Starship Asterisk*

starsurfer wrote:I used to feel that way about narrowband a long time ago. However, just because an image contains narrowband data doesn't necessarily make it false colour, it depends on the processing.

"False color" or "pseudocolor" generally refers to a mapping scheme whereby visible wavelengths in one band are mapped to a completely different band, or where invisible wavelengths are mapped into visible bands. "True color" mapping (in reality there is no such thing as true color) refers to mapping where visible wavelengths are mapped to new bands that correspond to similar wavelengths. All multiple channel images are mapped in some way.

The colour of OIII is generally blue but can also be green or turquoise. In many nebulae, there are areas where both Ha and OIII overlap and their colours are generally pink or purple.

Pure, bright Ha is perceived by the human eye as a deep, saturated red. Pure, bright [OIII] is perceived by the human eye as a saturated cyan. In broadband color images of most nebulas, we see neither of these in isolation, but rather we see them contaminated by essentially white light (blackbody emissions from stars). The more white, the more unsaturated they appear. And of course, as you note, mixing the wavelengths because of multiple emission bands can create entirely different colors.

Some people get surprisingly emotional about narrowband imagery. I booted someone off the APOD Facebook page the other day for being so nasty about it. I find the amount of negativity and distrust for narrowband to be very discouraging because to me it is incredibly beautiful. They just can't seem get over the (wrong!) idea that they are somehow being lied to as soon as they see the word or the often misunderstood "false color". I have an interest in mapping the entire EM spectrum to RGB images for some well-studied objects. Diffraction of white light is the most beautiful thing the human eye can perceive and yet, ironically, I think the visible spectrum is highly overrated.

geckzilla wrote:Some people get surprisingly emotional about narrowband imagery. I booted someone off the APOD Facebook page the other day for being so nasty about it. I find the amount of negativity and distrust for narrowband to be very discouraging because to me it is incredibly beautiful. They just can't seem get over the (wrong!) idea that they are somehow being lied to as soon as they see the word or the often misunderstood "false color". I have an interest in mapping the entire EM spectrum to RGB images for some well-studied objects. Diffraction of white light is the most beautiful thing the human eye can perceive and yet, ironically, I think the visible spectrum is highly overrated.

Consider the lowly honeybee, living out its existence in the correct knowledge that we humans can see only in false color...

Chris wrote:
Pure, bright [OIII] is perceived by the human eye as a saturated cyan.

So what is cyan? A greenish hue, like this one? A bluish hue, like this one? A smorgasboard of hues where you can pick greens of blues as you please, like this one?

I remain confused. I should probably ask, instead, what OIII cyan really is, and how greenish or how bluish it is.

Ann

If you had two beams of light at 500.7 nm and 495.9 nm you could reproduce the color of OIII. It peaks at those two wavelengths. Short of having a jar of it on your desk though you might never be satisfied.

Ann wrote:So what is cyan?

Cyan is the color lying directly between green and blue. On your monitor, it is formed by mixing green and blue at full intensity. Of course, all named colors are generally understood to represent a range of wavelengths, so people will quite correctly use "cyan" to identify a range that includes a degree of imbalance towards blue or towards green. You might be interested in playing around with this calculator which creates the nearest matching RGB value associated with a given wavelength. For [OIII] use 501 nm (or you can drop that a couple of nm to approximate the effect of the weaker 496 nm line). You'll see a green biased cyan color.

Of course, that's just the color of the pure emission, which you'll never see in nature. The actual color will, at the least, be less saturated, and probably altered by interference from other sources, as well. It's pretty meaningless to ask what color [OIII] is outside the context of a laboratory.

Chris Peterson wrote:this calculator

Useful thing, that.

geckzilla wrote:

Chris Peterson wrote:this calculator

Useful thing, that.

Very much. Thanks for posting it Chris.

Thanks a lot, Chris. Your calculator shows a very green hue, which I myself would never have called cyan. It is also far greener than anything I could find when I googled "cyan pictures".

Thank you very much! That's a very useful calculator.

Ann

Ann wrote:Thanks a lot, Chris. Your calculator shows a very green hue, which I myself would never have called cyan. It is also far greener than anything I could find when I googled "cyan pictures".

Just for reference, you can compare with 520 nm pure green, 490 nm pure cyan, and 440 nm pure blue... all of which are quite accurate when I compare them with my grating monochromator. To my eyes, 501 nm from the monochromator looks just a little less green, or more cyan, than what I see on my screen. But converting pure wavelengths into the limited RGB space of even a well calibrated monitor is an approximation at best.

I may have posted this reference before, but I find very useful the relationship between blackbody temperatures and RGB. Again, this is just chrominance; the actual perceived color is hugely affected by luminance as well, which typically reduces saturation (so we don't usually see stars colored this intensely, unless they are blurred in an image).

Thank you so much, Chris, that is so interesting!

Fascinatingly, according to that list you provided, the Sun, at 5,800 K, would be very slightly yellowish. I'm surprised. Would it really serve humanity best to see its chief source of light as colored, not white? And would it serve humanity best to see daylight as colored, not white?

Ann

geckzilla wrote:Some people get surprisingly emotional about narrowband imagery. I booted someone off the APOD Facebook page the other day for being so nasty about it. I find the amount of negativity and distrust for narrowband to be very discouraging because to me it is incredibly beautiful. They just can't seem get over the (wrong!) idea that they are somehow being lied to as soon as they see the word or the often misunderstood "false color". I have an interest in mapping the entire EM spectrum to RGB images for some well-studied objects. Diffraction of white light is the most beautiful thing the human eye can perceive and yet, ironically, I think the visible spectrum is highly overrated.

I also get incredibly emotional about narrowband but in a positive way! Seeing OIII narrowband always brings joy to my heart! The beautiful thing about narrowband is it opens up an invisible universe in the visible spectrum. I think a large part of the negativity comes from lack of knowledge about what narrowband is. So many objects are simply too faint to be visible in regular broadband RGB or BVR images. Narrowband reveals the narrow emission associated with particular ionized gases. The way this information is presented is an important topic, read more here: http://arxiv.org/abs/1308.5237

starsurfer wrote:

geckzilla wrote:Some people get surprisingly emotional about narrowband imagery. I booted someone off the APOD Facebook page the other day for being so nasty about it. I find the amount of negativity and distrust for narrowband to be very discouraging because to me it is incredibly beautiful. They just can't seem get over the (wrong!) idea that they are somehow being lied to as soon as they see the word or the often misunderstood "false color". I have an interest in mapping the entire EM spectrum to RGB images for some well-studied objects. Diffraction of white light is the most beautiful thing the human eye can perceive and yet, ironically, I think the visible spectrum is highly overrated.

I also get incredibly emotional about narrowband but in a positive way! Seeing OIII narrowband always brings joy to my heart! The beautiful thing about narrowband is it opens up an invisible universe in the visible spectrum. I think a large part of the negativity comes from lack of knowledge about what narrowband is. So many objects are simply too faint to be visible in regular broadband RGB or BVR images. Narrowband reveals the narrow emission associated with particular ionized gases. The way this information is presented is an important topic, read more here: http://arxiv.org/abs/1308.5237

Yeah, I think comprehension is vastly increased when the media is presented in multiple formats. Like, first showing how your eyes see it, then showing how an instrument sees it, and letting the user flip back and forth at their own pace.

I like to see the separate Ha and OIII images of a particular object to get a better understanding of its composition. My favourite narrowband astrophotographer is Don Goldman.