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Originally Posted by glend
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That's actually a really good point. Which would seem to conflict with the idea that some people are less sensitive to certain frequencies - which was posited as the reason why some experienced observers can't see the horsehead, even in large telescopes.
But now you mention it, the horsehead is a H beta nebula, which emits in at 486nm, which is in the *green part of the visual spectrum, yet all the RGB pictures show a red background nebulosity, which I'm struggling to make sense of.
But then wikipedia has this to say; "Infrared, as implied by its name, is generally considered to begin with wavelengths longer than visible by the human eye. However there is no
hard wavelength limit to what is visible, as the eye's sensitivity decreases rapidly but smoothly, for wavelengths exceeding about 700 nm. Therefore wavelengths just longer than that can be seen if they are sufficiently bright, though they may still be classified as infrared according to usual definitions. Light from a near-IR laser may thus appear dim red and can present a hazard since it may actually be quite bright. And even IR at wavelengths up to 1,050 nm from pulsed lasers can be seen by humans under certain conditions."
So if it's bright enough, you can even see longer light wavelengths, but is this a case of the Mesopic (colour vision) or Scotopic (luminance vision)?
Markus