The Hubble Palette and Digital Color Photos
Published on Feb 11, 2008 at 6:05 pm.
16 Comments.
Filed under astrophotography.
This wonderful photo was taken by Steve Tuttle. It is of an emission nebula designated IC 2177, and sometimes called the Seagull Nebula. It is actually a portion of a much larger region of excited gas. This is a region of space where stars are forming. The most massive stars form quickest. But, they also emit a copious amount of ultraviolet light. This UV light ionizes the gas in the vicinity of these new stars, and that gas then begins to glow as the atoms reacquire their electrons.
But, notice that Steve’s image looks far different from the other image that I linked to. That image of the larger region of space shows far more reddish colors, and even some blue right near the nebula. But, Steve’s image looks far different colors. Steve also has another image on his site of the same nebula, and I have reproduced it here, as well:
These are clearly images of the same object, but they look markedly different. So, what gives?
The answer is in how the images are processes. To understand this, we need to understand imaging. There are multiple processes at work in this part of space producing light. The dominant mechanism may be the electron capture and de-excitation of hydrogen, but this part of space contains other elements besides hydrogen. So, these other atoms are doing similar things to produce light. But, each element, when it produces light by de-excitation, produces only certain colors of light. These colors mix together to give a visual appearance to the nebula.
But, now we need to understand how we see light. Your eye perceives color using specialized cells called cones. There are three types of cones that detect light. Each type detects a different range of colors. One type of cone detects mostly bluish and purple light. Another type is typically designated as seeing green light, but it really detects light from blue to reddish-orange. The third type, generally designated as seeing red light, sees from red to green. The red and green cones overlap significantly, and it is a problem with one or the other that can give rise to red-green color blindness. Note that most red-green color blind people can see plenty of colors, but they see red and green light using the same cones and so those colors would look pretty much alike to them.
Imaging systems work in a very similar way. Color film uses three emulsions sensitive to three different ranges of light that cover the visual spectrum. These emulsions, though, typically don’t record all colors of light with the same efficiency, or with the same detection bands, as the cones in the eye, so color pictures often don’t look as if they accurately portray colors. Some colors are more vivid and some are less so than seen with the human eye. Professional photographers know techniques to adjust for this effect. With digital images, the detector is a CCD (Charge Coupled Device) rather than film. The problem with CCDs, though, is that they are not color sensitive. So, how do you get color pictures with CCDs?
There are basically three options for getting color CCD images. The first method used was to simply take three images using three different color filters. You could use, for example, a red filter, green filter, and blue filter. The images would then be what the object looks like in these colors. The computer can then process the images by stacking these colors onto one another, giving a color image. There have been a lot of discussions about this process, and I won’t go into them all. Most of the discussion has been about whether the color filters used should be those that give the most scientific data or whether or not they should match the color range of the cones. The real problem, though, is how the computer displays these colors, because light is really not just three colors. A color having a bit longer or shorter wavelength than the central peak of the filter will be equally dimmed. The CCD and computer don’t know whether or not the dimmer image is bacause it is not so bright, or because it is a color off of the peak. All three look the same to the CCD through the filter. The color is then portrayed as simply the color near the peak of the filter. So, displaying images in that way inherently alters their appearance. Still, the color representation is close enough to what you see for most people to be happy. And, of course, we are used to this way of doing things. That is how television sets display colors.
But, for digital cameras, it is tough to get three images of the subject with three color filters. So, digital cameras have to use a different technique. One way of doing that is to split the image into three colors using a prism. Then, three CCD chips are used to look at each color. This is a very expensive way of doing things, because it involves precise optics and three CCD chips. It also requires quite bright images, since the light is being split. Consequently, this method is seldom used. The third method is to simply put tiny microdot color filters on the CCD chip. That makes the chip much more expensive to make, but cheaper than three chips. So, only a third of the pixels see through each color filter. The electronics then add the images together to produce a single color photograph, in much the same way that images are made using three images taken through three different color filters. The major disadvantage of this method is that it effectively reduces the pixel resolution of the CCD chip by one third. However, you can take a single picture and get a color image. This is how most color CCDs work.
Astronomical objects seldom change much between exposures. And, since resolution is generally important, we usually simply take three images with three filters. That has the advantage, also, of having three separate images that you can then do all sorts of things with electronically when you add them, enhancing certain colors and bringing out extra detail in certain parts of an image. But, of course, that comes at the expense of “true color” images.
But, do you really need to make the images true color? There is often information that can be gained by making one or another part of the image dominate. For example, if you look at a nebula in one wavelength, say that given off by hydrogen as it deionizes, then you can see where the ionized hydrogen is in the nebula. And, you can even use different spectral lines to represent different colors. That is often done to produce the amazing images that you see from professional observatories or space based telescopes. It is important to note that these are virtually always false color images. A common color scheme is the Hubble palette (since many HST images are released using it). The Hubble palette is what Steve Tuttle used in the image at the top of this posting. The Hubble palette uses images from three narrow line filters, S II, Hα ,and O III, to be the red, green, and blue parts of the image.
S II is an ionized sulfur line. The S II filter allows only light from near 672.4 nm wavelength pass (1 nm =10-9 meters). This is a deep red color. The image taken with this filter is the red part of the Hubble palette.
The Hα spectral line, though, is also red. It is centered at 656.3 nm. That is most definitely red in color. However, the Hubble palette assigns this image to be the green part of the final image. Remember, the CCD does not really detect different colors, only different intensities. So, it doesn’t know whether the light making the image is red or green. And, the computer doesn’t care. So, it can display this second image as any color that it wants.
O III is doubly ionized oxygen (two electrons removed). O III filters generally pass two nearly spectral lines at 495.9 nm and 500.7 nm. Light of this color is green. So, O III images come from green light. However, in the Hubble palette, these images are displayed as blue.
When you put all of these colors together, then the image of IC 2177 looks like it does in top image. But, the Hubble palette is most definitely false color. Two slightly different red images make up the red and green parts of the image, and a green image makes up the blue part of the image (thanks to the wonders of computer processing). So, effectively, about half of the visual spectrum is stretched in color to cover the whole spectrum. But, the images produced in this manner are quite spectacular and very pretty. For this reason, they have been widely circulated, and so many people are familiar with how many celestial objects appear in this color scheme. But, many astrophotographers don’t really like the Hubble palette, because the images don’t look anything at all like the old images taken with film cameras. Those images had Hα as red and O III as green. A lot of people have seen these objects using just those filters, so the Hubble palette just looks weird.
But, the Hubble palette is not the only choice for displaying colors. Another color mapping scheme is the CFHT (Canada-France-Hawaii Telescope) system. CFHT uses Hα to be the red part of the image and O III as the green part of the image. This produces images that more closely match what people have seen before of just the Hα and O III parts of the image in their natural colors. The second photo above uses the CFHT palette. But, the CFHT system also uses S II. The S II image is the blue part of the final image. But, S II is actually also red light. So, that means that the final image still doesn’t look the way that it does if you could actually see it.
So, you might ask, why not use a filter for blue light? Well, one advantage of both of these systems is that they use narrow band images. That means that they look at light from only one wavelength, or very near that one wavelength. That means that you reject other wavelengths of light, including a lot of stray light produced artificially (light pollution) or naturally (moonlight). Thus, you can take cleaner images. Also, these wavelengths are from materials naturally found in interstellar space, so they are common in emission nebulae. These images also reject reflected light from other stars in the vicinity of the nebula, so you can see just the nebular structures. But, you might press the matter and ask why not use a narrow band filter for something emitting blue light. Well, the problem there is that there simply are not as many elements common in the interstellar medium that emit much blue light. Hydrogen can, and does, emit spectral lines in a very deep blue (almost purple) color, however the red emission by far dominates. And, an advantage of using different elements is that they can often dominate in different parts of the nebula. So, the blue hydrogen image would likely look just like a dimmer version of the red hydrogen image and would not produce any difference in the image that you couldn’t get by just adjusting the hue of the Hα image, whatever color it may be. Mercury has spectral lines in the deep blue and purple, but mercury vapor street lights also emit those same spectral lines, so the sky is awash in those colors. Whatever you are photographing would simply be washed out. Besides, there is not very much mercury in the interstellar medium. Another candidate might be molecular nitrogen, H2. But, that is also a problem, since the nitrogen in our own atmosphere would absorb that color. And, you would not expect to find molecular nitrogen anywhere in the vicinity of ionized hydrogen or sulfur. So, I guess that these false color images are probably more interesting scientifically.
Anyway, even if the images are not true color, they are very beautiful, so enjoy them.
-Astroprof
Images permission of Steve Tuttle’s Astrophotography
Ed Davies on February 12, 2008 at 5:18 am: 1
Thanks for this article, it reminded me look into why the Blue Snowball (NGC 7662) is blue after looking at it the other evening. OIII, apparently.
Dr. Stuart Savory on February 12, 2008 at 11:59 am: 2
Nice photos
And now an off-topic question here, because your contact-page doesn’t work:
How do free-falling astronauts weigh themselves in space? They need to keep fit and part of that regime is to track their weight(mass) I presume.
Astroprof on February 12, 2008 at 12:39 pm: 3
Hmm. I am not really sure how they weigh themselves, or if they do.
I had to deactivate the contact page due to massive amounts of spam on it.
Dr. Stuart Savory on February 12, 2008 at 1:11 pm: 4
I know they do, because a Russian cosmonaut told me he did so daily, but I forget to ask him how.
Presumably they oscillate between 2 springs and the frequency lets them deduce their mass ???
Stu
PS: You should like this cartoon:-
http://xkcd.com/21/
Astroprof on February 12, 2008 at 1:34 pm: 5
There are several ways that I can think of for them to measure their mass, but I honestly don’t know what they use. And, I am not going to say something just to have an answer since I have no idea! I don’t want to steer you in the wrong direction. But, it is an interesting problem!
That is a cool cartoon!
Observer on February 12, 2008 at 5:02 pm: 6
Very nice discussion. I use a lot of images in my constellation slideshows, and this is great background for that.
The Color of Space…? « Lindly’s Blog on February 15, 2008 at 1:27 am: 7
[…] The Color of Space…? Published February 15, 2008 color surfs Tags: color palette of space One of my favorite color inspiration collage is predominately made up of faded indigo and medium valued desaturated orange and many of the magazine clippings used to make this collage came from a n article in Science magazine featuring pictures from the planet Mars. Since Mars has been relatively close to earth these past few years-when sky gazing, I wasn’t sure if what I was seeing was the reflection of the sun or a direct indication of massive amounts of iron oxide. I have also been fascinated by the images that have been transmitted by the International Space Station and the Hubble telescope, as the color combinations often seem other-worldly and incredibly stunning! Turns out there is a Hubble Telescope color palette and this web site offers explanation= […]
Vito Caravito on April 16, 2008 at 9:45 am: 8
Thank you very much for the article. I saw a show recently on History Channel referring to the Hubble scheme and got the concept, but couldn’t remember the details, especially the wavelengths.
Alas, my skyshots are definately amatur-ish), but they do get me out at night.
Thanks again.
Astroprof’s Page » 100000 Orbits on August 11, 2008 at 9:31 am: 9
[…] see it in visual light, no matter how good of a telescope that you were using. The Hubble team uses a color palette that is not commonly used by other astronomers. The end result is an image that has a lot of […]
Prof. Francois on October 13, 2008 at 2:45 pm: 10
Even if the colours are a bit whack on the scopes’ palette processing, at least we get to marvel at Jah’s greatness and it is what it is, even if the colours are not 100%…Great understandings here…
Sally on October 17, 2008 at 12:21 am: 11
Thank you for this article.
Can you tell me if there is a certain scheme to which color is assigned to which element … Oxygen=blue, and Sulfur=red and Hydrogen is green? ..?
Are most all Hubble images color coded in this way? For some reason I thought the Hydrogen was always red.
I am looking for the Color Coding for the element Magnesium in the Hubble images.
Thank you for any assistance.
Sally
Astroprof on October 17, 2008 at 2:18 pm: 12
Sally, I am not sure that magnesium would have a particular color assigned in the Hubble images. The palette used is a color assigned to the images collected using a particular filter.
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LWK on July 22, 2009 at 5:05 pm: 14
(Followed the link from APOD July 22)
Nice article!
“… molecular nitrogen, H2.”
You surely meant N2! (You know, I’m a chemist… I spot these things!)
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