Color in Astrophotography (split: Hanny's Voorwerp)

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Color in Astrophotography (split: Hanny's Voorwerp)

Post by Ann » Mon Jun 28, 2010 7:31 am

Chris said:
The true color of the nebula is actually gray, since that's the only way the eye could ever perceive it.
Well, when you are speaking about a green nebula it's grey, eh? Bet you would have insisted that a red nebula is red, no matter how hopelessly grey it must always appear to our eyes!

(getting my pitchforks out)

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Re: APOD: What is Hanny's Voorwerp? (2008 Jun 25)

Post by rstevenson » Mon Jun 28, 2010 11:27 am

Is that a gray pitchforK? I see a hint of green in it, and there's some reflected blue, no doubt. But I can't really tell from the picture what its true colour is. I'd like to see a spectrographic analysis, please.

Rob

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Re: APOD: What is Hanny's Voorwerp? (2008 Jun 25)

Post by Chris Peterson » Mon Jun 28, 2010 3:01 pm

Ann wrote:Well, when you are speaking about a green nebula it's grey, eh? Bet you would have insisted that a red nebula is red, no matter how hopelessly grey it must always appear to our eyes!
"Color" describes a physiological response to some mix of wavelengths and intensities. If the intensities are low, the cones in the retina are simply not triggered, and we see only grays. Hanny's Voorwerp is not bright enough to trigger our color vision, so it truly is gray. It is only by integrating the light for a long period with some sort of instrument, and then converting that data to different wavelengths of light with another instrument that we are able to perceive (very indirectly) an approximation of how this object would appear to our eyes if they were more sensitive. But we can never see that directly or with any passive instrument like a telescope.
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Re: APOD: What is Hanny's Voorwerp? (2008 Jun 25)

Post by Ann » Tue Jun 29, 2010 6:08 am

Chris Peterson wrote:
Ann wrote:Well, when you are speaking about a green nebula it's grey, eh? Bet you would have insisted that a red nebula is red, no matter how hopelessly grey it must always appear to our eyes!
"Color" describes a physiological response to some mix of wavelengths and intensities. If the intensities are low, the cones in the retina are simply not triggered, and we see only grays. Hanny's Voorwerp is not bright enough to trigger our color vision, so it truly is gray. It is only by integrating the light for a long period with some sort of instrument, and then converting that data to different wavelengths of light with another instrument that we are able to perceive (very indirectly) an approximation of how this object would appear to our eyes if they were more sensitive. But we can never see that directly or with any passive instrument like a telescope.
I know that. My point is that you insist that emission nebulae are red, even though we can never see that they are red. I haven't heard you describe emission nebulae as gray!

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Re: APOD: What is Hanny's Voorwerp? (2008 Jun 25)

Post by Chris Peterson » Tue Jun 29, 2010 7:10 am

Ann wrote:I know that. My point is that you insist that emission nebulae are red, even though we can never see that they are red. I haven't heard you describe emission nebulae as gray!
Well, we all use shortcuts when precise terminology isn't required. Obviously when we talk about the colors of emission nebulas, we generally mean that the wavelengths of the emissions correspond to some particular color if they were bright enough to stimulate color vision. I generally describe emission nebulas as gray when discussing their visual appearance, and describe them as red, or cyan, or whatever is appropriate when discussing their appearance in images.

My comment about the actual color being gray was in response to dcrosby's questions about the technical reasons behind this object appearing green in images (and was only part of my response).
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Re: APOD: What is Hanny's Voorwerp? (2008 Jun 25)

Post by NoelC » Fri Jul 09, 2010 2:50 am

Chris Peterson wrote:"Color" describes a physiological response to some mix of wavelengths and intensities.
Where did you get the cockamamie idea to include intensitites in that definition? You can't hope to get people to take that as a given!

If you have an RGB camera that normally shows a reasonable facsimile of visual color in an image taken at "normal viewing" intensities, if that camera is then used to take a very long exposure, and shows an object that's normally too dim for us to see brightly and in a particular color (say, green or red), then that's what color that object is! When I mount my Canon camera on my telescope and point it at things you would say are gray and take very long exposures they come out in color.

If you used optics to concentrate the light from a dim object into a spot bright enough to trigger our color receptors, the color would be there.

There's no hard cutoff here at some arbitrary light level where color just magically "disappears". An argument for this is unscientific and smells of some ulterior agenda.

And you cannot possibly even define that light level. As soon as you try you can find a person for which the level you've picked is different.

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Re: APOD: What is Hanny's Voorwerp? (2008 Jun 25)

Post by Chris Peterson » Fri Jul 09, 2010 3:05 am

NoelC wrote:
Chris Peterson wrote:"Color" describes a physiological response to some mix of wavelengths and intensities.
Where did you get the cockamamie idea to include intensitites in that definition? You can't hope to get people to take that as a given!
I'm not sure what you are objecting to. Color is determined by the spectral characteristics of light, and by the intensity. Take a simple case of a monochromatic source- if you have two intensities, you have two colors. You can also have different mixes of wavelengths that produce the same color.
If you have an RGB camera that normally shows a reasonable facsimile of visual color in an image taken at "normal viewing" intensities, if that camera is then used to take a very long exposure, and shows an object that's normally too dim for us to see brightly and in a particular color (say, green or red), then that's what color that object is! When I mount my Canon camera on my telescope and point it at things you would say are gray and take very long exposures they come out in color.
If you take two images with different exposure times, both long enough to record color, the two images will be made up of entirely different colors unless you first normalize the intensities. If you use your camera on an astronomical target, there's a good bet that the colors you get in the image won't be very close to the colors your eye would see if it were simply more sensitive. The RGB filter matrix used by the camera produces a very different color space than the sensors in your eye, and the algorithms used by the camera for rendering color assume something like a continuum source- they do not do very well when you have narrow emission lines, as is often the case with astronomical objects.
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Re: APOD: What is Hanny's Voorwerp? (2008 Jun 25)

Post by NoelC » Fri Jul 09, 2010 5:48 pm

You say these things with such conviction! I'm surprised.

If we shoot a picture of a terrestrial object in bright light the colors come out in a reasonable semblance of what we see. Camera manufacturers work hard to achieve this.

If we shoot the very same terrestrial object in very dim light - e.g., a long exposure night shot where you can barely even SEE the object and can see no color with the unaided eye - the colors in the photo come out very close to what we see in the shot above. I've done this! It's based on the physics of light and the characteristics of the sensor - which don't change!

The camera is clearly picking up the same visual colors regardless of the light levels.

If I shoot pictures of a laser spot (narrow-band color), the color comes out the proper color in our photos as well - i.e., a picture of a red laser comes out red, a picture of a green laser comes out green. How could you say that is "not doing very well"?

Simple logic here.

When I shoot a picture of M42 with my camera, and it comes out looking like the below, you BETTER BELIEVE it's got glowing red gas and a slight turquoise color near the Trapezium. Trying to convince people otherwise just seems like tilting at windmills to me.
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Re: APOD: What is Hanny's Voorwerp? (2008 Jun 25)

Post by Chris Peterson » Fri Jul 09, 2010 7:07 pm

NoelC wrote:If we shoot a picture of a terrestrial object in bright light the colors come out in a reasonable semblance of what we see. Camera manufacturers work hard to achieve this.
Yes, they do.
If we shoot the very same terrestrial object in very dim light - e.g., a long exposure night shot where you can barely even SEE the object and can see no color with the unaided eye - the colors in the photo come out very close to what we see in the shot above. I've done this! It's based on the physics of light and the characteristics of the sensor - which don't change!
Yes, no argument.

But I'm talking about astronomical objects, which are usually dominated by narrow emission bands. Color cameras are very poor at reproducing accurate color with such objects- that is simply not what their color mapping algorithms are designed to do. When imagers make color images of astronomical targets using separate color filters, those filters have been very carefully designed to provide suitable overlaps across important emission lines, so that reasonably accurate colors can be achieved. The integral filters found on sensors designed for terrestrial imaging have not been designed this way.
If I shoot pictures of a laser spot (narrow-band color), the color comes out the proper color in our photos as well - i.e., a picture of a red laser comes out red, a picture of a green laser comes out green. How could you say that is "not doing very well"?
Because generally it is not. Your eyes are fooling you. Neither the camera nor the display device is capable of accurately representing either a red or green laser pointer. Those wavelengths are not in the gamut of either. Sure, the results are "red" or "green", but that is very different from being accurate. Furthermore, since the color is also defined by intensity, there is only one brightness level that will come closest to matching the laser- and it probably will be too dim.
When I shoot a picture of M42 with my camera, and it comes out looking like the below, you BETTER BELIEVE it's got glowing red gas and a slight turquoise color near the Trapezium. Trying to convince people otherwise just seems like tilting at windmills to me.
But I'm not trying to convince people otherwise. I'm just pointing out that even defining color accuracy for an object like this is practically impossible, and that the color in this particular image is probably only marginal. Certainly, the hydrogen alpha emission from this image, rendered on my calibrated monitor, is quite different from the Ha emission I see in my calibration light source. And in order to get the emission colors about right, it was necessary to distort the star colors (which the camera might otherwise reproduce with fair accuracy).

As long as people understand what pictures like this show- hydrogen and oxygen emission, and thermal continuum- there is no problem. If people start worrying about whether the color is "right", we're in trouble. Because there really is no "right" or "wrong" most of the time.
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Re: APOD: What is Hanny's Voorwerp? (2008 Jun 25)

Post by NoelC » Fri Jul 09, 2010 7:41 pm

there really is no "right" or "wrong" most of the time
On that we agree. No two astroimages ever seem to have the same colors, but you have to admit, in broad strokes the image doesn't have red where there should be green.

What I am questioning is your statements that refute that there is color at all, just because you say we can't see it in "normal" viewing conditions. Frankly it breaks down to just a variant of the old "if a tree falls in the forest" conundrum, except that here we have devices that can measure tree falls long distance and they say that it's happening.
Your eyes are fooling you
I think not. I do kind of know what I'm seeing as well as what I'm doing.

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Re: APOD: What is Hanny's Voorwerp? (2008 Jun 25)

Post by Chris Peterson » Fri Jul 09, 2010 8:08 pm

NoelC wrote:What I am questioning is your statements that refute that there is color at all, just because you say we can't see it in "normal" viewing conditions.
That's not quite what I'm saying. The point is, color is a physiological concept, not a physical one. Almost every DSO is too dim to stimulate the cones at all, so it really is correct to say that its color is gray. There is no purely optical way to make an extended object any brighter, so even with a telescope, the object is still gray. Beyond that, we can ask what sort of color we would see if our eyes could integrate longer than 100ms, or if the object had exactly the same spectral curve, but was brighter. And astroimagers approximate those colors all the time. Obviously, there is a concept of color for such dim objects, even if achieving what could reasonably be called "accurate" color is largely beyond our means.
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Re: Color in Astrophotography (split: Hanny's Voorwerp)

Post by NoelC » Sat Jul 10, 2010 3:06 am

So the disparity boils down to the "gray area" in the interpretation of the word "color". Fair enough.

You go right ahead and say that ionized hydrogen glows gray, and I'll keep saying it glows red (speaking of Ha emissions here), and that Hanny's Voorwerp is green. One of us will have fewer people scratching their heads than the other I think.

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Re: Color in Astrophotography (split: Hanny's Voorwerp)

Post by Chris Peterson » Sat Jul 10, 2010 3:53 am

NoelC wrote:You go right ahead and say that ionized hydrogen glows gray, and I'll keep saying it glows red (speaking of Ha emissions here), and that Hanny's Voorwerp is green.
I wouldn't normally say that either of these things "glows gray". I'd say just what you say, that Ha glows red (or pink) and OIII glows green (or cyan), and so forth. The only reason the matter came up was because we started talking about "true color", and it is important to remember that the term gets pretty soft when we are describing things that we are not able to see in color at all.

I would say that nearly every DSO is gray when viewed through a telescope.
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DN: Living in a Technicolor Universe

Post by bystander » Mon Aug 09, 2010 4:16 pm

Living in a Technicolor Universe
Discovery News | 09 Aug 2010
Ray Villard wrote:Last week astronomers released an arresting color picture of a pair of colliding galaxies, combined from separate images from all three of NASA’s Great Observatories in space. (STScI: Antennae Galaxies: A Galactic Spectacle)

The picture looks too good to be real. Or is it?

This is the most common question I get in e-mails and from audiences in science presentations. An urban legend has developed over the years that the colors in modern astronomical photos are made up in a paint-by-numbers game by publicity-hungry astronomers.

True, if you’ve ever had an opportunity to look at a nebula through an amateur telescope it looks like a gray cotton ball rather that a vibrant tie-dye composition.

Despite what you see in lavish Hollywood sci-fi movies, even if you could warp-speed out to a nebula it would still look grey. Why? Because the light from glowing gasses would be spread all over the sky and hence remain too weak to trigger our eye’s color receptors.
...
Collecting and analyzing these colors is one reason we go to the expense of building huge telescopes. They tremendously amplify our vision by orders of magnitude.
...
The way telescopes collect and assemble color information is fundamentally no different from consumer devices. Astronomers simply have to do it manually when it comes to astrophotography. Why? Because telescopes like Hubble do not have color cameras. Instead, they use black and white detectors that are more accurate, sharper, and more sensitive than a comparable color camera detector.

This is no different in concept than the Techincolor motion picture process that Hollywood perfected in the 1930s. All the cinematography was recorded on sharp black and white film and then optically filtered and printed onto color film stock to ensure color fidelity. Nobody has ever questioned Rita Hayworth’s true skin color

When images are combined astronomers take great effort to preserve the integrity of the information collected from far-flung objects. As with the Technicolor process, the color corresponds to the filters used for the initial set of exposures.
...
Many celestial pictures look garish because the universe is, well, garish. Hot gaseous clouds of hydrogen, helium, and oxygen glow with the same intense color saturation you would see on nighttime stroll down the Las Vegas strip.

The best astrophotography is comparable to Ansel Adams’ nature pictures. Adams worked to extract the maximum information and quality from his photographs. This required many hours in the darkroom for Adams to reconstruct an image that reproduced the broad tonal range and contrast that our eye perceives on a sunny day.

Astronomical colors today get even more problematic because space-borne telescopes like Chandra and Spitzer see in X-ray light and infrared light respectively ...

Besides making pretty pictures, when all this spectral information is properly combined it yields new insights into the nature of the objects.

So sit back and simply enjoy the natural beauty and wonder of the cosmos. We are privileged to live in a time when a marriage of computers, digital imaging and powerful telescope empowers us with a God’s-eye view of the heavens.

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Re: Color in Astrophotography (split: Hanny's Voorwerp)

Post by Ann » Mon Aug 09, 2010 5:40 pm

Many celestial pictures look garish because the universe is, well, garish. Hot gaseous clouds of hydrogen, helium, and oxygen glow with the same intense color saturation you would see on nighttime stroll down the Las Vegas strip.
That's what I like! :mrgreen:

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Re: Color in Astrophotography (split: Hanny's Voorwerp)

Post by Beyond » Mon Aug 09, 2010 8:54 pm

My take on this whole page is summed up by the following: If it looks like a duck and waddles like a duck, your eyes are fooling you, because it's Not a duck unless its in the water.
To find the Truth, you must go Beyond.