Random Excellence: Cassiopeia A

Supernova Remnant Cassiopeia A

Cassiopeia A is the most recent supernova in the Milky Way and is among the most-studied objects in the sky. The supernova occurred ten years before the death of Spinoza and twenty years before the publication of Newton's Philosophiæ Naturalis Principia Mathematica, but it was not widely observed by astronomers of the era.

Photo credit: NASA and the Hubble Heritage Team

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Mike

Featured Comment by Lawrence Siegel: "You might be interested to know that a large number of Hubble images are now on display in Baltimore's Walters Art Museum as part of a show entitled MAPS. The images are knockouts!"

Featured Comment by Thiago Silva: "It's a common misconception to assume that satellite and astronomical 'photos' are real color. In fact, 90% of the images you see are in fact composites of bands taken at different wavelength intervals, or single bands which are mapped to a color scale. Most of these images are in a spectral range invisible to us, and are assigned a 'false color' to be better interpreted by our eyes.

"An example:



"A perfect real color view, right? The city is gray, the vegetation is green, the water is blue. All good, except that this image is a composite of short-wave infrared, near-infrared and red channels. The SWIR is assigned to red, because soils reflect more in that range. NIR is assigned to green, because it's where vegetation reflects the most. And the red channel is assigned to blue, since only wavelengths in the visible range penetrate water bodies. Had this image been a real color [image], the vegetation would be almost black, the waters a murky brown, and the whole image would be very foggy as most of the atmospheric interference occurs in the blue range.

"These color assignments are still grounded on knowledge about optical and electromagnetic theory, and are defined to highlight features of interest to the scientists. As our eyes can see a much larger range of color than of shades of gray, we use it as a tool to analyze images. But they are not 'artistic' choices. Some of them look more 'artistic' than others, and those probably are the ones disclosed to the press, but it is wrong to assume there is no science behind the color mapping."

Featured Comment by Simon Griffee: "Hubble site —> Behind The Pictures —> Color as a Tool."

Comments

[Stephen Scharf]

Beautiful shot.

But I have to wonder..."random" excellence or "chaotic" excellence?

-Stephen.

[taran]

Nothing from nothing, but almost all astronomy photographs are products of extensive retouching and extremely haphazard color enhancement (these images are captured in monochrome by the Hubble if I am not mistaken). They are as much a product of the scientists painterly imagination as they are photos. While I can appreciate the reason for this, NASA needs funding after all, I would hasten to say NASA doesn't employ a single color scientist or even attempt to accurately represent color in any of these photos.

Just a heads up... I'm not knocking it (I would rather kids see these photos and ooh and ahh rather than be forced to look at black and whites of the same scene), but color wise, its the same as the "sci-fi" scene painter working with spray paint in camden market.

[Jonathan Irwin]

With respect to the previous comment, yes, there is a certain amount of manipulation required. This essentially amounts to (usually) stitching together a large number of individual frames to make one large image for each filter (same as a stitched panorama), assembling these into a colour map (very similar to the processing a digital camera does to convert the mono pixels from the CCD back into colour, only there the filters are a Bayer array rather than being done one-at-a-time), and then applying non-linear scaling (aka the curves tool). AFAIK digital photographers do all of these, and the funky colours are effectively just the product of messing with the white balance if the filters were something approximating "RGB".

As for "accurate" colour, the human eye is too insensitive to objects this faint to see any colour - so this is a bit of a misnomer since really we define "accurate" colour by what we would see. Yes, it is certainly true that there are some very "false colour" images out there representing wavebands that our eyes aren't even sensitive to, but I don't really see the difference between this and infrared photography, especially with the old Ektachrome "colour IR" film.

As for retouching, it's true some people will spot out certain image artefacts such as dead pixels and blooming, but there is almost never any pixel manipulation done.

It should be noted that I am an astronomer so this viewpoint may be slightly biased :)

[Ctein]

Well, I am a long-long-ago ex-astronomer, so I haven't been biased in decades [grin], and I will support Jonathan 100% on this. I'll go even further.

Astronomical photographs are almost never, ever retouched in any sense a photographer would understand the word, because it would be impossible for researchers to determine what was artifact and what was real. The majority of the content in most astronomical photographs has strong random or pseudo-random components to it. Unlike retouching an ordinary photograph, where you can usually figure out what is "real" and what isn't simply from looking at the content, you can't make such easy determinations in astronomical photograph. Any substantial retouching would simply corrupt the data.

(As an aside I would mention that there has been a serious problem and scandal with extensive retouching of microphographs, and that mess is still getting sorted out. Perhaps the earlier poster is only confused as to venue rather than entirely wrong.)

Furthermore, the color mapping that is done is almost always for important scientific reasons and, yes, NASA does employ color scientists (whatever that term actually means to the poster).

The earlier poster also seemingly does not understand the nature of color photography. With the sole and single exception of Lippmann photographs, all color photographs are made by recording monochrome light at multiple wavelengths and synthesizing a color image from that, either electronically or chemically. Nothing outside of Lippmann has ever recorded color.

Finally, I don't know what the earlier poster would consider "realistic" color in these cases but unless they are well trained in both astronomy and color science, their assumptions would almost certainly be wrong. Even under light-intensified conditions where the human eye can readily discern the colors in astronomical objects, we do not see them anywhere the same way photographic films do. Hydrogen nebulae, for example, are not red; that is an artifact of tri-color photography.

~ pax \ Ctein
[ please excuse any word salad. MacSpeech in training! ]
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-- Ctein's online Gallery http://ctein.com
-- Digital restorations http://photo-repair.com
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[taran]

To Johnathan:

Thank you for the informative reply. Please forgive my obnoxious assertions, you are of course right to point out that these are too faint for me to ascribe any color to them, or complain about realism... this is clearly a reflection of my less than erudite understanding of such captures, and my reluctance to believe such colors could ever exist in nature (but of course we know that the universe, by definition, includes all colors, visible/invisible/real/imagined). My comment arises out of some of these images which look extremely oversaturated and look extremely overprocessed. Maybe thats the way the galaxy looks, I don't get to look through these telescopes or play with the RAW files from the CCD's that make up these cosmic snapshots. If I did these photos I am pretty sure kids would stop being interested in science.

To Ctein:

"Perhaps the earlier poster is only confused as to venue rather than entirely wrong."

No, I am referring specifically to these astronomy photos, and entirely wrong. You say they aren't retouched in a conventional sense, I say that they are, in photoshop, in a very conventional way that makes them appealing to flickr users. :) Thats just what it seems like to me, but I stand corrected.

You are right about the not understanding color photographs though. I only need to read a few of your posts to know that :P

However, I don't think you need the intense background in astronomy or color science to critique these photos, as you mention. I am of the belief that anyone who has worked with a mass spectrophotometer or burned copper in a bunson burner (read high school science class) would have most of the knowledge one needs to extrapolate and assign color to the wavelengths captured via the Wide Field Array camera (or whatever its called inside the hubble), a conventional CCD according to the information I have read. Photons hit certain elements and give off colors should be most of what I need to assess "realism". My definition of course being rudimentary compared to yours.

Thanks for the helpful explanation, big fan of your posts.

[Ctein]

Dear Taran,

"You are right about the not understanding color photographs though."

No, I've looked at your website, and you understand color photographs just fine! Details of the underlying technology may be unfamiliar to you, but there's nothing wrong with your understanding of the medium.

I'm presuming that by now you followed the link to the Hubble website that explains how and why they do the color mapping they do. Something they don't mention there is that what we would call "realistic" color really isn't very useful in astronomy. Such photographs are created not for scientific reasons but, in fact, because they look pretty. Those are the ones that are most often scientifically inaccurate and merely designed to appeal to our senses.

Mapping emission wavelengths to the colors we see can be a tricky thing. In this context, "realistic" color of course means color that looks like what we see. But physically, that's pretty arbitrary. Colors don't look the same to my parrots-- they see four wavelength bands, and one of those bands is in the ultraviolet. Furthermore, in the three visible-light bands, they can discriminate between colors much better than we can; they have narrower color filters built into their eyes than we do. What's a useful description of color to a parrot is not very useful to us, but they're both physically realistic and accurate.

In an analogous way, useful astronomical color vision isn't the same as terrestrial color vision. Our human vision peaks strongly in the green and isn't very sensitive to red and blue wavelengths; that means that some of the "realistic" colors that we see don't have a good connection to physical reality. In astronomy, all wavelengths turn out to be interesting.

For example, grass reflects very, very strongly in the deep red (not just the infrared). If you happen to have a near-infrared filter in your kit, you can see this for yourself. When you hold it up to your eye, you see a very dim red image; if you look at foliage, it looks brighter than anything else. It looks "whiter" in the deep deep red than it does through a green filter. If our eyes rendered that with any kind of physical accuracy, we would describe grass is being orange, not green.

From a photographer's point of view, that would be a nonsensical thing to do. From the viewpoint of a scientist trying to study photosynthesis or the albedo of croplands, describing plants as "green" is just wrong. In the same way, mapping astronomical light so that it is human- vision color correct is "realistic" and visually appealing and looks natural to us, but it is actually usually wrong.

Even photographic film can get it wrong. Astronomical nebulae look red (usually) in photographs because the red hydrogen emission happens to be where the film is very sensitive to red light. The film is relatively sensitive to this color of light much, much more than our eyes. So the film says the nebulae looks red (the same way our imaginary panchromatic vision would say that grass was orange). But seen through large telescopes, most nebulae looked greenish to us.

Which is correct?

~ pax \ Ctein
[ please excuse any word salad. MacSpeech in training! ]
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-- Ctein's online Gallery http://ctein.com
-- Digital restorations http://photo-repair.com
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[taran]

Ctein, thanks very much for the thorough explanation, I am in your debt. Its almost bordering on a philosophical discussion, as you boldly point out, "correct", of course, being a relative term, with a definition floating in the liminal terrain between perception and reality. Reminds me of "Differance", by Jacques Derrida, the accuracy of color dependent on the feeble tools of language and layers of human thought, tainted by inherited prejudices and an illusory desire to believe our eyes aren't lying to us!

Thanks for the kind words regarding color btw. I have been privileged to be on the beta team of Alien Skin's "Exposure" for the last couple years. The goal, of course, is recreating nostalgic, disheveled, psychedelic, and inaccurate (but hopefully pleasing) color from the 35mm films and darkroom techniques many of us used to love but have lost in the move to digital.

BTW, completely off topic, but I would love it if you would review MacSpeech, somewhere.