Re: [APML] Filters and emission (was OK - here's your blue Veil Nebula)

[Aplanatic@aol.com]

Chuck,

Interesting points.

The problem with emission lines is that there is no way to reproduce the visual "sensation" of them with a monitor.  If I could construct a gas discharge tube such that equal intensities of H-a and O-3 were emitted, the "sensation" produced by such a tube would be neither pink nor grey, I would venture.

What would it be best approximated by?  Pink, grey, or some other subtle shade and hue?  I don't know without having such a gas discharge tube.

In lieu of doing the experiment and trying to match up the best approximation to the "sensation", I am willing to allow quite a range of colors (and therefore filter response functions).  Anywhere around pink, grey or slightly blue-green seems appropriate to me.

Your "ideal" filter does have one interesting attribute.  It more or less maximizes the total S/N for the O-3 and H-b lines.  Not a bad thing at all.

Dave Rowe

> Dave,
> 
> Like Mike's comment about no OIII, I put this up to discuss an
> issue I've never been quite able to resolve myself.
> 
> What you say is conventional wisdom. But I've mulled this over for
> sometime.... like 3 years<g> and played around in Photoshop to see
> what happens to the color when you put in different values.
> 
> My conclusion - unless I'm missing something still - is that the
> overlap areas of the filters do not add to produce a higher
> luminance when you create the RGB channels. Maybe they do on color
> print paper, but I have no experience with tricolor darkroom prints.
> 
> Look at a couple examples and maybe you can clarify it for me.
> 
> I think we can agree if you set RGB levels to the same value, the
> result will be a gray level. The question is what happens when you
> have an emission line sitting at the crossover point? Lets say you
> have an H-a line equal in intensity to the sum of the two OIII lines.
> What color would we see if we looked at that light? Gray? Pink?
> 
> My problem with the filters in the example above is that with 50%
> overlapping filters the intensity in the red channel will be twice that
> in the green and blue. If you plug that into the PS Color Picker
> you get pink. Is that really correct or should it be gray? If pink
> is correct, then that means it takes twice the level at OIII compared
> to H-a for it to be gray.
> 
> The filter bandpasses do two things for emission lines. They determine
> the intensity recorded over the bandpass and the slopes determine the
> hue of the emission line by what percentage falls in which filter. If
> the filter overlap point is less than 100%, then there is no way to
> record the correct intensity at that wavelength.
> 
> This is why I suggested such an "ideal" filter set.
> 
> Chuck <aa6g@aa6g.org>
> 
> ---------------------------------------
>  
> > Chuck,
> > 
> > I don't understand your reasoning for ideal filters.  Shouldn't the sum of the
> > three filters produce a unit response?  In other words, at the crossover
> > point, shouldn't each filter be about 50% transmission?  Am I missing
> > something?
> > 
> > Dave Rowe
> > 
> > 
> > 
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