Re: [APML]: Color Film and Filters

[Chuck Vaughn <aa6g@aa6g.org>]

Jack Schmidling wrote:
 
I'm going to assume some of your post is in jest and you're pulling our
collective chains a bit but for the benefit of some who may be less
experienced with filters I decided to make a few points.
 
> Contrary to popular opinion, one does not need access to a laboratory,
> fancy equipment or monochromatic light to be able to evaluate the
> effect that nebula filters have on color photos.

Actually all you need is the transmission curve of the filter and the
response curves of the film in question. 
 
> I next walked over to the pumpkin patch and much to my incredulous
> delight, there they were, 6 dazzling red pumpkins.

I've done that. It's a good way to illustrate the minus yellow and minus
orange aspects of a Deep Sky filter.
 
> As we only go to town once a week, it will be a while
> before I can prove my point but I suspect what my eye sees is
> what the film will see, otherwise, what good is color film?

What happened to your new color darkroon? :-)
 
> So again I ask,  does pointing this filter up into the night sky
> somehow validate all the phony colors?

For the benefit of others (because I know Jack knows this) the DS filter
will alter the color of stars photographed through it. But it's likely to
render most nebula just about the same as with no filter. Nebula emit
strongly at only a few emmission lines and the DS filter passes those
lines with little attenuation. Therefore the DS filter is effective at
reducing sky fog while not affecting the color of most nebula significantly.

Here's something I noticed when we were investigating designs for our H-
alpha interference filter. Interference filters have very high attenuation for 
_about_ 100nm on each side of the design bandpass. Above and below they begin
to transmit again. It appears that the Deep Sky filter takes advantage of this
characteristic in that high transmission occurs around the H-alpha line. I
don't think this is designed into the filter but merely a convenient accident.
This turned a broadband visual filter into a photographic nebula filter.
Those of you with UHC and OIII filters will notice you can see red through
them in the daytime and also red in bright stars in the telescope.

Red transmission could be eliminated by using a light blue (minus red) filter
as the substrate. You cannot see red through the Orion Ultra-block filter.
They do this. We used a red substrate on our filter to eliminate blue transmission.
It does pass IR around 850nm to 900nm, a problem for CCDs but not Tech Pan.

If we have any coating experts on the list you might want to confirm or repute
this.

Chuck  <aa6g@aa6g.org>