t actually green. It’s red. Granted, the SII sulfur line that’s mapped to red is a deeper red, but that doesn’t make Hα a good choice for green, in my opinion. There’s probably something significant about this configuration that makes seeing the structure of the nebula easier or otherwise scientifically worth the sacrifice of realistic color, but you would have to ask a better astronomer than me about that.
[R - SII, G - Hβ, B - OIII]
This is another spectral line image, which was basically my attempt at fixing my major gripe with the Hubble Palette and trying to come up with a spectral line configuration that more closely aligned with what would actually be seen. I replaced the Hα line in the green channel with another hydrogen line, Hβ, which does actually have a blue-green color. Unfortunately, I mixed up my wavelengths, and the OIII oxygen line, while also being blue-green, is more blue than Hβ and so probably should have gone in the green channel instead. Even so, this looks more similar to the broadband image and is probably closer to what you would actually see, but still has the same general limitation of all spectral-line images in that your eyes would see many more wavelengths and therefore different colors than shown here.
I don’t quite remember- I think it was [R - Hα, G - Hβ, B - OII] for both
These two images are spectral-line images made more just for fun/to alleviate boredom while we waited for the professor to get back to us on a solution to a problem we were having with our data, but they’re useful to illustrate the effects of artistic license on how the images turn out. When assembling a color image from the black-and-white components, you can adjust the intensity of the color applied to each layer and the saturation of the entire image to suit your purposes or aesthetic tastes. Thus, the final result does depend on what these values were set to, as evidenced here. The top image is one where I adjusted the colors, and the bottom is one where one of my project partners adjusted them. You can see how the top image is much more purple, indicating that I set the intensity of the blue layer up a bunch while my partner left it alone or turned it down. All the images in this post were tweaked in such a fashion.
so yeah TL;DR: space would probably still be pretty if we looked at it up close, but it doesn’t really look like how it does in all those famous space pictures
