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I think what we have to agree that the frequency's of light emitted by an doubly ionized oxygen are at 500.7 and 495.9nm. There is simply no argument here. You can discuss until the cows come home about your perception of these wavelengths but the realities of the physics involved and the actual frequencies emitted are not debatable.
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A nicely written, short synopsis of how the human eye sees colors compared to a standard OSC sensor camera. See here: https://pages.jh.edu/rschlei1/Photographic/violet/violet.html This from my thesis advisor (quite some time ago), Robert Schleif, Ph.D., he posted some years ago. Though very high level, there are some surprising things to learn about the biology of color sight. Note that the color sensing cells in the human eye do not have the same spectral sensitivities than color cameras. Also, note that for blue, cyan, magenta light, the human eye does not use the "green" (or Medium) + "blue" (short) sensors to sense these, but rather the blue (short) and the red (Long) cells! Not discussed in his short article is that in the range of blue, cyan, magenta, the very low sensitivity of the Short (edit: I meant Long/red here! Ugh!) sensor , (to light) down near the blue green area,must create different perceptions during low light, vs. high light vision situations. It is just a logical conclusion from the information in the article. I may be wrong about this. But that might well impact how the M57, Cats eye, and M42 were percieved visually, even though the viewers may have thought these were bright at the time. In any case, he alludes to some areas that someone could explore much more deeply if one so chooses. Finally, he describes how full visual color perception and descrimination could have come about if humans used just two color sensors, rather than three. And the example he highlights proves that to be true, in concept. Details aside regarding this level of processing, but the brain has additional inputs to the final perception as these relate to context. If you back out of that page you can find a couple other photo related interesting topics. Best, Alan |
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Christoph Nieswand: Let me add one more point. The B filters used in astrophotography nowadays often have a redder cutoff in the blue side (i.e., transmission stops at 410nm or even 420 nm rather than 400nm). I believe this is to suppress blue halos around stars caused by minor chromatic aberrations, but there could be other reasons for this design. A consequence of this is that the flux from continuum sources (stars) is suppressed in B, making the color of all stars shift to R and G. SPCC wants to correct this, since in principle it knows about the B filter you use (unless you gave it the wrong information). In other words, SPCC brightens B, relative to R and G. This would make the color of OIII shifts to cyan, rather than mostly green with a little blue. There also exist G filters that have a bluer red cutoff. This was initially to avoid the sodium spectral band in light pollution. This makes SPCC boost the green. So depending on how this effect competes with the above effect for the B filter, the color of OIII can shift to cyan, or even greener. Recently, I use a filter set whose blue filter does not capture the 500.7 nm line at all. So my OIII from broadband filters look almost purely green, even without a hint of blue. This is also incorrect. Anyway, how OIII appears in broadband images really depends on the bandpass of G and B filters. |
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John Hayes:Shine a white light through a OIII filter, there's your answer.. Generally, the emission is teal (bluish-green) and many nebulae show green in natural color... I feel smart, this is what I was going to recommend too! |
