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

[Philip Perkins <philip@astrocruise.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


At 14:21 18/10/01 -0700, you wrote:
>Hi Philip and list members:
>Your comment about the RG200 spectral sensitivity reminded me of a question
>I've been meaning to ask: simply, how to interpret these film sensitivity
>curves? For example, for the OIII line (I assume we're talking about the one
>at 500.7 nm) and RG 200 , it looks like the sensitivity in the
>magenta-forming layer is about 1.7 (log values) and the sensitivity in the
>yellow forming layer is about 0.9. If we scan an RG200 negative exposed to
>500.7 nm, will it not report a negative-of-green color there of about 2.6
>units (sum of 1.6 + 0.9)? So the O III green should register partly in the
>magenta-forming layer and partly in the yellow-forming layer. The sum of the
>two would be equal to (or better than)  the sensitivity of the cyan-forming
>layer  to H-alpha light at 656.3 (about 2.4 units).
>Or am I missing something fundamental here?
>Bert
>
>Bert Katzung
>katzung1@home.com
>www.astronomy-images.com
>
>----- Original Message -----
>From: "Philip Perkins" <philip@astrocruise.com>
>To: <astro-photo@seds.org>
>Sent: Thursday, October 18, 2001 10:54 AM
>Subject: Re: [APML] New Images From Last weekend
>
>
> > Michael,
> >
> > Great shots and well processed.
> >
> > RG 200 looks very encouraging.  I checked the sensitivity curves which are
> > here:
> >
> > http://www.kodak.com/global/en/consumer/products/pdf/e42.pdf
> >
> > These curves are very typical for a Kodak colour negative film.  In fact
> > they are very close indeed to a lower sensitivity version of PJM-2, PJ400,
> > and LE400.  This means some good things and some bad things.  The Ha
> > response is very good, which is typical, but the OIII response is very
> > poor, which is also typical.  However the OIII response is no worse than
> > the 'PJM-2 genre' and your results show that it improves dramatically upon
> > hypering (the IC 405 colour looks pretty decent).  The spec says that no
> > exposure adjustment is needed for exposures of up to 10 seconds which is a
> > sign of good reciprocity characteristics (same as other films in the
>'PJM-2
> > genre').
> >
> > This looks to me like a finer grained version of 'PJM-2 genre' films like
> > PJ400, LE400, etc, and it looks to me like the OIII response might improve
> > even further with hypering than those films.
> >
> > --Philip
> >
>
>
>
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Philip Perkins - philip@astrocruise.com
Wiltshire UK & Luberon France
Astrocruise - http://www.astrocruise.com


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