Re: [APML] RG200 characteristics. Was: New Images From Last weekend

["bert katzung" <katzung1@home.com>]

Hi Philip:
OK, we're definitely communicating on the H-alpha line: it's falling in the
middle of the cyan-forming layer where the sensitivity of the other layers
is zero, so the sensitivity of that layer alone will determine what
registers on the film. More cyan, I guess, will "print" as more (brighter)
red.

Where I lose my grip is in your line immediately after the Kodak URL. When I
look at the curves, I see a certain quantity of sensitivity in the yellow
(minus-blue) and a certain quantity of sensitivity in the magenta
(minus-green) layer. What I DON'T see is the weird result you refer to. I
just see two curves and I would like to *predict* from those curves whether
I will get good registration of bluish-green emission lines from that film.

Taking a different tack, perhaps the point I'm missing is in your comment
about the *relative* quantity of blue and green response. Are you saying
that if the yellow and magenta curves crossed at exactly 500.7 (like NHG800)
rather than at 485 (or whatever that RG200 cross-over wavelength is), then
the OIII color would be more accurate? That would be great, because then I
could simply look to see where the curves for any given film crossed. I
still wouldn't understand *why* the cross-over point wavelength was more
important than the summed sensitivity at 500.7, but at least I would have a
predictive tool I could use.

I hope I'm making my problem clear. Sometimes just formulating the question
makes the answer obvious, but that hasn't worked for me so far.

Thanks for your thoughtful input....
Bert

Bert Katzung
katzung1@home.com
www.astronomy-images.com

> Hello Bert,
>
> I think I understand what you are saying.  In the case of 656.3nm it's
only
> quantity that matters, because the whole of that spike can only fall in
the
> red (cyan forming) layer.  It can only evoke a response in the red
> layer.  But in the case of the 500.7nm spike it is more complicated
because
> that does not fall right in the middle of one of the colour forming
> layers.  In fact it falls almost exactly mid way between the blue and
green
> layer.  So if the film is responding correctly, that 500.7nm spike should
> produce a colour that is mid way between green and blue (sort of 'aqua
> marine' or whatever you want to call it:-)  but if you look at that curve
> for RG 200 again...
> http://www.kodak.com/global/en/consumer/products/pdf/e42.pdf
> ... you can see something wierd - that 500.7nm spike will produce a
> response that is almost wholly in the green with very little blue.  It
will
> produce a predominantly green colour, and when green is mixed with red
> (656.3nm) it produces a colour that veers towards the yellow (minus
blue) -
> in other words Ha regions that are orangey-red.  That's simply wrong and
is
> very typical of Kodak films.  If the 500.7nm spike produced the correct
> blue-green colour then, when mixed with the red, it would de-saturate it,
> producing a pinkish red.  That is correct, and it is the colour that we
see
> from tri-colour work, both CCD and film.
>
> So it is not just a case of quantity with the 500.7nm spike - you can't
> simply add up green and blue, and because the total is OK, say that the
> response is correct.  In this case it's also the quality that's critical -
> the _relative_ quantity of green and blue response.  And as mentioned
> above, they should be about equal.
>
> OTOH Fuji films often have a good OIII response and in the case of NHG II
> 800 it is terrific.  See here (scroll down the
> page):  http://www.astrocruise.com/xppfvnhg.htm  But just as if to deny
the
> astrophotographer his perfect film, these films often have a weak Ha
> response, as is the case with NHG II 800.  The exact opposite of the Kodak
> problem!  If only Kodak and Fuji could get together... there is no doubt
> whatsoever that technology exists to produce an astro film with perfect
> characteristics.
>
> (Much of the above owes nothing to me and everything to Chuck - I hope I
> have it reasonably correct)
>
> --Philip
>



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