colors? (APOD 13 August 2007)

[ldecola]

I see lots of images in full color but wonder what the 'true' colors of the objects might be? I.e. what would they look like to me say if I were visiting them?

[Case]

As I understand it, usually the astrophotographer will try to create accurate colors in his images. But the brightness possibilities in photography (very long exposures) is way more than the human eye can capture. Another thing is that the human eye won't see as much saturation at low light levels, making things more gray-like for 'visitors'.


An exception would be to use false color to make it easier to see certain details, either for aesthetically visuals or scientific purposes.

Infra-red and ultraviolet images show wavelengths the human eye could never see, but these images are usually marked as such.

[bystander]

I see lots of images in full color but wonder what the 'true' colors of the objects might be? I.e. what would they look like to me say if I were visiting them?

Depends on the object. If the explanation does not say "false color", it is probably at least an approximation of the true colors. Emission nebula are red in color, the predominant color of hydrogen-alpha emissions. Reflection nebula are blue in color because of the scattering of light by the dust clouds reflecting the light source, much the same as light scattering in our atmosphere colors the sky blue.

For example, in today's apod http://antwrp.gsfc.nasa.gov/apod/ap070813.html, the central part of the Trifid nebula is a stellar nursery, and the red emission nebula is created by hydrogen gas excited by the energetic young stars. The blue in the background is reflection of star light scattered by dust. These colors are probably enhanced somewhat, but are still close to what you might expect.

[jimmysnyder]

I see lots of images in full color but wonder what the 'true' colors of the objects might be? I.e. what would they look like to me say if I were visiting them?

What do these gas and dust clouds look like up close? Locally, I think they are indistinguishable from a good vacuum. you wouldn't see anything at all. The reason we can photograph them is because they are so thick, that the accumulated effect is enough to record. Even so, I think it requires a long exposure. Are there any such clouds that are visible to the naked eye for us on the Earth? If you were in the center of a red emission nebula, but not on a planet, would you see a red sky?

[iamlucky13]

I see lots of images in full color but wonder what the 'true' colors of the objects might be? I.e. what would they look like to me say if I were visiting them?

What do these gas and dust clouds look like up close? Locally, I think they are indistinguishable from a good vacuum. you wouldn't see anything at all. The reason we can photograph them is because they are so thick, that the accumulated effect is enough to record. Even so, I think it requires a long exposure. Are there any such clouds that are visible to the naked eye for us on the Earth? If you were in the center of a red emission nebula, but not on a planet, would you see a red sky?

That's always been my guess. Maybe away from the city lights you'd be able to perceive a greyish haze mixed among the stars, but you'd no doubt need a long exposure to bring out the fine details and the color.

I know, for example, you can see the Orion nebula with binoculars, but it is only visible as a light smudge. The same for the Andromeda galaxy, which spans larger than the moon, but is far fainter.

[jgabany]

I see lots of images in full color but wonder what the 'true' colors of the objects might be? I.e. what would they look like to me say if I were visiting them?

Hello Idecola:

This is a great question!

The human eye's retina contains two types of photoreceptors called rods and cones. There are about 120 million rods compared to approximately 7 million cones. Rods are more sensitive to light but only cones detect color. This is why we can make out objects that surround us, in dimly lit situations, but we cannot discern their hue. Light is comprised of three primary colors, red, blue and green. Of these, the cones in our eyes are most sensitive to the later, which makes some evolutionary sense if your ancestor's survival was dependant upon discerning plants.

Astronomical telescopes are essentially used for two purposes: 1) to help separate distant but closely spaced objects and 2) to collect a lot of light. The amount of light collected by even the world's largest telescopes is still insufficient for the cones in our eyes to detect color in faint nebula and galaxies other than green. Therefore, the full color of distant astronomical places, other than stars and planets, is something that still eludes direct observation. It should be noted, however, that there have been some rare claims of seeing other colors by a few observers who may simply have eyes with more color sensitivity.

But film and digital cameras do not have this type of color bias. Film emulsion contains crystals that are sensitive to each of the three primary colors of light and color digital cameras place microscopic red, green or blue filters on top of their pixels. Manufacturers use various schemes to place these filters, it should be noted, but here's the point: only a portion of the pixels in any color digital camera are dedicated to one color. Regardless, this enables cameras to detect color much more efficiently than human eyes. Digital astronomical cameras go one-step further- they use every pixel for each color.

Cameras specifically designed for taking deep space images are unsurpassed for detecting very faint light but they only produce results in black and white. To create a full color picture, astronomers, both professional and amateur, place a red, green or blue filter in front of the camera so that every pixel is limited to detecting one specific color reflecting or shining from the astro-subject. This, by the way, is a very time consuming process. To create a full color picture, the astronomer digitally combines separate red, green and blues images using commercially available software like Photoshop. Thus, the colors seen in deep space objects taken through a camera are very real and, unless mis-handled during processing, they are also accurate.


The picture that is on APOD today was produced using a total of over 8 hours of exposure. This amount of time enabled the light to be accumulated by the camera's pixels. However, if you were to travel close to this, or most astronomical places, the scene would look different. Most likely all you would see are stars and maybe hints of the nebulosity that has a teal hue in this picture. The rest would be essentially invisible- particularly the red colored areas because their material is very thin and our eyes are not red-sensitive.

The image comparison that Case included with his post does a good job of approximating how this particular scene would appear to our eyes at close distance. The only difference, I would propose, is that all hint of the red color still visible in his right panel would actually appear much less prominent and that which could be seen would be colored greenish-gray.


Jay

[ldecola]

Thanks, everyone, for your thoughtful responses, which I attempt to summarize.

On the one hand, astronomical tools produce renderings according to the following principles:
1) the lens geometry of telescopes magnify the viewing field, i.e. they separate distant, close objects,
2) the sizes of telescopes determines how much energy is collected over the viewing field,
3) cameras gather lots of energy over the time they are exposed, and
4) all of these (as well as processing, printing, projecting on a monitor, etc) inevitably affect the ultimate colors (RGB) on the viewed image.

On the other hand, the human visual experience is a complicated and ephemeral anatomical, neurological and psychological phenomenon that cannot easily be compared to what astronomical tools produce. So the idea that there may be a ‘true’ image is false – in fact we really don’t know even what the person next to us is seeing.

However, if significant amounts of ‘processing’ are applied, this should be noted in the caption.

A fellow geographer has stated the problem succinctly: “Not only is it easy to lie with maps, it’s essential. To portray meaningful relationships for a complex three-dimensional world on a flat sheet of paper or a video screen, a map must distort reality.” Monmonier, Mark (1991). How to lie with maps. Chicago, University of Chicago Press.

[craterchains]

Noe that is interesting, , , grins.