Re: [APML]: Color Film and Filters

[Chuck Vaughn <aa6g@aa6g.org>]

Jack Schmidling wrote:
> 
> Not at all.  Just applying a little objective experimentation to prove
> what seems intuitively obvious.  I have been wrong before so if you
> can convince me that pumpkins are really red, I am all ears.

Your comparing apples an oranges (or pumpkins). That is you cannot
compare a
daylight scene (a continuous spectrum) with an emission nebula (a
non-continuous spectrum in the extreme). Think of a typical nebula as a
red LED and a blue-
green LED, the green one often dimmer by far than the red one. There are
a 
few other colors of LEDs too, but they are likely to be very dim and
over
powered by the red and blue-green ones.

So what do you see when you look at these red and blue-green ones? Red
and
blue-green and not much else. Look at these same two LEDs through a Deep 
Sky filter and what do you see? The same as before because the DS filter
has little attenuation of the red and blue-green.

The DS filter makes the pumpkin red or the brown leaves red because it
subtracts orange and yellow leaving only the red component of the light
that made the pumpkin and leaves the color they really are. It didn't 
shift orange to red, only removed some of the colors.
 
> Seems to me it is the eye that we have to reconcile here not the film.
> When we talk about proper colors, are we not using the eye as the judge
> of what is proper?
> 
> Then why do the NA nebula and the Ring photograph red when, with or
> without a filter, the eye sees no red, no matter how big a scope one
> looks at them with?
>
> Not arguing for the sake of arguing but trying to understand why making
> a star red that isn't is cheating but doing the same to a nebula is
> somehow advancing the art.

The same answer applies to all three of the preceding paragraphs and
that
is the nebula really _is_ red. Since your eye has essentially no
response
to low light levels at 656nm you can't see it. It doesn't mean it's not
there. Color film that renders nebulae red simply has a flatter response
in the red as compared to the rest of the spectrum than your eye does.

When you photograph with Tech Pan the nebula recorded is the red image.
You wouldn't say Tech Pan is no good because it recorded a color you
couldn't see, would you? If you accept TP in this regard then you have
to accept color film. An example at the other end of the spectrum is
the Pleiades. The Pleiades have a high amount of UV light and the blue 
recording layer on film is quite sensitive to UV as is Tech Pan. That's
why the nebula is easy to photograph but difficult to see. Your eye
cannot
see UV. Actually the blue Pleiades is far more incorrect than a red
nebula.
What is UV supposed to look like when you can't see it? The film makes
it blue for you. At least it makes a red nebula red!

> > Here's something I noticed when we were investigating designs for our
> > H- alpha interference filter...
> 
> Just for the record, who is "our"?

To make a long story short, a few years back a couple friends of mine
and
I had some H-alpha interference filters specially made. Think of a UHC
filter except centered at 656nm instead of 501nm and you have it.

Chuck  <aa6g@aa6g.org>