Starship Asterisk*

MarkBour wrote:Thanks, neufer, for posting the article about the LAMOST / Guo Shoujing Telescope and its mission. Sounds like some really massive datasets to be gathered. As an earth-based telescope, will its spectra be limited, or is it expected that lots of great data will be gathered here? Are Chinese astronomers likely to make their data available freely, even to the West? Will this result in a more detailed map of the stars in our galaxy, with relative motions? Are there other places that similar/competing data are being gathered that astronomers here are more interested in?

Chris Peterson wrote:

Ann wrote:So, Art, are you telling me, sideways... that my middle name just might be Virginia and that there really is a big old man who is only seen around December 25 every year (December 24 in Sweden) and who wears red whenever you see him?

But only if you believe... really believe... that red is real.

Ann wrote:So, Art, are you telling me, sideways... that my middle name just might be Virginia and that there really is a big old man who is only seen around December 25 every year (December 24 in Sweden) and who wears red whenever you see him?

neufer wrote:

Chris Peterson wrote:

geckzilla wrote:
I get what people are trying to say by "there is no such thing as color" but I beg to differ. Color is how our brains translate certain wavelengths of EM radiation into a signal we understand.

The guy in the video is the only person I've ever heard claim that there's no such thing as color. And from the rest of his comments, it's clear that he's just saying that as a rhetorical device.

Yes, Ann, the not so terribly amazing Dr. B is wrong... there is color! Dr. B has been affected by the skepticism of a skeptical age. He does not believe what he sees. He thinks that nothing can be which is not comprehensible by his little mind. In this great universe of ours, man is a mere insect in his intellect as compared with the boundless world about him, as measured by the intelligence capable of grasping the whole of truth and knowledge. Alas! How dreary would be the world if there were no color! It would be as dreary as if there were no Anns. The eternal blue light with which childhood fills the world would be extinguished. Not believe in color! You might as well not believe in blue fairies!

Ann wrote:Chris, you wrote:

But visual observers more commonly report much less saturated color, as well as a range of yellows, oranges, and reds that are not typically seen in photographs. Camera images are a poor representation of the visual experience of color.

Okay, but so what? If most really bright meteors look green or blue when photographed, then surely the colors reflect some physical properties of the meteors. Surely, in some sense, these meteors are green or blue.

Chris Peterson wrote:

neufer wrote:

Click to play embedded YouTube video.

Do you see colors here? I see nothing but grays. But I have a wheel like this that I use in the classroom, and with that I see lots of colors. So again, a difference I think in how our eyes perceive things directly and how cameras capture them (in this case with the added complexity of video, so time becomes a factor, not just the sensor dynamics).

But visual observers more commonly report much less saturated color, as well as a range of yellows, oranges, and reds that are not typically seen in photographs. Camera images are a poor representation of the visual experience of color.

neufer wrote:Yes, Ann, the not so terribly amazing Dr. B is wrong...

Chris Peterson wrote:

geckzilla wrote:
I get what people are trying to say by "there is no such thing as color" but I beg to differ. Color is how our brains translate certain wavelengths of EM radiation into a signal we understand.

The guy in the video is the only person I've ever heard claim that there's no such thing as color. And from the rest of his comments, it's clear that he's just saying that as a rhetorical device.

geckzilla wrote:I get what people are trying to say by "there is no such thing as color" but I beg to differ. Color is how our brains translate certain wavelengths of EM radiation into a signal we understand. Say you had a transcript in a foreign language and had it translated into your native language. It's like saying your native language doesn't exist because the message existed originally in that foreign language.

neufer wrote:

Click to play embedded YouTube video.

https://en.wikipedia.org/wiki/Fechner_color wrote:

Click to play embedded YouTube video.

Click to play embedded YouTube video.

<<The Fechner color effect is an illusion of color seen when looking at certain rapidly changing or moving black-and-white patterns. They are also called pattern induced flicker colors (PIFCs). Not everyone sees the same colors.

The effect is most commonly demonstrated with a device known as Benham's top. It can also be seen in stroboscopic lights when flashes are set at certain critical speeds. Rotating fan blades, particularly aluminum ones, can also demonstrate the effect; as the fan accelerates or decelerates, the colors appear, drift, change and disappear. The stable running speed of the fan does not (normally) produce colors, suggesting that it is not an interference effect with the frequency of the illumination flicker.

The effect was noted by Gustav Fechner and Hermann von Helmholtz and propagated to English-speakers through Charles Benham's invention of his toy top. The perceptual mechanism of Fechner color is not entirely understood. When the disk is spun, arcs of pale color are visible at different places on the disk. One possible reason people see colors may be that the color receptors in the human eye respond at different rates to red, green, and blue. Or, more specifically, that the latencies of the center and the surrounding mechanisms differ for the different types of color-specific ganglion cells. The phenomenon originates from neural activity in the retina and spatial interactions in the primary visual cortex, which processes pattern recognition (von Campenhausen & Schramme, 1995). Research indicates that the blue/yellow opponent process accounts for all the different PIFCs (Schramme, 1992).>>

Ann wrote:

Chris Peterson wrote:

Tekija wrote:And add also that there is no such physical thing as color, there are only objects that reflect or emit certain wavelengths of radiation and combnations thereof, and how these wavelengths stimulate our photoreceptors varies - about 10 percent of men and a much smaller fraction of women have one of them mutated which greatly alters spectral sensitivity, and many women carry a heteozygous mutation that theoretically can enhance color perception because they then have four rather than normal three types of light sensitive pigments in their retina - and how this information when relayed to the visual cortex is interpreted also varies considerably.

And most of the information from the American Meteor Society about color isn't applicable to this image precisely because of that. What we see on camera images usually differs significantly from what we see with our eyes. I have thousands of witness reports which report completely different colors than what images show.

Still, Chris, very many different photographs show bright meteors as blue or green. I googled "bright meteors" and got many pictures of such meteors, where the majority appears to be predominantly blue or green.

Chris Peterson wrote:

Tekija wrote:And add also that there is no such physical thing as color, there are only objects that reflect or emit certain wavelengths of radiation and combnations thereof, and how these wavelengths stimulate our photoreceptors varies - about 10 percent of men and a much smaller fraction of women have one of them mutated which greatly alters spectral sensitivity, and many women carry a heteozygous mutation that theoretically can enhance color perception because they then have four rather than normal three types of light sensitive pigments in their retina - and how this information when relayed to the visual cortex is interpreted also varies considerably.

And most of the information from the American Meteor Society about color isn't applicable to this image precisely because of that. What we see on camera images usually differs significantly from what we see with our eyes. I have thousands of witness reports which report completely different colors than what images show.

Tekija wrote:And add also that there is no such physical thing as color, there are only objects that reflect or emit certain wavelengths of radiation and combnations thereof, and how these wavelengths stimulate our photoreceptors varies - about 10 percent of men and a much smaller fraction of women have one of them mutated which greatly alters spectral sensitivity, and many women carry a heteozygous mutation that theoretically can enhance color perception because they then have four rather than normal three types of light sensitive pigments in their retina - and how this information when relayed to the visual cortex is interpreted also varies considerably.

Chris Peterson wrote:

Ann wrote:Nice image! As a color commentator, I am of course very interested in the colors of the meteors. Since this is a composite image, the colors may not be absolutely comparable. I can't help noticing, however, that the brightest meteor is blue. Its true color may be more blue-green. What causes the color of the bright meteor and the different colors of the fainter meteors?

Color is complicated with meteors. Most of the color is produced by atmospheric gases, and in some cases meteoroid composition may affect color, too. In an image like this, the short trails near the radiant represent meteors that are traveling almost directly toward the camera. They are entering the atmosphere at a steep angle, and depending on their size may travel deeper and therefore encounter a wide range of atmospheric conditions. The meteors which are far from the radiant are shallower, and mostly higher. Their longer trails represent both a geometric effect, as well as probably longer duration (since they take a little longer to burn up at higher altitude).

The size of the meteoroid affects how hot it gets, as well as the physics of the heat production itself. The temperature impacts the spectrum, and therefore the color.

Add to all of this the various artifacts that can be created by the camera, such as what happens when your light source is made up of individual spectral lines but you collect the data through broad RGB filters, the effects of saturation in some color channels but not others, the variable exposure time created by a fast object moving across pixels (meteor exposure time is determined by meteor speed, not shutter time).

Really, given all the variables, it's remarkable that the colors are as consistent as they are!

Chris Peterson wrote:
the landscape was compostited into the final image.

ta152h0 wrote:Were these meteors all at once ? The background stars seem to be all points of light.

http://www.amsmeteors.org/fireballs/faqf/#5 wrote:

[b][color=#0000FF]The natural colors of a meteor's emission lines. This spectrum of a Perseid was photographed by Julian Nott (U.C. Santa Barbara) using a digital camera equipped with a 1000 lines/mm objective grating during the Aurigid MAC test flight on August 13, 2007. Photo: NASA/Julian Nott (www.nott.com).[/color][/b]

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American Meteor Society

<<The difficulties of specifying meteor color arise because meteor light is dominated by an emission, rather than a continuous, spectrum. The majority of light from a fireball radiates from a compact cloud of material immediately surrounding the meteoroid or closely trailing it. 95% of this cloud consists of atoms from the surrounding atmosphere; the balance consists of atoms of vaporized elements from the meteoroid itself. These excited particles will emit light at wavelengths characteristic for each element. The most common emission lines observed in the visual portion of the spectrum from ablated material in the fireball head originate from iron (Fe), magnesium (Mg), and sodium (Na). Manganese (Mn), Chromium (Cr), Copper (Cu) have been observed in fireball spectra, along with rarer elements. Silicon (Si) may be under-represented due to incomplete dissociation of SiO₂ molecules. The refractory elements Aluminum (Al), Calcium (Ca), and Titanium (Ti) tend to be incompletely vaporized and thus also under-represented in fireball spectra.

Vivid colors are more often reported by fireball observers because the brightness is great enough to fall well within the range of human color vision. These must be treated with some caution, however, because of well-known effects associated with the persistence of vision. Reported colors range across the spectrum, from red to bright blue, and (rarely) violet. The dominant composition of a meteoroid can play an important part in the observed colors of a fireball, with certain elements displaying signature colors when vaporized. For example, sodium produces a bright yellow color, nickel shows as green, and magnesium as blue-white. The velocity of the meteor also plays an important role, since a higher level of kinetic energy will intensify certain colors compared to others. Among fainter objects, it seems to be reported that slow meteors are red or orange, while fast meteors frequently have a blue color, but for fireballs the situation seems more complex than that, but perhaps only because of the curiosities of color vision as mentioned above.>>

Ann wrote:Nice image! As a color commentator, I am of course very interested in the colors of the meteors. Since this is a composite image, the colors may not be absolutely comparable. I can't help noticing, however, that the brightest meteor is blue. Its true color may be more blue-green. What causes the color of the bright meteor and the different colors of the fainter meteors?