ic 410 The Tadpoles Nebula - my 2nd attempt with Selenium toning

[wimvb]

A lifetime ago, before I had kids, I used to do photography the real way. The last step in the creative process used to be Selenium toning, to produce a warmer toned black and white print. The longer you had the print in the toning bath, the stronger its effect would be. I have tried to mimic the effect with a few monochrome H-alpha images. "Mimic", because I no longer have a dark room, and all my image processing is done on a computer, in PixInsight.

This is a starless H-alpha image of the Tadpoles Nebula, ic 410. The integration time was 4 hours and 48 minutes. Processing was done in PixInsight. To get the warm colour, I just combined two different stretches with ChannelCombination. The stronger stretch went into the red channel, and the weaker stretch went equally in both the green and blue channel. Once I had the RGB image, I applied further stretches and a bit of "dodging and burning" to enhance the contrast.

Edited December 27, 2023 by wimvb

[gorann]

Looks great Wim, especially for 4+ hours under our current conditions, although the PI work flow says nothing to a PS man like me, but I am sure a PS filter can do the same. Myself, I sometimes like to give Ha images a touch of red to indicate their true colour.

[wimvb]

1 hour ago, gorann said:

Looks great Wim, especially for 4+ hours under our current conditions, although the PI work flow says nothing to a PS man like me, but I am sure a PS filter can do the same. Myself, I sometimes like to give Ha images a touch of red to indicate their true colour.

Thanks, Göran. You know, the "true colour" of Hydrogen is a pinkish white. At least according to the gas discharge lamp I use when teaching atomic physics. But maybe in space the Hydrogen Beta line is so much weaker. If I also had a Oiii filter in my filter wheel, and the moon were less prominent, I would probably have chosen a HOO colour palette. "But since I don't, I didn't." 😉

[gorann]

9 hours ago, wimvb said:

Thanks, Göran. You know, the "true colour" of Hydrogen is a pinkish white. At least according to the gas discharge lamp I use when teaching atomic physics. But maybe in space the Hydrogen Beta line is so much weaker. If I also had a Oiii filter in my filter wheel, and the moon were less prominent, I would probably have chosen a HOO colour palette. "But since I don't, I didn't." 😉

Is not pink a mix of red and white light? Maybe your discharge lamp also produces some more broadband light that is mixed in with the Ha. 656 nm is near the middle of the red part of the spectrum.

[Mandy D]

2 hours ago, gorann said:

Is not pink a mix of red and white light? Maybe your discharge lamp also produces some more broadband light that is mixed in with the Ha. 656 nm is near the middle of the red part of the spectrum.

No, pink (magenta) is the result of mixing red and blue light. White is the result of mixing red, blue and green.

[vlaiv]

2 hours ago, gorann said:

Maybe your discharge lamp also produces some more broadband light that is mixed in with the Ha. 656 nm is near the middle of the red part of the spectrum

That is not very accurate rendition of spectrum. Compare to this one:

Ha is very deep saturated red / dark red. You can always see what Ha light looks like by actually looking thru the Ha filter.

[wimvb]

Methinks I should have expressed myself less casual. After the holiday break, I will just have to set up the gas discharge lamp and take a few pictures. Meanwhile, here's my very unscientific spectrum of the light source. Mind you, this was done with a CD spectrograph and a mobile phone camera in classroom lighting (overhead LED lamps) during day time.

In the meantime; yes, I'm aware that the blending of blue and red gives magenta. But that is under the assumption that the blue and the red have about equal intensities. The final colour that we perceive not only depends on the intensities of the individual constituents, but also on the overall brightness of the source, which depends on the pressure of the gas in the lamp, as is evident from the difference between high pressure and low pressure sodium lamps.

  😉😉😉

[vlaiv]

5 minutes ago, wimvb said:

Mind you, this was done with a CD spectrograph and a mobile phone camera in classroom lighting (overhead LED lamps) during day time.

Well, any sort of spectrograph is a good idea, but one must be careful when speaking about pure spectral colors in terms of capture and rendition.

We can't properly capture pure spectral colors with our cameras, and we can't properly display pure spectral colors with our computer screens.

Almost al spectral colors are outside of gamut of almost all capture and recording devices.

Mind you, this does not mean that we can't isolate particular wavelength with filters as we do in Ha/Hb/OIII, SII and similar (although fact that our targets emit individual wavelengths helps with this as filters are not quite narrowband although we call them that - think of Solar Ha that passes 1/100th bandwidth of what ordinary Ha filter passes).

In principle, we could capture spectral colors properly if we had sensor that has response that maps 1:1 to XYZ color space. Most sensors lack this capability.

Similarly, in order to have display that can reproduce all pure spectral colors - we would need to have display that can create any sort of spectrum in its pixels - which we now don't have.

As far as rendering spectrum goes - we need to resort to a trick. We know how to "generate" signal that represents pure spectral color (in XYZ color space), however, since our screens can't reproduce it - we need to do perceptual matching and select color that our display can produce that is closest in resemblance to what we would see with our eyes. 

Interestingly enough - although we can't display individual spectral lines - we can display some combinations of them (but not all).

Here we have "space of visible colors" with sub space of colors that can be displayed on computer screen that can display sRGB color space (central triangle).

There is interesting property of light and our vision that goes like this: If you take any number of points on above diagram - you can generate any color inside convex hull bounded by said points just by using light with color described by those points. This is why our computer screens can display any color inside above marked triangle - because pixels are able to produce R, G and B light with particular coordinates in above diagram (you even have primary illuminants in top right corner with x and y coordinates of each).

Pure spectral colors lie on outer boundary of color space (they are marked by their wavelength).

All of this shows that - if you want to produce particular spectral color - you need to include its source in color mix as you can't produce any pure spectral color as mix of any other two light sources - whole color space is convex and you can't draw a line from any two points that crosses the boundary unless one of the edges of this line is said point on the boundary.

It also shows that you can display Ha/Hb color mix in some ratios (but not all) - since 656nm - to 486 line passes thru marked triangle and you can see what sort of colors such mix can produce - from red to purple, pink and blue. In fact - if we use "strict" way of producing colors - we can't have red version as line falls outside of triangle - but if we do above trick and select similar colors that do lie in triangle and are close on diagram (and more importantly visually) then we can get almost all combinations.

By the way - here is image of color spectrum produced with unknown (but I'm assuming incandescent light) source on gray background (which does tend to wash out colors a bit):

This is also a good capture (but again, we have some background):

 

[gorann]

2 hours ago, Mandy D said:

No, pink (magenta) is the result of mixing red and blue light. White is the result of mixing red, blue and green.

I disagree Mandy and think you are mixing up pink with purple (=magenta). In my world pink is a pale red colour and Wiki says "Pink is the colour of a namesake flower that is a pale tint of red. It was first used as a color name in the late 17th century." Here is what "pure pink" looks like in a colour scheme:

 

[gorann]

2 hours ago, vlaiv said:

Ha is very deep saturated red / dark red. You can always see what Ha light looks like by actually looking thru the Ha filter.

Yes, I thought of suggesting holding up a Ha filter to a daylight sky.

Edited December 28, 2023 by gorann

[Mandy D]

7 minutes ago, gorann said:

I disagree Mandy and think you are mixing up pink with purple (=magenta). In my world pink is a pale red colour and Wiki says "Pink is the colour of a namesake flower that is a pale tint of red. It was first used as a color name in the late 17th century." Here is what "pure pink" looks like in a colour scheme:

 

 

Magenta is definitively not purple. It is a very deep pink. If you add 100% blue, 100% red and 75% green you will have a softer pink, more like the example you produced.

Anyway, my point is that you make white light and secondary colours by mixing coloured lights, not the other way around.

[gorann]

3 minutes ago, Mandy D said:

Magenta is definitively not purple. It is a very deep pink. If you add 100% blue, 100% red and 75% green you will have a softer pink, more like the example you produced.

Anyway, my point is that you make white light and secondary colours by mixing coloured lights, not the other way around.

I am quoting Wikipedia again (https://en.wikipedia.org/wiki/Magenta), The first line says "Magenta (/məˈdʒɛntə/) is a purplish-red color"🙂

[gorann]

14 minutes ago, Mandy D said:

Magenta is definitively not purple. It is a very deep pink. If you add 100% blue, 100% red and 75% green you will have a softer pink, more like the example you produced.

Anyway, my point is that you make white light and secondary colours by mixing coloured lights, not the other way around.

And I now mixed red with 50% white in Photoshop and got what I this is pink:

[Mandy D]

30 minutes ago, gorann said:

And I now mixed red with 50% white in Photoshop and got what I this is pink:

What you have actually done is to mix white (which is 33% red, 33% green and 33% blue) with an additional 50% red, because the white is a balanced mix of red, green and blue. You cannot carry out your analysis in the way you are attempting to. White does not have a solitary wavelength associated with it, whereas red, blue and green can and do.

[gorann]

32 minutes ago, Mandy D said:

What you have actually done is to mix white (which is 33% red, 33% green and 33% blue) with an additional 50% red, because the white is a balanced mix of red, green and blue. You cannot carry out your analysis in the way you are attempting to. White does not have a solitary wavelength associated with it, whereas red, blue and green can and do.

Yes of course I know that but red, blue and green are not solitary wavelengths

[vlaiv]

11 minutes ago, Mandy D said:

What you have actually done is to mix white (which is 33% red, 33% green and 33% blue) with an additional 50% red, because the white is a balanced mix of red, green and blue. You cannot carry out your analysis in the way you are attempting to. White does not have a solitary wavelength associated with it, whereas red, blue and green can and do.

Things are even more complicated than that.

White is not color of light - there is no white light. White is construct of our mind. Our brain selects what it thinks is best suited to be white at any particular time.

Light that we perceive as white in one set of conditions can seem to be yellowish white under different circumstances and even bluish white under yet another conditions.

That is why we talk about "white points" rather than white light when we discuss color spaces. It is light that is most likely to be perceived as white under set of conditions that color space is designed for.

Similarly - there are not red, green and blue colors. I mean, there obviously are such color - but unless you specify color more correctly - you are very ambiguous of what color you are actually talking about.

When you say that white is 33% blue - which blue are you exactly talking about:

 

Here is a neat trick by the way, I'm going to repeat above graph:

Provided that I marked Pink correctly (I'm a male and as such take no responsibility in naming colors correctly ), It can be produced by either taking Orange and Blue (marked with arrows) or Green and Some strange color between red and blue that looks like Pink but is much more saturated in certain ratios.

Like I explained - any color on above diagram can be produced with any other colors (we are talking about light and mixing of the light of certain spectra) provided that wanted color lies inside convex hull created by vertices of other colors that we use to generate it.

There is nothing inherently special in "red", "green" and "blue". We can do the same with Teal, Orange and that saturated color between red and blue that looks like Pink

Well, in fact - there is something a bit special with red, green and blue - it turns out that most of color gamut can be covered with correct choice of blue like color, red like color and green like color:

Here is wide gamut RGB color space defined by CIE - it uses 700nm, 525nm and 450nm pure spectral lines as "R", "G" and "B" components - and thus is able to produce most visible colors - with only three wavelengths!

 

[gorann]

59 minutes ago, Mandy D said:

What you have actually done is to mix white (which is 33% red, 33% green and 33% blue) with an additional 50% red, because the white is a balanced mix of red, green and blue. You cannot carry out your analysis in the way you are attempting to. White does not have a solitary wavelength associated with it, whereas red, blue and green can and do.

PS. My only point is that pink is far from magenta but is white light with a red bias. Magenta is purple so red light mixed with blue. As @vlaiv states then there are many shades of red, blue and green, so there are many shades of pink and purple.

[Mandy D]

5 minutes ago, gorann said:

PS. My only point is that pink is far from magenta but is white light with a red bias. Magenta is purple so red light mixed with blue. As @vlaiv states then there are many shades of red, blue and green, so there are many shades of pink and purple.

"There are many shades of pink ..." <--- You have stated it here, yourself. Magenta is one of those shades. It is a very deep and saturated pink and was the simplest example I could produce at short notice. The major point I am making is that whte does not have a wavelength associated with it, so does not appear in the spectrum, even though such spectrum can be produced from white light, which is the proof of it's composition. Vlaid has it nailed as human construct, but even that is not the full story.